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Biodeterioration of Wood in Structures

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  • UnderstandingBiodeterioration of Wood in Structures


    P.I. MorrisWood Preservation Scientist

    Composites and Treated Wood ProductsForintek Canada Corp.

  • NoticeThis document was prepared by Forintek Canada Corp. The analysis, interpretation andrecommendations are those of the author and do not necessarily reflect the views of BCBEC or thoseofficers of BCBEC that assisted in the review and publication of this document.

    Neither BCBEC, nor Forintek, nor its members nor any other persons acting on its behalf, make anywarranty, express or implied, or assume any legal responsibility or liability for the completeness ofany information, apparatus, product or process disclosed or represent that the use of the disclosedinformation would not infringe upon privately owned rights. Any reference in this document to anyspecific commercial product, process or service by tradename, trademark, manufacturer or otherwisedoes not necessarily constitute or imply its endorsement by BCBEC, Forintek or any of its members.

    Care has been taken to review the research summarised in this document, but no attempt has been madeto repeat or check experimental results. It is the responsibility of the reader to apply professionaljudgement in the use of the information contained in this text, to consult original sources or, whenappropriate, to consult with a specialist in biodeterioration and protection of wood.

  • Understanding Biodeterioration of Wood in Structures ii

    Table of Contents

    1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

    2 The Structure and Composition of Wood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2.1 Wood as a Structural Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2.2 Wood as a Food Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

    2.2.1 Sapwood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.2.2 Heartwood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.2.3 Wood Moisture Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2.4 Fungi or Insects Carried over from the Living Tree . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2.5 Summing up Wood as a Food Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

    3 Wood-Inhabiting Organisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.1 Fungi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

    3.1.1 Moulds and Staining Fungi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.1.2 Soft-Rot Fungi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1.3 Wood-Rotting Basidiomycetes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

    3.2 Bacteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.3 Termites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.4 Wood-boring Beetles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.5 Carpenter Ants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.6 Marine Borers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

    4 Microbial Biodeterioration Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 4.1 The Colonisation Sequence Leading to Decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 4.2 Conditions Required for Decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 4.3 Growth and Decay Rates of Brown-Rot Fungi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

    5 Some Considerations for Prevention and Remediation of Decay and Termite Damage . . . . . . . . . . . . . . . 16 5.1 Protection by Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 5.2 Considerations for Repair and Remediation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

    6 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

    7 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

  • Understanding Biodeterioration of Wood in Structures 1

    1 Introduction

    Decay of wood buildings in BC is hardly a newproblem. In 1945, H.W. Eades of the WesternForest Products Laboratory (now Forintek)wrote: In the coastal region of BritishColumbia, much preventable loss results fromdecay in wooden structures, particularly in andaround residences. Common causes are faultyventilation, exposure of wood parts to moistureand contact of these with the ground. The bestmethod of prevention lies in proper design andconstruction, with factors which induce decayborne in mind. (14)

    Improvements to design and construction are theprovince of architects, engineers, buildingscientists, and construction trades. However,these professions have not always had readyaccess to knowledge of the factors which inducedecay. This document seeks to provide theessential basic knowledge and to direct thereader to appropriate reference works for moredetailed information.

    2 The Structure and Compositionof Wood

    2.1 Wood as a Structural Material

    If an engineer were to design a material he would almost certainlystart with strong polymer chains (cellulose)aligned to form fibres (micro fibrils). Tomaximise the strength to weight ratio, he mightconstruct small tubes (cells) by embeddingcross-banded mats of these fibres in a resinmatrix (lignin), glueing these cells together in athree dimensional structure to the required size.An engineer would also want to maximise

    aesthetics, cost effectiveness, strength-to-weightratio, workability and thermal properties. Theywould plan to manufacture the material from arenewable resource using minimal energy andwater and produce minimal pollution. Ideallythe material would be reusable, recyclable andbiodegradeable at the end of its multiple servicelives. Fortunately, nature has already done the designand manufacturing of this material for us. Wecall it wood.

    Wood does have a number of properties whichcould bear improvement but a variety ofprocesses are available to make it resistant tomould, rot, insects, fire, ultraviolet light anddimensional change.

    For the tree, the wood in the trunk performs threemajor functions (one structural and twometabolic). Structurally, it holds the canopy offoliage above other vegetation which wouldotherwise compete for light. Metabolically, ittransports water and nutrients to and from thefoliage and it stores food reserves for the tree.In the natural forest, the trees with the bestgenetic ability and environmental advantagebecome dominant and the most desirable forharvesting. In a similar way, we use some of thebest tree trunks, in their simplest worked form,as utility poles to support overhead power andcommunication lines. At the other extreme ofcomplexity, we also take the wood structureapart and put it back together again asengineered wood products for buildingconstruction. In between are a wide variety ofwood products used to construct ourcommunication pathways, transportation

  • Understanding Biodeterioration of Wood in Structures 2

    systems, places of work, recreational facilitiesand our homes. Although we use wood for itsstructural properties, we can not avoid theconsequences of the metabolic functions of thetree trunk. Wood products still absorb waterand provide a source of food.

    The negative perception of current harvestingtechniques in BC forests, has to be balancedagainst the positive environmental benefits ofusing wood. Wood is the only constructionmaterial which is a net consumer and store ofcarbon dioxide rather than producing CO2during manufacture. The use of wood insteadof other materials therefore has a doublybeneficial effect in reducing global warming.Wood systems, such as platform frameconstruction, can be very efficient in their useof materials. At the end of their initialservice life, wood products are becomingincreasingly re-used or recycled. Re-userequires very careful deconstruction, whereasrecycling can reduce wood to the flake orfibre level and rebuild it into new structuralmaterials. Wood systems are thus capable offulfilling the principles of the three Rs:reduce, reuse and recycle. Furthermore, theyare also very amenable to the much older,and frequently forgotten, fourth R: namelyrepair. It is very common in Vancouvertoday to see wood frame houses built in theearly part of the 20th century being lifted up,restored, extended and put back onto newfoundations. Unfortunately, it is also commonto see condominiums having to be rebuiltafter as little as three to five years.

    2.2 Wood as a Food Source

    We see wood as an almost ideal constructionmaterial but a lot of organisms, specificallycertain bacteria, fungi, insects, crustacea andmolluscs, see it as a conveniently orientedseries of holes made of food (Figure 1).

    Figure 1: Scanning electron micrographshowing the cellular structure of asoftwood

    These organisms make wood biodegradable andare crucial to the breakdown of woody materialon the forest floor or in water. Without certainfungi and, in warmer climates, termites, forestecosystems could not function. These organismslive by consuming complex organic materialswhich form the structural and non-structural

  • Understanding Biodeterioration of Wood in Structures 3

    components of wood. However, they can not which is physiologically dead but can still haveuse wood as food unless they also have their an important structural function.oxygen, water and temperature needs satisfied(see Section 4.2). The wood itself also has tobe susceptible to attack, that is, it must not benaturally durable. The susceptibility of wood tobiodeterioration is related to its chemicalcomposition.

    At the chemical level, the major component ofwood cells is lignocellulose, that is: celluloseand similar long chain polymers of sugar unitsencased in lignin. Fortunately, only a limitedrange of organisms are capable of breakingdown cellulose which provides most of thestrength to wood. Lignin is a three dimensionalresin of interlinked phenolic units which evenfewer micro-organisms can degrade.Hardwoods generally have less lignin thansoftwoods and the lignin they contain is alsomore readily degraded making them particularlysusceptible to attack by some types of fungi(Section 3.1). The breakdown of lignocelluloseby fungi is the process we call wood decay.

    At a larger scale, tree trunks are made up of aseries of concentric cylindrical layers (Figure2). On the outside of the tree is the bark whichis physiologically dead but is critical inprotecting the living tissue of the tree from attackby wood-destroying organisms. Beneath the 2.2.1 Sapwoodbark, the underbark (phloem) conductscarbohydrates and amino acids down from theleaves to fuel the extension of roots, the outwardexpansion of the tree during the growing seasonand for storage during dormant periods. Thenext layer is the growth layer (cambium) whereexpansion of the tree occurs. The cambium layerdivides to produce phloem cells on the outsideand the needle shaped sapwood (xylem) cellson the inside. After a certain number of years,the sapwood becomes converted to heartwood,

    Figure 2: The layers of the living tree

    During the early part of the growing season, thegrowth layer produces large, relatively thinwalled, sapwood cells, we call early-wood.Later in the growing season, it lays downsmaller, thicker-walled cells we call late-wood.The change in ratio of cell wall to water-filledor empty cell space (lumen) gives the annulardensity variation visible as annual rings.

    Sapwood conducts water from the roots to theleaves and can absorb water by capillary action.

  • Understanding Biodeterioration of Wood in Structures 4

    The cells are the longitudinal water-conductingpathways (Figure 1). These pathways are thereason wood is more permeable to water along As more sapwood is layed down on the outsidethe grain than across the grain. Passage of of the tree, the inner rings of sapwood arewater from cell to cell occurs through narrow gradually converted to heartwood. In most treeapertures (pits), many of which are fitted with a species, the conversion of sapwood tomembrane-supported plug (torus) which heartwood involves blocking pathways betweenautomatically closes if the water is removed cells which dramatically reduces thefrom one side. In addition to longitudinal permeability of the heartwood to moisture. pathways, there are radially aligned bundles ofcells (rays) which conduct water and nutrients In many tree species heartwood formation alsobetween the sapwood and the underbark. The involves the manufacture of natural toxictree also stores starches, lipids and proteins in materials which provide some degree ofthe rays. These non-structural, relatively protection against fungi and insects. Treeeasily digestible materials are one reason why species vary in the natural durability of theirthe sapwood of all tree species is much more heartwood (Table 1) and the level of durabilitysusceptible to biodeterioration than the also varies with the age of the tree. heartwood. There is very little nitrogen inwood but wood-rotting fungi are very efficient in Table 1: Natural Durability of Majorthe use and recycling of nitrogen. Somemicronutrients (minerals) needed by fungi mayalso be scarce but may be obtained from soil ormoist building materials in contact with thewood. For example, iron plays a key part in thedecay process but there is not much iron inwood. It is however found in soils, bricks,rockwool insulation and fasteners so that fungiare able to solubilize this iron for their own use.

    In the living tree, the sapwood is protected bythe bark and by active wound responses whichcan deliver natural phytotoxins to the site of awound. The tree is able to wall off thedamaged area by blocking cell lumens and pits.Once the tree is weakened or killed, thesapwood is perishable, in other words,susceptible to attack by bacteria, fungi andinsects. Wood in construction is stripped ofsome of its natural defenses and is thereforemore susceptible to decay.

    2.2.2 Heartwood

    North American Softwoods

    Predominant HeartwoodType Durability

    Douglas Fir heart moderateWestern Hemlock heart non-durableWhite Spruce heart non-durableEngelmann Spruce heart non-durableBlack Spruce heart non-durableRed Spruce heart non-durableSitka Spruce heart non-durableLodgepole pine heart non-durableJack Pine heart non-durableRed Pine sap non-durableSouthern Pine sap moderatePonderosa Pine sap non-durableAmabilis Fir heart non-durableAlpine Fir heart non-durableBalsam Fir heart non-durableW. Red Cedar heart durableYellow Cypress heart durableWestern S-P-F heart non-durableEastern S-P-F heart non-durableHem-Fir heart non-durable

  • Understanding Biodeterioration of Wood in Structures 5

    The older the tree, the more natural toxins laid living, water-conducting sapwood is typically atdown when each successive annual ring is a moisture content (mc) of 150% to 200% whileconverted to heartwood. This means that the the dead heartwood is around 30-40% mc.least durable heartwood is in the centre of the Some species, such as western hemlock and thetree and the most durable is the outer heart. true firs (e.g. grand fir and Pacific silver fir),This original pattern is exaggerated by the have a wet heartwood, 50% - 80% mc and someactivities of non-wood-rotting fungi which break have additional wet pockets associated withdown the extractives in the oldest heartwood in bacteria. the centre of the tree. These non-wood-rottingfungi can be followed by wood-rotting fungi In the living tree the cellulose in the cell wallswhich may also have some tolerance to remains fully swollen with water with the waterextractives. These factors account for the content of the cell walls around 27% of the ovenpresence of columns of decayed wood in the dry weight. The point where the walls are fullycentre of older Western red cedar trees with an swollen but the cells are otherwise empty ofintact outer shell of heartwood. Many of these liquid water is known as the fibre saturationnatural toxins are also water soluble. point. Once wood begins to dry below thisConsequently, even our most durable Canadian point, it begins to shrink. Rapid drying can resultspecies, Western red cedar, yellow cypress in stresses within the wood which may be(Alaska yellow cedar) and Eastern white cedar, relieved by the wood splitting (checking). Kilnwill suffer from loss of durability and decay drying of structural lumber reduces theover time. Second-growth trees which may be moisture content to a target average of 16%harvested after 80 years may have laid down with a maximum of 19%. Exposure to rainless extractive originally but they may also have during transport and storage can, however raiseretained more of the extractives in the central this moisture content to much higher levels.heartwood if there has been no time for Wood which has equilibrated with the moisturedetoxification to occur. This has not been content of exterior air typically ranges from 12%adequately investigated. to 18% mc. Canadian Building Codes require

    The heartwood of many tree species is darker in the time the building is closed in. Wood incolour than sapwood, but the colour is not heated buildings typically ranges from 4% tonecessarily related to the natural durability. For 10% moisture content. While green (not kilnexample, contrary to some expectations, paler dried) lumber is still used in construction, kiln-coloured western red cedar heartwood tends to dried lumber is increasingly the material ofbe more durable than the darker coloured choice.material. Yellow cypress is just as durable aswestern red cedar.

    2.2.3 Wood Moisture Content

    Wood moisture content is expressed in terms of infected the standing tree (13). These maythe weight of water as a percentage of the oven- remain viable and spread during storage if thedried weight of the wood. In the standing tree wood is not treated with a prophylactic anti-

    that wood be below 20% moisture content at

    2.2.4 Fungi or Insects Carried over fromthe Living Tree

    Green lumber may contain decay fungi which

  • Understanding Biodeterioration of Wood in Structures 6

    sapstain treatment (8). Untreated green lumber isalso susceptible to colonisation by fungi. Fungiwhich colonised in the standing tree or the In the tree, the heartwood has some level oflumberyard can remain active if green lumber durability and the sapwood is protected. Oncedoes not dry out once it is built into a structure. the tree is felled the sapwood becomesThe effects of this can be seen in the fact that perishable while the heartwood remainsalmost all of the fungi causing serious problems durable. The level of heartwood durabilityin buildings in California were also found in depends on the wood species, the age of the treegreen Douglas fir in lumberyards (9). and the position within the tree.

    Live larvae or pupae of wood-boring beetles,bark beetles or wood wasps which attackweakened living trees may also be found ingreen lumber. They do not tend to re-infest cutlumber so the infestation does not spread. Inmost cases, however, organisms that attackstanding trees do not survive the process ofmanufacturing into wood products. The initialwet heat involved in conventional and highertemperature kiln drying are effective at killingall fungi and insects. Even the most resistantwood-rotting fungi can be killed by 75 minutesat 65 C, 20 minutes at 80 C, or 5 minutes ato o

    100 C in the centre of the product, provided theo

    atmosphere is close to saturated with moisture(6). Increasingly complex life forms die atlower time/temperature combinations.Pinewood nematodes are killed at a temperatureof 54 C for 30 minutes or if heated to 70 C.o o

    Wood-boring beetles can be killed if the woodis heated to 50 C. Being a good insulator theo

    wood will remain at these target temperaturesfor some time. The temperatures typicallygenerated in the manufacturing of structuralpanels and engineered wood products arenormally adequate for sterilisation. Air dryingand lower temperature kiln drying methods, suchas dehumidification, may not kill theseorganisms.

    2.2.5 Summing up Wood as a Food Source

    Further Reading:

    More detail on wood as a substrate forbiodeterioration may be found in thepublications by Eriksson et al (16), Eatonand Hale (15) and Zabel and Morrell (28).

    3 Wood-Inhabiting Organisms

    A wide range of organisms can live in anddegrade wood. In temperate climates, fungicause the most damage to wood products. In thetropics, termites are the more serious problem.Carpenter ants can be a minor problem insouthwestern BC and the Pacific Northwest ofthe USA. Wood-boring beetles are regarded asa serious concern in some regions of the worldbut have not proved to be a major problem inCanada. Marine borers can cause problems forwood stored or used in the oceans.

    3.1 Fungi

    The filamentous fungi (as opposed to yeasts) area group of organisms which feed on organicmatter and grow by extension and branching oftubular cells (hyphae), in a manner somewhat

  • Understanding Biodeterioration of Wood in Structures 7

    similar to the roots of plants. This form of divided into moulds, stainers, soft-rot fungi, andgrowth is called mycelium (Figure 3).

    Figure 3: Mycelium and mycelial cords of awood-rotting basidiomycete on drywall

    Fungal reproduction involves the production ofmicroscopic spores which are smaller than thepollen grains of plants. These spores are spreadby air currents, insects or by water splash. Theamount and types present in the atmosphere varywith the season and the weather. For example,spore production of many fungi increasesdramatically in the fall. A light rainfall canincrease the spore load in the air but a heavyrainfall can wash most of the spores to theground. The fungi which live in wood can be

    wood-rotting basidiomycetes. These are listedin the order of increasing levels of damage ofwood and in the order in which they colonisewood (see section 4.1).

    3.1.1 Moulds and Staining Fungi

    These fungi typically produce spores onstructures which can only be identified under themicroscope. They normally require a highrelative humidity to produce spores. They areparticularly prevalent in exterior air in latesummer and fall when diurnal temperaturevariations favour the formation of fog and dew.This is also the season when plants are droppingoverripe fruit and dying back. Inside the home,under the right conditions, this type of funguswill grow on a wide variety of organic materialsincluding food, dead or dying houseplants,natural fabrics and drywall. They can also growon dust and dirt on otherwise inert surfaces.Mould and staining fungi live on non-structuralcomponents of wood and are thus moreprevalent in sapwood. Moulds have colourlessor pale coloured mycelium and coloured spores.En mass, these spores may appear white, black,grey, green, olive, brown, yellow or othercolours, depending on the fungus species.Mould spore production and the associateddiscolouration, is often confined to the surfaceand can be washed or planed off. However,some moulds are known to cause respiratoryproblems in sensitive individuals.

    Staining fungi have brown- or black-colouredmycelium, giving the wood a characteristicblack or blue appearance. These fungi, whileunsightly, generally cause very little damage. If

  • Understanding Biodeterioration of Wood in Structures 8

    the wood dries out, they go dormant. If it doesnot they are normally replaced by wood rottingfungi. The major economic impact of staining Closely related to the mould and stain fungi, thefungi is in the disfigurement of the appearance soft-rot fungi are capable of breaking downof wood products. This can reduce the value of lignocellulose and therefore of causing severeraw lumber and require costly refinishing on strength loss. Some types erode the cell wall,wood in exterior exposure. In the unusual event others tunnel within it. Confirmation of soft-rotthat staining fungi remain active longer than must be made using the light microscope. Thesenormal, they can cause reduction in impact fungi typically attack wood in permanently moiststrength. conditions such as soil or cooling towers. They

    Staining fungi can be subdivided into two inwards and some are tolerant of heartwoodgroups: those causing sapstain and those causing extractives and wood preservatives. They canbluestain in service. Sapstain fungi are early cause severe damage to preservative-treatedcolonisers of freshly felled logs and they rapidly wood in ground contact if insufficientpenetrate deep into the sapwood. Bluestain preservative is used. However, in untreatedfungi tend to grow mainly near the surface of the wood above ground they are normally succeededwood. They have the capability to penetrate very quickly by wood-rotting basidiomycetes.through paint films and their pigment protects Hardwoods such as oak and maple arethem from ultraviolet (UV) light. Some can also particularly susceptible to soft rot. Since we uselive on the products produced by the breakdown softwood in construction, soft rot causesof wood by UV. Consequently, they are a problems only insofar as it places limits on theparticular challenge to transparent film-forming service life of preservative-treated wood infinishes which do not block UV light and/or can ground contact.not accommodate the movement of wood inresponse to humidity. Staining fungi growparticularly well on surfaces which are keptmoist by a partially intact surface film but These are the fungi which cause most of theremain exposed to UV light. If wood is left damage found in buildings. Basidiomycetesunfinished, UV light, in combination with water, typically produce large fruiting structures,causes the breakdown of the lignin which is then toadstools, brackets or conks, which are obviouswashed away by rain. The exposed cellulose to the naked eye. A good example of a fruitingreflects light giving a silvered appearance but body of a wood-rotting basidiomycete is thestaining fungi darken this to a grey "weathered" oyster mushroom now sold in an increasinglook. number of produce stores. Although many

    3.1.2 Soft-Rot Fungi

    attack relatively slowly from the surface

    3.1.3 Wood-Rotting Basidiomycetes

    basidiomycetes in temperate regions produceannual fruiting bodies in late summer and fall,spore production may continue through a mildwinter. When moisture and temperature

  • Understanding Biodeterioration of Wood in Structures 9

    conditions are favourable, some wood-rotting rot fungi on the basis of the colour change theybasidiomycetes will produce fruiting bodies in cause on wood.winter, spring or early summer. Yet others,particularly those which grow on live trees,have perennial fruit bodies which grow largereach year. The mycelium of most of theeconomically-important wood-rottingbasidiomycetes is white, pale yellow or buff.Several of them produce mycelial cords (Figure3) consisting of bundles of hyphae which extendby growing at the tip. These cords are capableof growing considerable distances over inertmaterials and, in some cases, transportingmoisture with them. They are more resistant todrying out and competition than a loose web ofmycelium. They also concentrate the energies ofthe fungus facilitating the invasion of newsubstrates. The power to invade new substratesis termed inoculum potential. Mycelial cordshave more inoculum potential than mycelium,which, in turn, has much more inoculum potentialthan a spore. Infection of wood structuresabove ground probably occurs mainly throughspore germination, though subsequent spreadfrom component to component can be bymycelium or mycelial cords. In or near theground, infection by mycelium or mycelial cordsis much more likely.

    The term wood-rotting basidiomycetes (WRBfor short), although cumbersome, is at leastprecise. Many wood-rotting fungi (for examplethe soft-rot fungi) are not basidiomycetes andmany basidiomycetes do not decay wood. TheWRB decay wood at a much faster rate than thesoft-rot fungi. Their destructive effects can bereadily distinguished with the naked eye and theycan be divided into white-rot fungi and brown-

    White-rot fungi: These fungi tend to attackhardwoods such as aspen and maple. Theydegrade both lignin and cellulose leaving thewood bleached to a paler colour. Decay maystart in small pockets and may result in a stringytexture to the wood (Figure 4). Strength loss iscomparatively slow to occur. Figure 4: Bluestain (black) and white rot

    (bleached patches) on OSB

    Brown-rot fungi: These fungi tend to attacksoftwoods such as pines, spruces, hemlocks andfirs. They degrade primarily the cellulose butthey do modify the lignin leaving a brown,

  • Understanding Biodeterioration of Wood in Structures 10

    oxidised appearance. As the first step in the cause the most damage in Canada but the list isdecay process, the cellulose is chopped into likely to have much in common with the listshort lengths by a non-enzymic process resulting developed for the United States (12). The WRBin rapid early strength loss. This also allows the most often identified with decay in buildings inwood to crack across the grain as it shrinks in the USA were Meruliporia incrassata,response to the breakdown and removal of the Gloeophyllum trabeum, Tapinella panuoidescellulose. The cross-grain cracks and (Figure 6), Antrodia vaillantii, Coniophoralongitudinal cracks combine to give the puteana, Serpula lacrymans, Postia placentacharacteristic cubical cracking pattern (Figure and Antrodia serialis. In Canada, we can5). In some cases the wood may appear charred. probably substitute Gloeophyllum sepiarium, aBrown-rot fungi are the most economically species with a more northerly range, forimportant agent of destruction in wood Gloeophyllum trabeum.buildings in temperate climates.

    Figure 5: Brown rot showing the typicaldarkening and cubical cracking

    No extensive studies have been done todetermine which wood-rotting basidiomycetes

    Figure 6: A fruiting structure of the brown-rotfungus Tapinella Panuoides on aVancouver condominium

    All of these are brown-rot fungi, two of them, M.incrassata and S. lacrymans, have the capability

  • Understanding Biodeterioration of Wood in Structures 11

    to transport water from wet wood to relatively considerable strength loss over the course ofdry wood over inert surfaces. This allows them many decades. They can also attackto spread more rapidly under conditions which preservative-treated wood, but again, this is aare marginally favorable for decay. very slow process. Many archeological artifacts

    Although these agents of biodeterioration are a attacked by bacteria. Bacterial attack can benuisance to us in our attempts to use wood for seen under the light microscope but electronour own purposes, they also have the potential to microscopy is required for detailed diagnosis ofrecycle wood at the molecular level when wood the various types of bacterial decay. Bacterialproducts reach the end of their useful service decay is not regarded as of great economiclives. significance.

    Wood-rotting-basidiomycetes aregenerally more sensitive to woodpreservatives than soft-rot fungi orbacteria. There are, however, anumber of brown-rot fungi which aretolerant of moderate amounts ofcopper, and some which are tolerant ofarsenic. Many of the white-rot fungiand a few of the brown-rot fungi aretolerant of organic preservatives,possibly through the action of theenzyme or non-enzymic systems theyuse to break down lignin.

    3.2 Bacteria

    Bacteria tend to colonise wood with a highmoisture content, either fresh from the tree,water sprinkled for long-term storage prior tosawing, submerged in lakes or wet soil. Somebacteria simply live on the non-structuralcomponents in sapwood and may increase thepermeability of wood by destroying pitmembranes. Other bacteria are capable ofdegrading lignocellulose but the rate of decay isvery slow. They can, however, cause

    recovered from lake sediments have been

    3.3 Termites

    Termites are primitive social insects related tocockroaches. Though often called white ants,they are not closely related to ants. They do,however, superficially resemble ants and live inlarge colonies with a variety of specialisedcastes including a Queen (and King). Based ontheir effect on wood in construction, termites canbe divided into three groups: dampwoodtermites, subterranean termites and drywoodtermites. Dampwood termites live in damp anddecaying wood, and the damage they cause is ofrelatively little economic significance.Prevention of decay is the main problem here.Subterranean termites cause the majority of theeconomically important damage to woodproducts throughout the world. They typicallyrequire some connection to moist soil through asystem of galleries. In order to cross inertsubstrates to access wood, they will buildshelter tubes from earth, wood fragments, faecalexcretions and salivary excretions. Formosansubterranean termites have, however, beenknown to start colonies out of ground contact, forexample, around water tanks in high-rises inHawaii. This particularly voracious species hasalso been introduced into the continental UnitedStates via several Gulf Coast ports. It is also thespecies which is prevalent in southern Japan.

  • Understanding Biodeterioration of Wood in Structures 12

    Drywood termites attack dry sound wood and do by the nitrogen and starch content of the wood.not colonise via the soil. A mated pair of The sapwood of hardwoods is particularlywinged reproductives can fly in and start a new susceptible to beetle damage. A number ofcolony directly in wood in buildings. In North beetles and weevils preferentially attack rottedAmerica, drywood termites are confined to the wood. Sometimes their emergence holes, bittenextreme southern United States and Mexico. through an otherwise sound surface, are the first

    In Canada, subterranean termites are only been known to cause economically importantconsidered a serious pest in some southern damage in wood structures in Canada.Ontario cities (particularly downtown Toronto)and in some of the drier areas of BritishColumbia (Eastern Vancouver Island, the GulfIslands, the Sunshine Coast and theOkanagan).

    3.4 Wood-boring Beetles not eat wood, they merely excavate it to live in.

    There are a large number of types of wood- vegetation and any sugary household foods.boring beetles, weevils, wasps and bees (2, 18). They prefer softer woods or those that have beenFor the species which cause most structural softened by decay and other soft buildingdamage in buildings, it is only the grubs which materials, such as insulation. Unchecked,feed on the wood. After pupation, the adults carpenter ants can cause serious structuralemerge by biting their way out of the wood. damage. The noise of their excavation can beSince they do not consume wood at this stage, highly irritating particularly in buildings withthey are not killed by surface application of expanded polystyrene insulation. Removal ofpreservatives, many of which are stomach woody debris, adequate soil/wood separationpoisons. Such preservatives will however (or use of treated wood in ground contact) andprevent reinfestation by the next generation. general sanitation of buildings is normallySome beetles and wood wasps lay their eggs adequate to discourage carpenter ants. Aonly through bark and are only problematic in mixture of sugar and borax has been used as atheir unexpected emergence from wood home made poison bait to reduce infestation.products. Wood wasps are particularly largeand alarming in appearance. Typically suchspecies do not re-infest wood in service.

    Other wood-inhabiting beetles can causeconsiderable damage particularly where, overmany decades, generation after generation canreinfest the same piece of wood unnoticed. Atypical example would be trusses and enclosedwooden staircases in older European houses.The rate of growth of many species is controlled

    signs of decay. Wood boring beetles have not

    3.5 Carpenter Ants

    Carpenter ants are easy to identify because theyare large (workers 6 - 10mm) and completelyblack. They are very common in B.C. They do

    They emerge from the wood to forage for

  • Understanding Biodeterioration of Wood in Structures 13

    3.6 Marine Borers

    Two types of marine borers are important in thecoastal waters of Canada, shipworms (Bankiasetacea and Teredo navalis) and gribble(Limnoria quadripunctata and Limnoriatripunctata). The damage from these organismsis unlikely to be found in buildings unless thelumber has been cut from logs which have beenstored too long in sea water. Shipwormdamaged wood has occasionally been used asdecorative panelling. Shipworms are molluscswhich settle on the wood as larvae and tunneldeep into it by rasping with their serrated shells.They leave tunnels up to 12 mm in diameter anda major infestation can destroy untreated pilingin less than a year. Gribbles are crustaceanswhich leave very small tunnels (1mm) seldomextending more than 12 mm from the surface ofthe wood. Severe damage can occur overseveral years as the weakened surfaces arecontinually abraded by the action of waves andflotsam. Both types of borer are controlled bygood quality preservative treatment. Bramhall(1) provided an excellent review of the situationon the West coast of Canada.

    Further reading:

    More detail on the organisms involved inbiodeterioration of wood can be found inthe textbooks written by Cartwright andFindlay (6), Eaton and Hale (15), andZabel and Morrell (28). Two other texts,Bravery et al (2), and Levy (18) are veryuseful for identifying which organism iscausing a particular problem.

    4 Microbial BiodeteriorationProcesses

    Of all the organisms which use wood as a foodsource, the most important, in the temperateregions, are the wood-rotting basidiomycetes. Instructural softwoods, the brown-rot fungi areparticularly important. Wood that isrecognisably rotten is the product of a sequenceof events with the participation of a successionof microorganisms.

    4.1 The Colonisation Sequence Leadingto Decay

    Although they can decay fresh, previously sterilewood, WRB are commonly prevented fromcolonising by competition from a sequence oforganisms, including: bacteria, mould fungi,staining fungi and soft-rot fungi. This sequencecan take weeks or months to complete,depending on the conditions prevailing in thewood. As discussed above, the bacteria, mouldfungi and staining fungi live on non-structuralcomponents of the wood and can not break downlignocellulose. They are, however, adapted torapidly colonising and exploiting readilydigestible carbohydrates, lipids and proteins insapwood. To prevent other fungi fromcompeting with them, many fungi produceantibiotics. When the bacteria, moulds andstaining fungi have used up all the non-structural components, they stop growing andthe soft-rot fungi and WRB can take over. Thewood-rotting fungi can compete at this stagebecause the only remaining carbon source islignocellulose and only they can use it. Unlessthe WRB are excluded by unfavourableconditions, such as high moisture content or

  • 0 10 20 30 40Temperature oC







    Understanding Biodeterioration of Wood in Structures 14

    wood preservative, they will out compete the non susceptible to decay through chemicalsoft-rot fungi. (Note: most of the research on the treatment.colonisation sequence has been done onsapwood. Heartwood which has relatively little The temperatures that we regard as ideal forreadily digestible component and some natural comfort are also perfect for the growth oftoxins may have a different sequence) wood-rotting fungi (Figure 7). Such fungi do

    If wood becomes stained and stays at the right commonly lie between 20 C and 30 C. A fewmoisture content, it will almost certainly be WRB grow fastest at 34 - 36 C and these arecolonised by wood-rotting fungi some time later. typically the species (Gloeophyllum species)It is difficult to predict how long this stage will that are dominant on exterior wood productstake. It depends on the chances of a WRB exposed to sunlight. All WRB are stopped byarriving on the wood when the wood is in the temperatures higher than 46 C but they may notright conditions for colonisation and also on the be killed until the temperature reaches 60 C. Asability of the WRB to compete with the fungi with many biological systems, a ten degree dropalready present. The cellar fungus, Coniophora in temperature results in a halving of the growthputeana is one of the most aggressive in rate. Growth and decay does not cease until theovercoming the resistance of the mould and temperature is very close to 0 C.staining fungi. Gloeophyllum sepiarium, themost common decayer of millwork and decking,is relatively weak.

    In the absence of experimental data, we mustassume that colonisation and decay processesstart where they left off when lumber is driedand re-wetted. It is also safest to assume that thecolonisation sequence does not start fromscratch when lumber that has suffered blue-stainprior to kiln drying is re-wetted.

    4.2 Conditions Required for Decay

    Wood-rotting fungi have the same basic needs asother organisms: a food source, an equabletemperature, oxygen and water. As discussedabove, the wood provides almost everything afungus or insect needs for food. However, theheartwood of some species is less susceptiblethan others. Furthermore, wood can be made

    vary in their temperature optima, but theseo o





    Figure 7: A typical temperature response for awood-rotting basidiomycete

    Oxygen is ubiquitous and is not normally alimiting factor. Painting or otherwise sealingthe wood surface will not exclude oxygen fromthe wood. Although decay is an oxidativeprocess, WRB are relatively tolerant of lowoxygen levels. A normal atmosphere contains21% oxygen and reduction in growth of WRB

  • Understanding Biodeterioration of Wood in Structures 15

    does not occur until the concentration drops between 40 and 80% mc. The upper limits varybelow 1 - 2%. When buried in anaerobic lake between 100 and 250% mc depending on thesediments, wood is not attacked by WRB or wood density. Once water fills more than 80%soft-rot fungi, but it can still be degraded very of the space in the wood, diffusion of gas isslowly by some bacteria. slowed to the point that CO builds up to

    Water is the key limiting factor in decay of may stop growing under these conditions butwood in structures and, in temperate climates, they are not necessarily killed.moisture control is key to the durability ofwood systems (Table 2). While some moulds Once WRB are established, the minimumcan colonise wood at moisture contents between moisture content for decay to proceed is15 and 20% little or no sporulation occurs. Most around 22 - 24%, so 20% is frequently quotedmoulds require moisture contents above 20% for as a maximum safe moisture content for wood.growth and sporulation. Infection by spores of The National Building Code requires theWRB probably does not occur at wood moisture content of wood to be below 19%moisture contents below about 29%. At when the building is closed in. Drying the woodtypical interior temperatures, this corresponds to to below 15% will stop the decay process buta relative humidity (RH) around 96%. In order will not necessarily kill the decay fungus unlessto germinate, spores need to absorb moisture a sufficiently high temperature has been used infrom the wood and below the fibre saturation the drying process. WRB can survive for up topoint, the wood exerts too much suction on the nine years in wood at moisture contents aroundwater. The mycelium and mycelial cords of 12%. If the wood wets up again, the decayWRB can colonise wood below the fibre process can restart.saturation point, possibly down to 20% mc,provided they are growing from a substrate ata higher moisture content. Certain WRB, suchas the true dry rot fungus, are capable oftransporting water through mycelial cords overinert surfaces and depositing it into dry wood.The true dry rot fungus (Serpula lacrymans) isvery rare in Western North America but commonin the East and in Europe. It shows a distinctpreference for wood in contact with bricks andmortar. In Western North America there isanother water- conducting fungus (Meruliporiaincrassata) with more limited capabilities.

    Once WRB are established, the optimummoisture contents for decay of wood lie


    inhibitory levels and oxygen is limiting. WRB

    4.3 Growth and Decay Rates of Brown-Rot Fungi

    While their growth rates in the laboratory arewell known, the growth rates of brown-rot fungiunder field conditions have not been extensivelystudied. Our best estimate is that, under idealtemperature, moisture and nutrient conditions,WRB will grow between 1 and 10mm per dayover the surface of wood (23) and at asomewhat slower rate within solid wood (10) inthe longitudinal direction (parallel with themajority of cells). The rate of growth withinsolid wood across the grain will be slower still.

  • Understanding Biodeterioration of Wood in Structures 16

    The rate of strength loss and weight loss causedby brown-rot fungi has been much moreextensively studied. As discussed above, thelignocellulose breakdown mechanism used bythe brown rot fungi causes rapid strength lossbefore decay is obvious to the naked eye. Somestrength properties are more sensitive thanothers. Compression perpendicular to the grain,important for the bearing strength of plates andbeams may be reduced by up to 60% in oneweek under ideal conditions in sapwood. Underthe same conditions, Douglas fir heartwood,which is moderately durable, might only lose25% of compression perpendicular to grain in aweek. For compression parallel, important forstuds, sapwood might lose up to 40% andDouglas fir heartwood up to 15% in one week.Zabel and Morrell (28) provide a detaileddiscussion of the effect of white-rot and brown-rot fungi on all the strength properties ofsoftwoods and hardwoods.

    Wood-rotting fungi break down thecarbohydrates in wood to carbon dioxide andwater, thus the moisture content of woodincreases by tens of percentage points as decayprogresses. This makes it somewhat moredifficult to stop the decay process by drying outthe wood. Some form of accelerated drying isnormally needed. Interestingly, the WRB(Gloeophyllum species) commonly foundattacking wood in exterior applications(decking, windows, etc.) cause a greaterincrease in moisture content than predicted fromthe breakdown of carbohydrate alone (25). Thissuggests that they are capable of extractingadditional moisture from the air. These fungi arealso particularly tolerant of drying out and hightemperatures (6, 28).

    Further reading:

    The figures quoted in this section were alltaken from Cartwright and Findlay (6), orZabel and Morrell (28). These textbooksprovide references to the originalpublication from which these data are taken.More specific details on the woodbreakdown mechanisms of fungi areprovided by Eriksson et al. (16).

    5 Some Considerations forPrevention and Remediation ofDecay and Termite Damage

    5.1 Protection by Design

    In the temperate zones, where insects whichattack dry wood are of little significance, woodproducts and systems can last indefinitely,provided they are kept dry. There are numerousexamples of historic wooden structures whichare thousands of years old. Most often quotedare the temples of Japan and the stave churchesof Scandinavia (Figure 8).

  • Understanding Biodeterioration of Wood in Structures 17

    Figure 8: An 800 year old Scandinavian stavechurch

    These would have been built with only the bestof materials using the true wood (heartwood) ofthe most durable local trees. When thosestructures were built, the sapwood was normallycut off and discarded. Such temples andchurches were designed with pitched roofs toshed the rain and they were constructed withgreat attention to detail. Being places ofworship, they have almost certainly been wellmaintained over the years. They will also haveundergone periodic repairs. With all thesefactors taken into consideration, it seems we canbuild wooden structures to last as long as welike. The primary consideration is to keep thewood dry and, where moisture can not beavoided, use durable wood products: naturally

    durable or pressure-treated. Where termitesare a problem, specific design elements must beincluded to exclude them from the structure.

    More detail on construction of woodbuildings for durability can be found inpublications by the Canadian Wood Council(5), Dost and Botsai (11), Lstiburek andCarmody (19), the National Forest ProductsAssociation (21), Scott (22), Trechsel (24),Wilcox et al. (26) and the Wood ProtectionCouncil (27).

    More detail on protecting wood fromtermite damage can be found in publicationsby the UK Building Research Establishment(4), the National Forest ProductsAssociation (21), and the Wood ProtectionCouncil (27).

    Most wood products used in full exteriorexposure should be constructed from naturallydurable or pressure-treated wood if they are toprovide a service life, safety and maintenancerequirement acceptable to the end user.

    Useful handbooks on using pressure-treated wood subject have been written byCassens et al. (7), McDonald et al. (20)and the Wood Protection Council (27).

  • Understanding Biodeterioration of Wood in Structures 18

    5.2 Considerations for Repair andRemediation

    Since decay and termite damage are usually faradvanced when they are discovered, the primaryconsideration is normally the restoration of therequired strength to the structure. The secondconsideration is the restoration of theappearance. The third, and often overlooked,consideration is the prevention of recurrence ofthe problem. This requires an understanding ofthe causes of the problem in the first place. Thisshould include the search for primary andsecondary sources of moisture. Since fungi needmoisture contents at or above the fibre saturationpoint, the initiation of decay may have beenassociated with a water leak which has sincebeen eliminated. However, once established,decay can progress at moisture contents as lowas 25% so it can be maintained by a moistatmosphere. All sources of moisture must beeliminated. It is also critical to remove allinfected wood, not only that which is obviouslydecayed but 60cm beyond the end of the visibledecay along the length of the damagedmember. As discussed above, visibly rottenwood is the last stage in the decay process, thusWRB may be present in wood which is notapparently affected. Normally decay in part ofthe cross-section of a wood member means thatthe entire cross-section has to be replaced.

    Eradication of infection by WRB is mademore difficult in buildings with brick, blockor masonry construction where the true dry-rot fungus can survive in wood concealedwithin solid walls. In a study by Bracknell(3) of incidences of the true dry-rot fungusin UK buildings, 52% had a history of dryrot and had undergone previous attempts toeradicate it.

    If it is not 100% certain that all the fungalinfection has been removed and all moisturesources eliminated, consider using pressure-treated wood, to CSA O80 standards, toreplace rotted members. It may also beprudent to treat, in situ, the remainder of theoriginal wood. A diffusible, borate-based,preservative is recommended for this either inthe form of soluble rods placed in drilled holesor as a spray application to exposed surfaces.Soluble rods are best used where it is expectedthat the wood will stay wet for some time.Where the moisture source is known, the rodsshould be placed as close as possible to thepoint where the water will enter the wood.Soluble rods can provide enough preservative tohalt existing infections but a surface applicationcan only hope to prevent WRB and insects fromtransferring out of infected wood onto newwood.

    In the case of subterranean termites, theirmeans of entry to the building should beeliminated by redesign and/or repair.Restoration of other elements of the originaltermite management system may also benecessary. Where reinfestation seems likely,

  • Understanding Biodeterioration of Wood in Structures 19

    pressure-treated wood to CSA O80 standardsshould be used for any repairs.

    Where the structure being restored hassome historic importance, the historicqualities should not be compromised bythe restoration process. It is often desiredto retain as much as possible of theoriginal material, thus new wood or othermaterials may be attached to existingdecayed structures. Attempts may be madeto restore the strength of rotted wood andfill rot pockets. As a result, existinginfections may not be completelyeradicated and intensive remedialtreatment may be needed. Ideally, nothingshould be done to a historic structure thatcan not, in future be reversed if moreappropriate conservation technology isdeveloped.

    For further reading on remediation andrepair of decay in buildings refer to Freas(17) and the publications of the USNational Pest Control Association, DunnLoring VA.. Some additional informationis provided by Eaton and Hale (15),Bravery et al. (2), and Levy (18) . Therecommendations provided by Scott (22)are designed for British housing and aresomewhat out of date. Design andconstruction principles for primaryconstruction which are also valid for repaircan be found in booklets by the WoodProtection Council (27) and the NationalForest Products Association (21).

    The final word:

    Annual losses from decay in lumber used forconstruction purposes in Vancouver, and BritishColumbia generally, assume large proportions;much of this loss is preventable. The dampclimate of the coastal region is especiallyconducive to decay, and it is not sufficientlyrealized that timber construction methods whichmay be suitable in other parts of Canada (forexample the Prairie Provinces, where theweather is hot and dry in the summer and quitecold in the winter) are not suited to this region(14).

    6 Acknowledgements

    This document is based on an annual lecture tothe Masters in Pest Management course at SimonFraser University in British Columbia. Thepreparation of this document was funded, in part,by the British Columbia Building EnvelopeCouncil whose contribution is gratefullyacknowledged.

    Forintek Canada Corp. would also like to thankits industry members, Natural Resources Canada(Canadian Forest Service) and the Provinces ofBritish Columbia, Alberta, Quebec, Nova Scotiaand New Brunswick for their guidance andfinancial support for this research.

  • Understanding Biodeterioration of Wood in Structures 20

    7 Bibliography

    Note: Publications marked E are written from apredominantly European perspective and may beless relevant for North America.

    1. Bramhall, G. 1966. Marine borers andwooden piling in British Columbiawaters. Department of ForestryPublication No. 1138. Available fromForintek Canada Corp. Vancouver BC68p.

    2. Bravery A. F., R.W. Berry, J. K. Carey andD.E. Cooper. 1987. Recognisingwood rot and insect damage inbuildings. Building ResearchEstablishment Report. Department ofthe Environment , Princes RisboroughLaboratory, Princes Risborough,Aylesbury, Bucks, UK. 120p. E

    3. Bricknell, J.M. 1984. A statistical analysisof dry rot attacks in buildings. Proc. aconv. Brit Wood Preservation Assoc.1984. 64-75. E

    4. Building Research Establishment. 1976.Overseas Building Notes. Termitesand tropical building. OverseasDivision, Building ResearchEstablishment, Department of theEnvironment, England. No. 170. 16p.

    5. Canadian Wood Council. 1995. WoodReference Handbook. Canadian WoodCouncil, Ottawa Ontario. 561p.

    6. Cartwright, K. St. G. and W.P.K. Findlay.1950. Decay of timber and its

    prevention. Chemical Publishing Co.New York. USA. 294p. E

    7. Cassens, D. L.; W.C. Feist; B.R. Johnson;R.C.DeGroot. 1995. Selection and useof preservative-treated wood. ForestProducts Society. Madison, Wisconsin.USA. 104p.

    8. Clark, J.E. and R.S. Smith. 1987. Source ofdecay in sawn hem-fir lumber. Reportto the Canadian Forest Service.Forintek Canada Corp. Vancouver BC.29p.

    9. Dietz, M. and W.W. Wilcox. 1997. The roleof pre-infection of green Douglas-firlumber in aboveground decay instructures in California. ForestProducts Journal. 47(5):56-60.

    10. Dobry, J. and V. Rypacek. 1991. Axialgrowth rates of wood-destroying fungiin relation to wood sample dimension.Mat. u Org. 26 (4) 259-268.

    11. Dost, W., E.E. Botsai. 1990. Wood:Detailing for performance, GRDAPublications, Mill Valley, CA 190p.

    12. Duncan, C.G., F.F. Lombard. 1965. Fungiassociated with principal decays inwood products in the United States.U.S. Forest Service Research paperW0-4. Dept. of Agriculture,Washington. D.C. 30p.

    13. Eades, H.W. 1932. British ColumbiaSoftwoods: Their Decays and NaturalDefects. Forest Service Bulletin No.80. Department of the Interior. 126p.(may be difficult to obtain)

  • Understanding Biodeterioration of Wood in Structures 21

    14. Eades, H.W. 1945. Cause and prevention Agriculture, Forest Service, Forestof decay in wooden buildings, with Products Laboratory, Madison,particular reference to the coastal Wisconsin. USA. 81p.region of British Columbia. Circular61, Department of Mines and 21. National Forest Products Association.Resources Canada, Lands, Parks and 1988. Design of wood frameForest Branch, Dominion Forest structures for permanence. NationalService. 12p. Forest Products Association,

    15. Eaton, R. A. and M.D.C. Hale. 1993.Wood: decay pests and protection. 22. Scott, G.A. 1968. Deterioration andChapman and Hall, London U.K. 546p. preservation of timber in buildings.

    16. Eriksson, K.E.L, R.A. Blanchette and P.Ander. 1990. Microbial andEnzymatic Degredation of Wood andWood Components. Springer-Verlag,New York, USA. 407p.

    17. Freas A. 1982. Evaluation , maintenance andupgrading of wood structures: A guideand commentary. American Society ofCivil Engineers, New York, NY, USA.428p.

    18. Levy, M.P. (undated, 1979?) A guide to theinspection of existing homes for woodinhabiting fungi and insects. USDepartment of Housing and UrbanDevelopment. 104p.

    19. Lstiburek, J., and J. Carmody, 1991.Moisture control handbook: new, low-rise, residential construction.Springfield, Va.: U.S. Department ofCommerce. National Technicalinformation Service. 1991. 247p.

    20. McDonald, K.A.; R.H. Falk; R.S. Williamsand J.E. Winandy. 1996. Wooddecks: materials, construction, andfinishing. U.S. Department of

    Washington, D.C. 17p.

    Longmans, Green and Co. Ltd., LondonUK. 148p E

    23. Thornton , J.D., and G.C. Johnson. 1986.Linear extension rates of SerpulaLacrymans within a simulated wallcavity, Internat. Biodet. 22(4): 289-293.

    24. Trechsel, H. R., editor . 1994. Moisturecontrol in buildings, ASTM ManualSeries: MNL 18. 485p.

    25. Viitanen, H and Ritschkoff, A-C. 1991.Brown rot decay in woodenconstructions: Effect of temperature,humidity and moisture. SwedishUniversity of Agricultural Sciences,Department of Forest products. ReportNo. 222. 55p.

    26. Wilcox. W.W., E.E. Botsai and H. Kubler.1991. Wood as a building material: Aguide for designers and builders. JohnWiley & Sons. 215p.

    27. Wood Protection Council. 1993. WoodProtection Guidelines, Protectingwood from decay fungi and termites.

  • Understanding Biodeterioration of Wood in Structures 22

    Wood Protection Council, NationalInstitute of Building Sciences. 53p.

    28. Zabel, R.A. and J.J. Morrell. 1992. WoodMicrobiology: Decay and itsPrevention. Academic Press. SanDiego, California, USA. 476p.

  • Understanding Biodeterioration of Wood in Structures 23

    Table 2: Guide to Moisture Conditions for Colonisation, Growth and Damage by Moulds,Brown-Rot fungi and White-Rot Fungi .1

    Moisture Spore Strengthcontent production loss

    Colonisation Growth

    0 - 14% None None None None

    15 - 19% Mould spores (few species) Very slow Neligible None

    20 - 24% Mould spores Slow Minimal NoneBrown rot, mycelial cords Slow Slow Very slow2

    25 - 29% Mould spores Moderate Moderate NoneBrown rot, mycelial cords Moderate None Slow2

    Brown rot, mycelium Slow None Slow2

    White rot, mycelium Very slow None Very slow2

    30 - 49% Mould spores Fast Prolific NoneBrown rot, mycelial cords Fast Limited FastBrown rot, mycelium Fast Limited FastWhite rot, mycelium Slow Limited SlowBrown rot, spores FastWhite rot, spores Slow




    50 - 89% Mould spores Fast Prolific NoneBrown rot, mycelial cords Fast Limited FastBrown rot, mycelium Fast Limited FastWhite rot, mycelium Fast Limited FastBrown rot, spores FastWhite rot, spores Fast




    90 - 160% Mould spores Fast Prolific NoneBrown rot, mycelial cords Slow LimitedBrown rot, mycelium Slow LimitedWhite rot, mycelium Fast LimitedBrown rot, spores SlowWhite rot, spores Fast





    > 160% not possible None Minimal

    Notes:1 These are guidlines for typical conditions and there are no hard and fast moisture content

    boundaries. Wood moisture contents are rarely uniform and there is a great deal of variation inmoisture requirements among species of fungi. The critical moisture conditions also vary withtemperature and wood density. Temperature fluctuations can cause condensation leading to anincrease in wood moisture content.

    2 Growing from moist wood or soil3 In most cases, spores only produced from large fruiting structures

    Table of Contents1. Introduction2. The Structure and Composition of Wood2.1 Wood as a Structural Material2.2 Wood as a Food Source2.2.1 Sapwood2.2.2 Heartwood2.2.3 Wood Moisture Content2.2.4 Fungi or Insects Carried over from the Living Tree2.2.5 Summing up Wood as a Food Source

    3. Wood-Inhabiting Organisms3.1 Fungi3.1.1 Moulds and Staining Fungi3.1.2 Soft-Rot Fungi3.1.3 Wood-Rotting Basidiomycetes

    3.2 Bacteria3.3 Termites3.4 Wood-boring Beetles3.5 Carpenter Ants3.6 Marine Borers

    4 Microbial Biodeterioration Processes4.1 The Colonisation Sequence Leading to Decay4.2 Conditions Required for Decay4.3 Growth and Decay Rates ofBrown -Rot Fungi

    5 Some Considerations for Prevention and Remediation of Decay and Termite Damage5.1 Protection by Design5.2 Considerations for Repair and Remediation

    6 Acknowledgements7 Bibliography