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Trees and Houses: A Question of Function
Don Cameron, Experimental CS1RO Division of Building and Ivan Earl, Treemasters Pty Ltd Published by Cement and Concrete Association of Australia 1982
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1-1-0LA .E- t(c))..oce_
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DAC007/140782/1
ti-maLav PARTNERS PTY. LTD.
CONSULTING ENGINEERS
E'5 PACIFIC HIGHWAY, PYMBLE, NSW 2073
TREES AND HOUSES : A QUESTION OF FUNCTION
Don Cameron, Experimental Officer, CSIRO Division of Building Research
and Ivan Earl, Treemasters Pty Ltd
INTRODUCTION
New houseowners usually wish to plant their own choice of shrubs and
trees in their new gardens. Frequently, replanting of an existing
established garden is required. Fast-growing trees are often planted
haphazardly to quickly change the garden's appearance. Although trees
and shrubs are desirable on a suburban block, if the garden has not been
planned carefully, much time, effort and money can be wasted. In
particular, the function of adjacent buildings may be impaired if the
interrelationship of trees and the environment is not appreciated.
TREES AND THE ENVIRONMENT
Soil Water
Trees need water. The fine hair-like roots at the extremities of a
tree's main root structure collect water stored in the soil. The type of
soil determines the amount of water it can store in two ways. Sleet
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CEMENT AND CONCRETE ASSOCIATION OF AUSTRALIA
Firstly, soil permeability controls both the amount and the extent of
penetration of rainfall. Sands and gravels are very permeable whereas
heavy clays only accept water very slowly. However, heavy clays can dry
out sufficiently to form an extensive pattern of cracks to some depth
below the ground surface. This cracking greatly increases the clay's
2
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CONSULTING ENGINEERS
895 PACIFIC HIGHWAY, 3 PYMBLE, NSW .3"..
permeability until sufficient water enters the cracks, then the clay
swells and the cracks close.
Secondly, the amount of water able to be held in a soil is controlled by
how strongly the water is held by the soil particles. In simple terms,
the water-holding forces are stronger for soils with smaller particles.
Of the different soil types, clays have the smallest particle size,
followed by silts, loans, sands and gravels*. So clays can contain
relatively large amounts of water.
water Uptake of Trees
Almost all water taken up by the roots is transpired through the leaves
of the tree. Water is required by the leaves to produce sugars for tree
growth. Indeed, water lost through transpiration can be related to tree
growth rate.
'Footnote: The three major soil types can be roughly identified by first
wetting the soil, kneading it (compacting it between the fingers) and
then describing the feel of the soil. Generally speaking, clove feel
either sticky or puggy, and leave stains on the hands after kneading;
silts feel smooth to silky and sometimes seem spongy when they are
kneaded; gysnds feel coarse or gritty and tend to fall apart during
kneading. If the sand is fairly coarse, the grains may be seen.
Tree species vary in their ability to produce sugars for a given amount
of water. One species may require relatively large volumes of water to
produce a growth rate similar to another species which requires
relatively little water.
A broad indicator of a tree's potential water demand is its total leaf
area, which governs its ability to transpire moisture to the atmosphere.
The amount of exposed leafage is largely determined by the size and
shape of the tree canopy and the thickness of the foliage, both of which
are species characteristics.
Water uptake is also influenced by the environmental conditions, such as
amount of sunlight, nutrient supply and availability of soil water.
Although water demand is difficult to estimate, some examples have been
cited, e.g. poplars 60 000 litres per year (Ward') and apple trees 20
000 litres per year (Bush'). A rough indication of the relative water
demand of commonly grown South Australian trees can be derived from a
large scale study of tree root intrusion into sewers, undertaken by the
South Australian Engineering and Water Supply Department. Table 1,
listing particularly 'thirsty' tree species, originated from this study.
The table shbuld be used only as a rough guideline; it is not fully
comprehensive and it may require revision as further data become
available.
Jree Root Development
The roots of most trees are able to adapt to different site conditions.
On sloping sites, the majority of roots develop up-slope to provide
stability for the tree. Soil type also has a pronounced effect on root
growth. Deep loamy soils which hold adequate supplies of moisture
encourage fine fibrous root systems. In heavy, dry clay soils, the roots
must penetrate a far greater volume of soil and so they tend to be much
thicker and longer with fewer fibrous roots. Otherwise the tree may
become stunted. Root growth is unlikely through either solid rock or
high water tables and is discouraged in poorly aerated or heavily
compacted soil. Water tables are areas below ground where the soil is
very wet. Holes dug through water tables soon fill up with water. In
Melbourne, however, water tables are generally so low that they have
little effect on root development.
Some trees have deep tap roots, which give them stability in the early
stages of their growth. However, the lateral root system which later
develops soon becomes the primary water collector. These lateral roots
may extend sideways from the trunk between 0.4 and 2.1 times the height
of the tree (Yeager'). The depth of root penetration in a clay soil is
usually between 1 and 2 m.
surrounding soil and, in extreme cases, uplifting of the adjacent
structure. Structures lighter than houses, such as fences and paths are
normally affected.
Secondly, a more common and more serious problem occurs when a tree
extracts moisture from a clay soil below a house footing and the soil
shrinks. The concrete footings bf the house will settle in much the same
pattern as the underlying soil unless they have been specially designed.
As the footings settle, brick walls crack, timber frames distort, and
doors and windows may cease to function effectively.
The five major factors which determine the risk of damage due to
shrinkage settlement are as follows:
(a) Soil type
Only clays show appreciable shrinkage on drying. Soils, however,
are normally a mixture of sand, silt and clay.
The more clayey a soil is, the more shrinkable it is.
TREES NEAR BUILDINGS
Introduction
Trees can damage houses in two ways. Firstly, a problem arises with
trees which develop large, deep boles (e.g. liquidambars, figs and
willows). The bole is that section of the tree where the trunk and roots
meet below ground level. If such trees are located against the building
perimeter, the growth in root diameter may cause compacting of
As well, clays vary in mineral composition and can therefore
respond differently to drying. The grey-brown basaltic clays of
Melbourne's western suburbs are more highly shrinkable than the
yellow-brown and grey clays derived from mudstones of the majority
of the eastern suburbs.
(b) Soil profile
The soil profile is the vertical arrangement of soil and rock
layers below the surface.
The highest risk situation is where shrinkable clays extend from
the ground surface to at least 2 m depth. If the clays are overlaid
by sands, silts or other material which does not shrink appreciably
when dried, then the risk of tree-affected settlement lessens as
the depth of this inert layer increases. If this depth exceeds 1.0
to 1.5 m, the risk could be considered to be negligible.
Where clay extends from the surface to a layer of bedrock or
permanent water table at less than 2 m depth, the thickness of the
clay layer will determine how much shrinkage settlement can be
expected. As the thickness of the clay layer decreases, so the
level of risk will decrease accordingly.
7
tree root growth patterns and the variations in trees' demand for
water. Fortunately, recent (1980) amendments to the Victorian
Uniform Building Regulations have resulted in the adoption of
stiffer footings for houses, which should lessen the risk of
damage. Where the risk of damage to a house by an expected tree
planting is deemed to be high, then the designer should consider
the higher categories of footing types offered in the Regulations.
Concrete slab-on-ground floors have performed much better in
Melbourne than conventional footings because they are generally
stiffer and less prone to rotational movements. The edge beams of
the slab are structurally tied into the floor and the internal grid
of stiffening beams. However, slabs can be affected by severe
shrinkage settlement and so some care must still be exercised.
(c) Tree species and proximity to building
The species of tree determines its potential water *uptake, the
pattern of root development for a given site and the tree's
hardiness.
The proximity of a tree to a building determines whether the soil
below the footing is within range of the roots of the tree in
question.
(d) Footing design
Footings can be designed to reduce potential movement. The footing
can be either stiffened with additional concrete and reinforcement
or based at a depth below the drying zone of the tree roots.
However, both alternatives require the designer to assess the
effect of specific tree plantings. The information he requires is
unfortunately difficult to obtain because of the complexities of
(e) Age of tree relative to age of house
Most problems occur when trees are planted after construction of a
house. Where a building is located near a mature tree without
either severing the roots of that tree during excavation works or
substantially altering the water supply to the soil, then the risk
of building damage may be considered to be negligible. If, however,
the roots are severed and root development is regenerated, some
damage may follow. Trees like poplars, ashes and willows develop
root fibres at the severed roots that eventually develop into
larger roots and so shrinkage settlements may occur 10 to 20 years
after construction. Most native trees respond poorly to damage of
their major arterial roots. They are more likely to die than
regenerate. A tree expert should be consulted in such cases to
consider the likelihood of root regeneration .
In any case, large trees within 2 to 3 m of the proposed building
line should normally be removed because of potential instability
problems. The ground adjacent to the felled tree should be
thoroughly soaked prior to construction to reduce the potential
effects of subsequent ground swell on the building.
Experiences in Melbourne
Studies of tree damage throughout Melbourne have not been comprehensive
enough to establish with any certainty whether particular species are
more dangerous than others. Present indications are that almost any
medium to large tree can cause damage if conditions are right. All trees
use water. Even if their water demand is relatively low, they can cause
damage if they are close enough.
However, a listing can be prepared, on a subjective basis only,of trees
which are commonly linked with building damage. Table 2 was prepared in
this fashion by Treemasters Pty Ltd. The reasons why some trees are. more
frequently involved than others may simply be that, in some cases,
because of their form or shape, they are commonly placed closa to
buildings (e.g. pin oak, bamboo leaved willow). Table 2 does not
represent a rigorous scientific study and should only be used as a rough
indicator of possible troublesome species.
Most cases of damage have been confined to the eastern and south-eastern
suburbs, where the clay soils are favourable for tree growth. The heavy
clays of the north and west, although highly shrinkable, appear to be
less of a risk since gardens are more difficult to establish in these
areas.
The incidence of damage is not always predictable. Situations are
encountered where one would expect damage but none has occurred.
The majority of houses throughout the eastern and south-eastern suburbs
consist of single storey brick veneer construction supported by lightly
stiffened concrete footings founded at only 450 mm. Cases of damage to
this class of structure have been reported most frequently where trees
were located closer to the building than 0.5 times their height. Where a
row of trees lines the side of a house, the wall may tilt towards the
trees so that adjoining walls separate and crack. More commonly, trees
at corners of buildings cause the corners to settle. The damage becomes
evident during dry spells. Thereafter cracks may close during winter,
only to widen the following summer. The degree of damage generally
varies from slight to moderate. Moderate damage may mean that doors and
windows have ceased to function properly.
PREVENTION OF DAMAGE
Preventative action should match the risk of damage, as outlined in
Table 3. The level of•risk is assessed by consideration of all site
conditions.
From our experiences in Melbourne, it would seem that the incidence of
tree damage may be reduced most effectively by implementing simple
planting rules for all species of trees. Table 3 gives planting rules
for low, moderate and high risk sites, specifying that trees be planted
at distances of 0.5, 0.75 and 1.0 times their mature height from the
10
building respectively'. Alternative preventative actions are given where
trees are required or have been planted closer than these rules permit.
Soaking of the ground to prevent excessive loss of soil moisture and
hence shrinkage is most economically achieved by using a drip feed
system rather than a continuous spray of water. Garden mulches can be
used in conjunction with soaking to prevent evaporative water losses.
Foliage pruning reduces leaf transpiration losses and so temporarily
reduces the water demand of the tree.
Cut-off walls are merely a physical barrier between the roots and the
building. In order to be successful, they must be impenetrable and of
adequate depth and width to permanently dissuade roots from developing
on the building side of the wall. For long term protection, lightly
reinforced concrete of 150 mm thickness should be used. Depths of 1.5 m
are commonly recommended but each site requires individual
consideration, preferably by a foundation engineer.
GETTING ADVICE
Wherever possible, horticultural or arboricultural authorities should be
consulted in the initial planning of a garden to ensure the correct soil
'Footnote: These planting rules apply for relatively light planting
densities. Heavier plantings such as rows or 'bush' gardens cause fierce
root competition and more extensive ground drying.
11
preparation, the appropriate selection of compatible plant material,
themaking of a manageable garden and the minimization of the risk of
damage to buildings. A foundation engineer may be required to help
assess the risk of damage to buildings and to suggest appropriate
preventative action if trees need to be placed close to the structure,
i.e. for most sites within 0.75 times their expected mature height on a
clayey site.
Information can be obtained from other sources such as nurseries and
current literature, but some caution is required when evaluating its
worth. Host nurserymen, if experienced, are extremely knowledgeable and
are a wealth of information on all aspects of trees. If there is some
doubt regarding the value of the information from the nursery, a
suitable botanical reference book should suffice as a cross-check.
Gardening books often quote a range of expected mature heights of trees
for Australian conditions, but don't give estimated tree growth for
specific local conditions. As briefly explained earlier, owing to the
complexities of nature it is almost impossible to predict growth
patterns for a certain area unless local experience with that species is
available. Therefore, when considering where to locate trees in a
garden, it would be best to assume the maximum potential growth will be
achieved unless there is suitable local knowledge to the contrary.
IN SUMNARY
Trees can cause damage to houses. There are many complex factors
involved when assessing the risk of damage for a particular situation.
As a general rule of thumb for most sites, if you plan to have trees
TABLE 1 PARTICULARLY 'THIRSTY' TREE SPECIES
Mature Botanical name Common name
height
H(m)
Anoophora costae Araucaris heterophvlla (and similar species) Casuarina cunninghamiang Casuarina glaucq gEicma species Cupressus species Eucalyptus bridgesianq Eucalyptus canaldulensis Eucalyptus citriodorq Eucalyptus cladocalyx Eucalyptus cornutA Eucalyptus diversicolor Eucalyptus globulus Eucalyptus leucoxylog Eucalyptus maculate Eucalyptus occidental's Eucalyptus rubidA Eucalyptus viminalis Ficus species Fraxinus oxvcarna Fraxinus "Raywood" (unless grafted or budded onto a rootstock of Fraxinus ornus (Hanna ash))
Grevillea robust Phoenix species pins species platanus species populus niora (and similar species) Quercus robur (and similar species) Robinia pseudoacacia Salix babylonicA (and similar species) $alix chilensis "Fastigiate" $chinus molls Tamaris aphYllA Ulmus procerA (and similar species)
Smooth-barked apple Norfolk Island pine River sheoak Swamp ehmoak Cedars Cypress But-but River red gum Lemon-scented Sugar gum Yate Kerr' Tasmanian blue Yellow gum Spotted gum Flat-topped yate Candlebark Manna gum Figs Desert ash
Claret ash Southern silky oak Date palms Pines Planes Black poplar English oak False acacia, Weeping willow Chilean willow Pepper tree Athel tree English elm
15 - 24 30 - 60 12 - 30 12 - 15
variable variable
24 - 30 gum to 15
15 - 30 9 - 18 to 60
gum 30 - 60 4.5 - 7.5 18 - 30
9-30 9-60 to 30 9 - 15
9 - 15 15 - 30
variable to 30
15 - 36 to 24 to 20
Black locust 9 - 15 9 - 15
6 - - 15 to 6 to 30
12 13
closer to your house than 0.75 times their expected mature height and
you know the soil is essentially clayey, then a foundation engineer or
tree expert should preferably be consulted. Once the risk for the site
is determined, then the appropriate planting rule should be adopted, if
other preventative measures are not considered. The rules are as
follows. For low, moderate and high risk sites, trees should be planted
a minimum distance of 0.5, 0.75 and 1.0 times their expected mature
height from the building respectively. Alternatively, action may be
taken which will allow closer plantings without causing damage to
neighbouring structures. Again, the preventative action should match the
level of risk: it may range from regular tree maintenance, construction
of concrete cut-off walls or re-design of footings.
REFERENCES
1. Ward, W.H. (1947). The effects of fast growing trees and shrubs on
shallow foundations. Journal of the Institute of Landscape
Architects, No. 11, pp. 7-16.
2. Bush, R. (1943). 'Tree Fruit Growing. VI'. (Penguin Books: London.)
3. Yeager, A.F. (1935). Root systems of certain trees and shrubs grown
on prairie soils. Journal of Agricultural Research, V51, pp. 1085-
1092.
The information contained in this table has been derived from:
- Baker, P.D. Tree root intrusion into sewers - Progress Report No. 2: Analysis of root chokes by species. Engineering and Water Supply Dept, SA, Sewerage Branch, Aug. 1978.
- Lord, E.E. (1970). 'Shrubs and Trees for Australian Gardens. 4th Edn. (Lothian Publishing Co.: Aust.)
TABLE 2 TREES FREQUENTLY ASSOCIATED WITH BUILDING DAMAGE IN MELBOURNE
(From Treemasters Pty Ltd)
Botanical name Genera Species
Common name
RISK OF DAMAGE'
PREVENTATIVE ACTION
Minimal Regular watering below the drip line of the tree (line below outer extremity of foliage)
Low 1. Locate trees at a distance from the building equal to 0.5 times their estimated mature height; or
2. Employ water reservoirs and/or regular foliige pruning
Moderate 1. Redesign footings to accomodate settlement, or 2. Employ cut-off walls, or 3. Locate trees at a distance from the building equal
to 0.75 times their estimated mature height; or 4. Regular deep root pruning of close trees if they
are of suitable hardy species.
High 1. Redesign footings to accomodate settlement, or 2. Employ cut-off walls, or 3. Locate trees at a distance from the building equal
to 1.0 times their estimated mature height
'Assessed from site conditions
15
14 TABLE 3 ACTION REQUIRED TO AVOID DAMAGE
Acacia dealbata decurrens
Aux =gunk Alnus iorrulensis Cuoressus torulosa Eucalyptus. botrvoidea
Citriodora gladocalvx globulua =Alta nicholi1 Oda rshmata viminalis OMEN M ravvoodii robust.% atvraciflug Nora var. Italica palustris babylonicet ghilensia pacera grocer* var. Louis van Houtte
Silver wattle Black wattle Box elder Evergreen birch Bhutan cypress Mahogany gum Lemon-scented gum Sugar gum Tasmanian blue gum Spotted gum NSW peppermint gum Swamp gum Swamp mahogany Manna gum Desert ash Claret ash Silky oak Sweet gum Lombardy poplar Pin oak Weeping willow Bamboo leaved willow English elm
Golden elm
Fraxinus
Grevillea Liouidambar Pooulus Quercus 2olix
Ulmuq
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DIRECT" pwisicAL 11.1-TaRFRat4CE• e))1 ,•
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WATER AVAILABLE TO TREE ROOTS
FORMS OF DAMM,E. 6\( TREE ROOTS
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LOW RISK
EXAMPLES OF RISK OF SHRINKM,E. SETTLEMENT
(For Sii-es where-Eke floildih85 are supporl-ecl by Shallow 54-rip Foo+inss)
DEW:MI.4114G FOOT'114G
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PLANTING RULE FOR MEDIUM RISK SITES.
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