De s i g n e r s a n d b u i l d e r s o fresidential chimneys some-times fail to realize the se-

verity of the conditions to whicha chimney is exposed. As a result,efflorescence, staining, water pen-etration, and deterioration of ma-sonry chimneys may occur, un-less precautions are taken.

Most problems associated withmasonry chimneys, whether aes-thetic or more substantial, are re-lated to water. Chimneys can beexposed to significant amounts ofwater under a variety of climaticconditions. Both the warm rains ofthe South and the cold wind-driv-en rains and snow of the North-east can be damaging to a mason-ry c h i m n e y, p a rt i c u l a r l y i f t h ec h i m n e yi si m p ro p e r l yc o n s t ru c t e dor poorly maintained. In addition,the thermal cycling that occursdue to varying outdoor tempera-tures and normal chimney usagecan create a potentially damagingenvironment.

To withstand the rigors of suchan environment, a chimney mustb e p ro p e r l y d e s i g n e d a n d c o n-structed of the best materials.

Chimney exteriorThe materials used to construct

exterior chimney walls are sub-ject to the most severe exposure.If brick is used, it should conformto the requirements of ASTM C

2 1 6 S p e c i f i c a t i o n f o r F a c i n gB r i c kfor Grade SW (severe weathering).Because they have a higher com-pressive strength and are less ab-sorptive than Grade MW (moder-ate weathering), Grade SW brickmeeting this standard are durablee v e n i f f ro z e n w h i l e s a t u r a t e dwith water (Ref. 1).

ASTM C 216 requirements limitthe boiling-water absorption forGrade SW to 17% and the cold-water absorption to 13.3% for anaverage of five brick. The boiling-w a t e r a b s o r p t i o n re f l e c t s t h e

moisture absorbed under pres-sure, such as from driving rain orfreezing effects. The cold-waterabsorption is a measure of themoisture absorbed by a unit undernormal conditions.

H o w e v e r, i n a re a s w i t h l a rg ea m o u n t s o f r a i n o r n u m e ro u sfreeze-thaw cycles, consider usinge v e n l e s s a b s o r p t i v e m a s o n ryunits—Class SX paving brick con-forming to ASTM C 902 Specifica-tion for Pedestrian and Light Traf-fic Paving Brick. Because they arelaid horizontally, pavers typically

Figure 1 Chimney crown

2" min.

2" min.

1" space

Expansion joint and sealant

Sloped concretecrown

Drip

Vent

Flashing

Chimneywall

Clay flue liner

21⁄2" min.

Building a better chimneyUse proper detailing and the best materials to avoidmoisture-penetration problems

By Christine A. Subasic

are subject to more severe weath-er exposure than brick in verticalapplications. Consequently, theabsorption limits found in C 902are considerably lower than thosein C 216 (Ref. 2).

ASTM C 902 limits cold waterabsorption to 8.0% and boilingwater absorption to 10.3% for anaverage of five units. As a result,chimneys built with C 902 brickabsorb less water and should pro-vide better durability than thosebuilt from C 216 brick.

Mortar used on the exterior of achimney can be Type N or S mor-tar made from portland cementand lime or from masonry cement.In areas with wind pressures over25 psf, the Brick Institute of Amer-i c a ( B I A ) re c o m m e n d s Ty p e Sportland cement-lime mortar forits high tensile strength. Underthese conditions, chimneys madew i t h o t h e r m o rt a r s a re m o reprone to cracks at the brick/mor-tar interface.

Chimney interiorThe interior of a masonry chim-

ney typically is constructed withclay flue liners conforming to there q u i re m e n t so fA S T M C 3 1 5 S p e ci-fication for Clay Flue Linings. Be-cause of their exposure to mois-ture from condensing flue gases

and the exterior elements, onlynon-water-soluble refractory mor-tars should be used for laying theflue, as specified by the NationalFire Protection Association stan-d a rd N F PA 2 1 1 ( R e f . 3 ) . T h e s emortars should meet the require-ments of ASTM C 105 Specificati o nfor Ground Fire Clay as a Ref r a c t o ryM o rt a r f o r L a y i n g U p F i re c l a yB r i c k o r A S T M C 1 9 9 Test Method

for Pier Test for Refractory Mort a r s ,m e d i u m - d u t y.

Chimneys used to vent high-effi-ciency gas appliances are subjectto highly acidic condensate. Useclay flue liners and an acid-resis-tant or chemical-resistant mortarin these chimneys for best perfor-mance. Do not use portland ce-m e n t o r c a l c i u m a l u m i n a t e c e-m e n t b e c a u s e o f t h e i r l o w a c i dre s i s t a n c e .

To accommodate the wide tem-perature changes experienced bythe flue, include a 1-inch air spaceb e t w e e n t h e e x t e r i o r c h i m n e ywalls and the flue. In high seismica re a s , t h i s s p a c e i s f i l l e d w i t hgrout for structural reasons, butreinforcement placed within thisspace helps to accommodate thethermal stresses that occur.

CrownBetter chimney design always in-

cludes a crown to protect thechimney walls and interior fro mrain and snow. A portland cementc o n c rete crown is re c o m m e n d e d .Whether prefabricated or cast-in-p l a c e , a c o n c re t e c ro w n s h o u l d b ea t l e a s t 2 i n c h e s t h i c k w i t h a s l o p eof 3 inches or more to pro m o t edrainage, as shown in Figure 1 .T h e c h i m n e y c ro w n s h o u l d p ro-ject at least 21⁄2 inches past the faceof the chimney wall below and in-

Figure 2 Intersection with the roof

Base flashing

Counter- flashing

Base flashing Tab

Figure 3 Through-wall counterflashing

Chimney wall

Counterflashing

Roof3" min. lap

4" min.

4" min. Base flashing

Clay flue liner

corporate a drip. This re d u c e sw a-t e r ru n d o w no nt h e chimney walls.

Leave a 3⁄8-inch gap between thechimney crown and the flue to al-low for thermal expansion of theflue. The gap should be filled witha backer rod and an elastomericsealant, such as a silicone or apolysulfide sealant. Extend theflue at least 2 inches or the mini-m u mc o d e - re q u i re dd i s t a n c ea b o v ethe top of the crown.

To promote drying of the chim-n e y i n t e r i o r, B I A re c o m m e n d splacing vents in the mortar jointsdirectly below the crown flashing,spaced at 24 inches on center (Ref.4). Brick vents placed in mortarhead joints, combined with weepholes at flashed locations, encour-age air circulation in the cavity be-tween the chimney and the flue.

Don’t use a mortar wash in lieuo f a c h i m n e y c ro w n . A m o rt a rwash does not withstand the mois-ture and temperature changes aswell as a concrete crown, nor doesit provide an overhang to protectthe chimney walls.

FlashingFlashing should be placed at

three locations in a masonry chim-ney—directly beneath the crown,at the intersection of the chimneyand the roof, and at the base ofthe chimney.

Beneath the crown. Flashingp l a c e db e n e a t ht h ec h i m n e yc ro w n

protects the chimney walls from

any moisture that might penetratet h e c ro w n a n d p re v e n t s w a t e r

from entering the space betweenthe crown and the flue. The flash-

i n g s h o u l d e x t e n d b e y o n d t h e

chimney walls to form a drip edge.

It should pass through the chim-

ney walls into the space betweenthe walls and the flue. The flash-

i n g s h o u l d b e t u rn e d u p w a rdagainst the flue and extend up to

the top of the crown (see Figure 1).

A backer rod and sealant placedb e t w e e n t h e f l a s h i n g a n d t h e

crown provides a tight barrier towater penetration.

C h i m n e y - ro o f i n t e r s e c t i o n .Proper flashing is critical at thec h i m n e y - ro o fi n t e r s e c t i o nb e c a u s eit prevents water from penetrat-ing into the interior of the struc-ture. Typically, roof or base flash-ing is installed first. Extend theb a s e f l a s h i n g a t l e a s t 4 i n c h e shorizontally along the roof and atleast 4 inches up the face of thechimney.

Then the counterflashing is in-s t a l l e d . I t s h o u l d l a p t h e b a s eflashing a minimum of 3 inches(see Figure 2). BIA recommendsextending the counterflashingthrough the chimney wall, up intothe space between the chimneyand the flue (see Figure 3). Thisp ro v i d e s m a x i m u m p ro t e c t i o nfrom water that penetrates thechimney walls and can be particu-larly important on tall chimneysthat have large areas of exposedwall (Ref. 5).

However, in dry regions withlittle rainfall, the counterflashingmay be terminated in the mortarj o i n t s . I n t h i s c a s e , t h e m a s o ns h o u l d r a k e o u t t h e j o i n t t o re-ceive the flashing 3⁄4 to 1 inch. Thenthe flashing is inserted into thejoint and mortared or caulked in

place. But use this detail cautious-ly because it does not providedrainage for water entering thechimney space.

I n t h e n o rt h e a s t e rn U n i t e dStates, where driving rains are se-vere, tray or pan flashing may beused. A solid sheet of flashing, of-ten made of lead, is cut to matchthe chimney cross-section. Holesare then cut for the flue(s).

The tray flashing is laid in place,g e n e r a l l y t w o t o f o u r c o u r s e sabove the highest point where thechimney intersects the roof. Theflashing is turned up into the cavi-ty around each flue and down toform a drip on the exterior. Thisprovides a continuous barrier toany moisture that penetrates thechimney walls above. To providedrainage, locate weep holes di-re c t l y a b o v e t h e f l a s h i n g a t 2 4inches on center.

Chimney base. If the chimneyis built on an outside wall of theresidence, the base of the chim-ney must also be flashed. Placethrough-wall flashing between thebottom course of brick and thefoundation. If the top of the foun-d a t i o n i s b e l o w g r a d e , p ro v i d eflashing at a level above the gradeline. Weep holes should be placedabove grade at the flashing.

CricketA cricket should be used when

the chimney penetrates the roofbelow the ridge line and the faceof the intersecting chimney wallparallel to the roof line is over 30inches long. Framed of wood andc o v e re d w i t h ro o f i n g m a t e r i a l ,

Source: Masonry Institute of America (Ref.6)

Chimney cricket

Roof pitch H

1:1 1⁄2 W

2:3 1⁄3 W

1:2 1⁄4 W

1:3 1⁄6 W

1:4 1⁄8 W

W = Cricket widthH = Cricket heightCricket dimensions

H

1"

Figure 4

crickets are used to divert waterfrom the roof around the chimney.But they do not eliminate the needfor flashing and counter flashingaround the chimney walls. Crick-ets should be flashed and count-er-flashed like a typical roof inter-section (see Figure 4).

Water-repellent coatingsThe use of water-repellent coat-

ings when constructing a mason-ry c h i m n e y i s n o t w i d e s p re a d .More often, water repellents areconsidered when existing chim-neys have problems. In the caseof newly constructed masonrychimneys, such coatings are ap-propriate in a limited number ofcircumstances.

In regions with large amounts ofrainfall, the chimney exterior maybenefit from treatment with a wa-ter-repellent coating, regardlessof the type of masonry unit used.When relatively absorptive ma-sonry units are used, water-repel-lent coatings often can reducethe amount of efflorescence andstaining and the likelihood of oth-er water-related problems. Moreabsorptive units include somemolded brick, concrete brick, andc o n c re t e m a s o n ry m a d e f ro mlightweight aggregates.

T h et y p e o fw a t e r- re p e l l e n tc o a t-ing to use depends on the type ofmasonry and the desired chimneyappearance. Use a clear water re-pellent for most brick chimneys;recommended types include silox-anes, silanes, and blends.

For concrete brick and light-w e i g h t c o n c re t e m a s o n ry, a nopaque water- repellent coatingis used more often. Choose spe-cific brands based on past suc-cessful perf o rmance in a similar

a p p l i c a t i o n .

Structural requirementsC o n s t ru c t i n g a s t ru c t u r a l l y

sound chimney depends on antic-ipating structural loads from windor seismic forces, as well as theshape and path of the flue. Build-ing codes provide information onminimum wall thicknesses, mate-rials, and other structural require-ments. In some cases, high lateralloads may necessitate bracing atthe floors and roof and reinforcingof the chimney. Offset or slopedflues must not exceed the limitsof structural stability.

When slopi n g a f l u e , t h e m a x i-m u m a n g l e s h o u l d b e 3 0 d e g re e sf ro m t h e v e rt i c a l . I n a d d i t i o n ,when corbeling is used, an imagi-nary line drawn down through thecenter of the upper, offset portionof the chimney must fall withinthe lower chimney walls.

Additional limits are placed oncorbeling when it is used as a dec-orative accent at the chimney top.The maximum projection of thedecorative corbeling should notexceed the nominal thickness ofthe chimney wall. For a typicalbrick chimney, this limits totalcorbeling to 4 inches. Corbeling ofindividual units should not ex-ceed one-half the nominal unitheight or one-third the nominalunit bed depth.

MaintenanceYearly inspection of both the in-

terior and exterior of a masonrychimney is recommended for bestperformance. The interior of thechimney flue should be checkedf o r a c c u m u l a t e d c re o s o t e a n dother debris and cleaned as nec-essary. A visual inspection of theexterior chimney walls and crown

will help prevent many water-re-lated problems. Inspection shouldinclude:■ Examining the chimney crown

for cracks or other deteriora-tion

■ Examining the sealant betweenthe top of the crown and flue

■ Examining the chimney wallsfor signs of water penetration,such as efflorescence, staining,cracking, or spalling

■ Examining the mortar joints fordeterioration

■ Ensuring weep holes are clearof any blockageMaking needed repairs will pre-

vent major problems in the future .B u i l d i n g a b e t t e rc h i m n e y m e a n s

c h o o s i n gt h eb e s tm a t e r i a l sf o rt h ejob, including a concrete chimneycrown, flashing at the proper loca-tions, and designing for structuralstability in accordance with coderequirements. Properly designed,constructed, and maintained, aresidential chimney should last alifetime.

Christine A. Subasic, P.E., is a consultingarchitectural engineer based in Olney, Md.

References1. ASTM C 216 Specification for FacingBrick (Solid Masonry Units Made From Clayor Shale), 1994, ASTM, 100 Barr HarborDr., West Conshohocken, PA 19428.2. ASTM C 902 Specification for Pedestrianand Light Traffic Paving Brick, 1993, ASTM.3. NFPA 211 Chimneys, Fireplaces, Vents,and Solid Fuel-Burning Appliances, 1992,National Fire Protection Association, 1 Bat-terymarch Park, Quincy, MA 02269.4. “Flashing Chimneys,” Engineering & Re-search Digest, August 1992, Brick Instituteof America, 11490 Commerce Park Dr., Re-ston, VA 22091.5. “Residential Chimney Design and Con-struction,” Technical Notes on Brick Con-struction 19B Revised, January 1988, BIA.6. James E. Amrhein, Residential MasonryFireplace and Chimney Handbook, 1995,Masonry Institute of America, 2550 BeverlyBlvd., Los Angeles, CA 90057.

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