Building a Better Chimney

Building a better chimney

Use proper detailing and the best materials to avoid moisture-penetration problems

By Christine A. Subasic 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 k moisture absorbed under pres- for Grade SW (severe weathering). sure, such as from driving rain or Because they have a higher com- freezing effects. The cold-water e s i g n e r s a n d b u i l d e r s o f pressive strength and are less ab- absorption is a measure of the residential chimneys some- sorptive than Grade MW (moder- moisture absorbed by a unit under Dtimes fail to realize the se- ate weathering), Grade SW brick normal conditions. verity of the conditions to which meeting this standard are durable H o w e v e r, i n a re a s w i t h l a rg e a chimney is exposed. As a result, e v e n i f f ro z e n w h i l e s a t u r a t e d a m o u n t s o f r a i n o r n u m e ro u s efflorescence, staining, water pen- with water (Ref. 1). freeze-thaw cycles, consider using etration, and deterioration of ma- ASTM C 216 requirements limit e v e n l e s s a b s o r p t i v e m a s o n r y sonry chimneys may occur, un- the boiling-water absorption for units—Class SX paving brick con- less precautions are taken. Grade SW to 17% and the cold- forming to ASTM C 902 Specifica- Most problems associated with water absorption to 13.3% for an tion for Pedestrian and Light Traf- masonry chimneys, whether aes- average of five brick. The boiling- fic Paving Brick. Because they are thetic or more substantial, are re- 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 laid horizontally, pavers typically lated to water. Chimneys can be exposed to significant amounts of Expansion joint and sealant water under a variety of climatic conditions. Both the warm rains of Sloped concrete the South and the cold wind-driv- crown 2" min. en rains and snow of the North- east can be damaging to a masonry c h i m n e y, p a rt i c u l a r l y i f t h e 2" min. c h i m n e yi si m p ro p e r l yc o n s t ru c t e d or poorly maintained. In addition, the thermal cycling that occurs Drip due to varying outdoor tempera- tures and normal chimney usage Vent can create a potentially damaging 1 2 ⁄2" min. environment. Flashing To withstand the rigors of such an environment, a chimney must b e p ro p e r l y d e s i g n e d a n d c o n - Chimney structed of the best materials. wall 1" space Chimney exterior Clay flue liner The materials used to construct exterior chimney walls are subject to the most severe exposure. If brick is used, it should conform Figure 1 Chimney crown to the requirements of ASTM C are subject to more severe weath- er exposure than brick in vertical applications. Consequently, the Clay flue liner Chimney wall absorption limits found in C 902 are considerably lower than those in C 216 (Ref. 2). Counterflashing ASTM C 902 limits cold water absorption to 8.0% and boiling water absorption to 10.3% for an average of five units. As a result, chimneys built with C 902 brick absorb less water and should pro- 3" min. lap Roof vide better durability than those 4" min. Base flashing built from C 216 brick. Mortar used on the exterior of a chimney can be Type N or S mor- 4" min. tar made from portland cement and lime or from masonry cement. In areas with wind pressures over 25 psf, the Brick Institute of Amer- i c a ( B I A ) re c o m m e n d s Ty p e S portland cement-lime mortar for its high tensile strength. Under Figure 3 Through-wall counterflashing these conditions, chimneys made w i t h o t h e r m o rt a r s a re m o re prone to cracks at the brick/mor- and the exterior elements, only for Pier Test for Refractory Mort a r s , tar interface. non-water-soluble refractory mor- m e d i u m - d u t y. tars should be used for laying the Chimneys used to vent high-effi- Chimney interior flue, as specified by the National ciency gas appliances are subject The interior of a masonry chim- Fire Protection Association stan- to highly acidic condensate. Use ney typically is constructed with d a rd N F PA 2 1 1 ( R e f . 3 ) . T h e s e clay flue liners and an acid-resis- clay flue liners conforming to the mortars should meet the require- tant or chemical-resistant mortar re q u i re m e n t so fA S T M C 3 1 5 S p e ci- ments of ASTM C 105 Specificati o n in these chimneys for best perfor- fication for Clay Flue Linings. Be- for Ground Fire Clay as a Ref r a c t o r y mance. Do not use portland ce- cause of their exposure to mois- M o rt a r f o r L a y i n g U p F i re c l a y m e n t o r c a l c i u m a l u m i n a t e c e- ture from condensing flue gases B r i c k o r A S T M C 1 9 9 Test Method m e n t b e c a u s e o f t h e i r l o w a c i d re s i s t a n c e . To accommodate the wide tem- Base flashing Tab perature changes experienced by the flue, include a 1-inch air space b e t w e e n t h e e x t e r i o r c h i m n e y walls and the flue. In high seismic a re a s , t h i s s p a c e i s f i l l e d w i t h grout for structural reasons, but reinforcement placed within this space helps to accommodate the thermal stresses that occur. Counter- flashing Crown Better chimney design always in- cludes a crown to protect the chimney walls and interior fro m rain and snow. A portland cement c 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 e a 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 e of 3 inches or more to pro m o t e Base flashing drainage, 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- 1 Figure 2 Intersection with the roof ject at least 2 ⁄2 inches past the face of the chimney wall below and in- corporate a drip. This re d u c e sw a- chimney walls to form a drip edge. place. But use this detail cautious- t e r ru n d o w no nt h e chimney walls. It should pass through the chim- ly because it does not provide 3 Leave a ⁄8 -inch gap between the ney walls into the space between drainage for water entering the chimney crown and the flue to al- the walls and the flue. The flash- chimney space. low for thermal expansion of the i n g s h o u l d b e t u rn e d u p w a rd I n t h e n o rt h e a s t e rn U n i t e d flue. The gap should be filled with against the flue and extend up to States, where driving rains are se- a backer rod and an elastomeric the top of the crown (see Figure 1). vere, tray or pan flashing may be sealant, such as a silicone or a A backer rod and sealant placed used. A solid sheet of flashing, of- polysulfide sealant. Extend the b e t w e e n t h e f l a s h i n g a n d t h e ten made of lead, is cut to match flue at least 2 inches or the mini- the chimney cross-section. Holes m u mc o d e - re q u i re dd i s t a n c ea b o v e crown provides a tight barrier to are then cut for the flue(s). the top of the crown. water penetration. The tray flashing is laid in place, To promote drying of the chim- C h i m n e y - ro o f i n t e r s e c t i o n . n e y i n t e r i o r, B I A re c o m m e n d s Proper flashing is critical at the g e n e r a l l y t w o t o f o u r c o u r s e s placing vents in the mortar joints c 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 e above the highest point where the directly below the crown flashing, it prevents water from penetrat- chimney intersects the roof. The spaced at 24 inches on center (Ref. ing into the interior of the struc- flashing is turned up into the cavi- 4). Brick vents placed in mortar ture. Typically, roof or base flash- ty around each flue and down to head joints, combined with weep ing is installed first. Extend the form a drip on the exterior. This holes at flashed locations, encour- b a s e f l a s h i n g a t l e a s t 4 i n c h e s provides a continuous barrier to age air circulation in the cavity be- horizontally along the roof and at any moisture that penetrates the tween the chimney and the flue. least 4 inches up the face of the chimney walls above. To provide Don’t use a mortar wash in lieu chimney. drainage, locate weep holes di- o f a c h i m n e y c ro w n . A m o rt a r Then the counterflashing is in- re c t l y a b o v e t h e f l a s h i n g a t 2 4 wash does not withstand the mois- s t a l l e d . I t s h o u l d l a p t h e b a s e inches on center. ture and temperature changes as flashing a minimum of 3 inches Chimney base. If the chimney well as a concrete crown, nor does (see Figure 2). BIA recommends is built on an outside wall of the it provide an overhang to protect extending the counterflashing residence, the base of the chim- the chimney walls. through the chimney wall, up into ney must also be flashed. Place through-wall flashing between the Flashing the space between the chimney and the flue (see Figure 3). This bottom course of brick and the Flashing should be placed at p ro v i d e s m a x i m u m p ro t e c t i o n foundation. If the top of the foun- three locations in a masonry chim- from water that penetrates the d a t i o n i s b e l o w g r a d e , p ro v i d e ney—directly beneath the crown, chimney walls and can be particu- flashing at a level above the grade at the intersection of the chimney larly important on tall chimneys line. Weep holes should be placed and the roof, and at the base of that have large areas of exposed above grade at the flashing. the chimney. wall (Ref. 5). Beneath the crown. Flashing However, in dry regions with p l a c e db e n e a t ht h ec h i m n e yc ro w n Cricket little rainfall, the counterflashing protects the chimney walls from A cricket should be used when may be terminated in the mortar the chimney penetrates the roof any moisture that might penetrate j o i n t s . I n t h i s c a s e , t h e m a s o n t h e c ro w n a n d p re v e n t s w a t e r below the ridge line and the face s h o u l d r a k e o u t t h e j o i n t t o re- of the intersecting chimney wall from entering the space between 3 ceive the flashing 4⁄ to 1 inch. Then parallel to the roof line is over 30 the crown and the flue. The flash- the flashing is inserted into the inches long. Framed of wood and i n g s h o u l d e x t e n d b e y o n d t h e joint and mortared or caulked in c o v e re d w i t h ro o f i n g m a t e r i a l ,

1" Roof pitch H

1 1:1 ⁄2 W

1 2:3 ⁄3 W

1 H 1:2 ⁄4 W

1 1:3 ⁄6 W

1 1:4 ⁄8 W W = Cricket width Cricket dimensions H = Cricket height Chimney cricket Figure 4 Source: Masonry Institute of America (Ref.6) crickets are used to divert water a p p l i c a t i o n . will help prevent many water-re- from the roof around the chimney. lated problems. Inspection should But they do not eliminate the need Structural requirements include: for flashing and counter flashing ■ Examining the chimney crown C o n s t ru c t i n g a s t ru c t u r a l l y around the chimney walls. Crick- for cracks or other deteriora- sound chimney depends on antic- ets should be flashed and count- tion ipating structural loads from wind ■ Examining the sealant between er-flashed like a typical roof inter- or seismic forces, as well as the section (see Figure 4). the top of the crown and flue shape and path of the flue. Build- ■ Examining the chimney walls ing codes provide information on for signs of water penetration, Water-repellent coatings minimum wall thicknesses, mate- such as efflorescence, staining, The use of water-repellent coat- rials, and other structural require- cracking, or spalling ings when constructing a mason- ments. In some cases, high lateral ■ Examining the mortar joints for ry c h i m n e y i s n o t w i d e s p re a d . loads may necessitate bracing at deterioration More often, water repellents are the floors and roof and reinforcing ■ Ensuring weep holes are clear considered when existing chim- of the chimney. Offset or sloped of any blockage neys have problems. In the case flues must not exceed the limits Making needed repairs will pre- of newly constructed masonry of structural stability. vent major problems in the future . chimneys, such coatings are ap- When slopi n g a f l u e , t h e m a x i- 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 propriate in a limited number of m u m a n g l e s h o u l d b e 3 0 d e g re e 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 e circumstances. f ro m t h e v e rt i c a l . I n a d d i t i o n , job, including a concrete chimney In regions with large amounts of when corbeling is used, an imagi- crown, flashing at the proper loca- rainfall, the chimney exterior may nary line drawn down through the tions, and designing for structural benefit from treatment with a wa- center of the upper, offset portion stability in accordance with code ter-repellent coating, regardless of the chimney must fall within requirements. Properly designed, of the type of masonry unit used. the lower chimney walls. constructed, and maintained, a residential chimney should last a When relatively absorptive ma- Additional limits are placed on lifetime. sonry units are used, water-repel- corbeling when it is used as a dec- lent coatings often can reduce orative accent at the chimney top. The maximum projection of the Christine A. Subasic, P.E., is a consulting the amount of efflorescence and architectural engineer based in Olney, Md. decorative corbeling should not staining and the likelihood of oth- exceed the nominal thickness of er water-related problems. More References the chimney wall. For a typical absorptive units include some 1. ASTM C 216 Specification for Facing brick chimney, this limits total Brick (Solid Masonry Units Made From Clay molded brick, concrete brick, and corbeling to 4 inches. Corbeling of or Shale), 1994, ASTM, 100 Barr Harbor Dr., West Conshohocken, PA 19428. c o n c re t e m a s o n r y m a d e f ro m individual units should not ex- 2. ASTM C 902 Specification for Pedestrian lightweight aggregates. ceed one-half the nominal unit T h et y p e o fw a t e r- re p e l l e n tc o a t- and Light Traffic Paving Brick, 1993, ASTM. height or one-third the nominal 3. NFPA 211 Chimneys, Fireplaces, Vents, ing to use depends on the type of unit bed depth. and Solid Fuel-Burning Appliances, 1992, masonry and the desired chimney National Fire Protection Association, 1 Bat- appearance. Use a clear water re- Maintenance terymarch Park, Quincy, MA 02269. pellent for most brick chimneys; 4. “Flashing Chimneys,” Engineering & Re- Yearly inspection of both the in- search Digest, August 1992, Brick Institute recommended types include silox- terior and exterior of a masonry of America, 11490 Commerce Park Dr., Re- anes, silanes, and blends. chimney is recommended for best ston, VA 22091. For concrete brick and light- 5. “Residential Chimney Design and Con- performance. The interior of the struction,” Technical Notes on Brick Con- w e i g h t c o n c re t e m a s o n r y, a n chimney flue should be checked struction 19B Revised, January 1988, BIA. opaque water- repellent coating f o r a c c u m u l a t e d c re o s o t e a n d 6. James E. Amrhein, Residential Masonry is used more often. Choose spe- other debris and cleaned as nec- Fireplace and Chimney Handbook, 1995, cific brands based on past suc- essary. A visual inspection of the Masonry Institute of America, 2550 Beverly Blvd., Los Angeles, CA 90057. cessful perf o rmance in a similar exterior chimney walls and crown