Masonry Magazine August 1993 Page. 9
Proper Chimney Caps
The top of a chimney warrants close attention to cap details which prevent moisture penetration. Poor construction results in costly repairs later.
CHIMNEY CAPS require special attention to prevent unwanted moisture penetration. The Brick Institute of America receives numerous complaints about "disintegrating" brick near the top of chimneys. Many of these complaints are due to chimney caps which do not effectively stop moisture from penetrating the brick below. When excess amounts of moisture continue to enter masonry, problems such as efflorescence and spalling are likely to occur. Defective chimney caps may also contribute to moisture related problems near the base of the chimney and at the interior fireplace.
Why Chimney Caps Leak
The top of a chimney warrants close attention to cap details which prevent moisture penetration. Chimneys project above the roofline and are completely exposed and highly vulnerable to the effects of the weather. Chimney caps are often poorly detailed and constructed, requiring costly repair later.
One of the most common and least effective chimney caps is constructed by placing mortar on top of the finished brick. The mortar placed in this location usually shrinks, causing cracks to occur around the flue liner and in the body of the mortar itself. These openings provide easy access for water to saturate the brick masonry below (Figure 1).
Other causes for water entering through chimney caps are:
* No flashing or improperly installed flashing under the cap.
* Slope of cap less than three inches per foot from the flue liner to the edges of the chimney.
* No sealant between the cap and the flue liner.
* Insufficient projection past the exposed chimney wall faces or the lack of a drip on the cap's underside located at least one inch from the chimney's wall face.
* Metal caps which do not extend down the face of the chimney at least four inches.
* No sealant between a metal cap and the chimney face.
The repair of a chimney cap should take place at the first sign of moisture related problems. If a faulty cap isn't repaired, problems are likely to worsen. If a cap is cracked or disintegrated, it must be removed and replaced or covered with a metal chimney cap. This also applies to flashing. Stainless steel, galvanized steel, copper or plastic should be used as flashing. Aluminum or lead materials aren't recommended. Unless a prefabricated flashing is used, the flashing will have to be cut to the required shape. All joints must be lapped at least six inches and sealed with a mastic or soldered if metal. The flashing must extend from the top of the flue liner to the top of the chimney and run completely under the cap to the exterior face of the chimney wall (Figure 2).
Chimney caps are typically made of pre-cast concrete, stone, cast-in-place concrete, or metal. Non-metal caps should not be anchored to the flue liner. The cap and flue liner must be
Cap Materials and Details