Whether you have a pre-cast or cast-in-place concrete crown or cap, it must be capable of effectively shedding water and properly sealing the top of the chimney. The chimney crown cannot allow water to enter the chimney. If the cap fails over time, the moisture wears away the brick and mortar, which can cause serious structural problems. The end result is a short useful life for the chimney. Thus, a correct installation is vital.
Chimney crowns are made of four types of materials: concrete slabs/reinforced concrete, brick/mortar, metal and stone. Concrete is the most popular choice; it is very durable as long as it is installed properly. Brick covered with mortar is also used, but crowns made with mortar have a history of poor performance. Stone offers a superior performance over the other materials. It is usually made of a single solid block of stone. It is the most expensive option for a chimney crown.
The most popular material used to for brick chimney crowns is concrete. Concrete crowns can be prefabricated off-site and delivered or cast-in-place. Cast-in-place crowns are poured into a form right on top of the chimney. Traditionally, masons used handmade-wooden forms, which were very time-consuming to set up. Today, adjustable steel forms are used. Steel forms can be assembled in a fraction of the time compared to the old wooden forms.
The minimum thickness for crowns is 2 inches. The Brick Institute of America requires a 2½-inch drip cap or overhang. The drip cap or overhang is designed to protect the face of the brick wall. This is the part that the crown rests on. Use stainless steel or galvanized steel on the face. Never use an aluminum drip cap because aluminum interacts with the mortar and will corrode over time.
The crown should also slope away from the chimney flue toward the crown edge 3 inches for every foot. It’s important to make sure that there is an opening between the chimney crown and liner. This will allow the liner to expand and contract.
There should be a gap between the chimney crown and liner, filled with a flexible sealant, which allows for liner expansion. Without that gap during the cold season, exhaust gases warm up the liner, which expands, pushing against the much colder chimney crown, and causes it to crack.
A very inexpensive type of chimney crown is known as a chimney wash. It is made up of a layer of mortar installed over bricks. This type of crown is not recommended because, after a winter or two, it will be ineffective. You will pay for making the choice of a chimney wash in costly repair bills.
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