Concrete Caps / Crowns - Why do they fail?

Bill Hussel
Concrete Cap Diagram

Masonry brick and stone chimneys deteriorate faster than any portion of the home because of their extreme weather exposure on all sides. For the latitude of Madison Wisconsin, we receive the most freeze/thaw cycles than any other area of the United States.

There are many reasons why the top portion of a masonry chimney – commonly called "chimney crown" fails prematurely and allows severe water leakage which in turns deteriorates the rest of the chimney and possibly does damage to the roof area and below inside the home:

1.  Severe freeze/thaw cycles as explained above – especially for the Madison latitude.

2. Horizontal masonry surfaces allow water to stand and collect without running off as compared to a vertical surface. The entire surface of a chimney crown is horizontal with the exception of a slight slope.

3. Most chimney masons when constructing a chimney simply use the same materials for the chimney crown as for laying the bricks/stone. This material is called a "mortar mix" and is only 20% as strong as "Portland concrete" which should be used for the crown. 

4.  Also for most chimney masons, they do not take the time to build a form and pour the concrete crown 4 inches thick.  Instead, they simply pour a layer of mortar mix, including feathering the mortar at the outer edge to a 1/8" thickness. The outside edge should be a minimum of 4 inches thick. Also, by skipping the construction of a chimney crown form, there is no overhang to shed the water away from the chimney. This overhang should be a minimum of 1 inch wide and should include a drip edge groove underneath to keep water away from the sides of the chimney.

5. Another cardinal sin when most new chimneys are constructed is that the mortar is bonded directly to the flue tile with no"slip joint" material to allow the flu tiles to expand and contract while the fireplace is in use. The entire flu tile system expands significantly when heated. A "bond break" should be installed between the top surface of the bricks/stone and the bottom of the poured concrete crown since concrete expands at different rates compared to brick or stone.

6. Lastly, the poured concrete cap must be reinforced with steel and/or fiberglass fibers to prevent cracking.

The Wisconsin building code states: CHIMNEY CAPS: Chimneys shall be provided with precast, or cast in Place concrete chimney caps. Chimney caps shall have a minimum thickness of 2 inches, shall slope outwards away from the flue, and shall provide a 1 inch overhang and drip edge on all sides. A slip joint shall be installed between the flue tile and the concrete cap. The slip joint shall be filled with 1/4 inch felt or similar material and shall be caulked with high temperature caulk or similar material to prevent water infiltration. (Issued May – 2005.)

The attached diagram shows the proper construction of a poured chimney cap as described above.