Expert: Bart Ogden - 10/2/2006

Question
Bart, thanks for answering my previous question about the mortar cracks on my new chimney crown.  I looked at the web site you suggested and I couldn't find a reference for using concrete versus mortar.  

What is the reason mortar alone doesn't work?  I plan to take this subject up with the county building inspector.  I would like to be able to point to something in writing that says my chimney was built incorrectly.  That would give me more leverage with the mason who did the job.

Thanks again.

Answer
Hi David,

From Tech Note 19-A:  "Chimney Caps. There are, as discussed in the materials section, two options regarding chimney caps: 1) prefabricated, and 2) cast-in-place. Prefabricated caps generally provide superior performance as compared to the cast-in-place type. Regardless of which type cap is used, it should be thoroughly primed, backed, and sealed at the cap and flue liner interface to reduce the potential for water penetration.

Prefabricated caps are set in place on a mortar bed. There should be a bond break between the brickwork and the setting bed to allow the cap to respond to the differential movement it will encounter without distressing the brickwork. Figure 3 depicts a typical prefabricated cap. From this figure, general configurations and waterproofing methods may be obtained.

Cast-in-place caps should conform to the shape and minimum dimensions shown in Fig. 3. Feathering the cap to the edge should be avoided since this substantially reduces the thickness at the edge and therefore the potential for deterioration is increased. Waterproofing requirements are different since shrinkage of the concrete as it cures is a certainty. Flashing is highly recommended for cast-in-place caps. The flashing may also be considered as the bond break material. Adequate reinforcement should be placed in the cap to help control cracking due to shrinkage and thermal movements. Additional reinforcement may be necessary in the portion of the cap that overhangs the face of the chimney. Figure 3 shows one method of forming a cast-in-place chimney cap.

When using a chimney cap that does not overhang the face of the chimney, the last two courses of the chimney brickwork should be corbeled out to form a drip to help reduce the amount of water allowed to run down the face of the chimney. The flue liner should extend a minimum of 2 in. (50 mm) above the top of the cap, see Fig. 3."

Mortar is commonly used and it always fails. By using a BIA-type crown assembly it has more longevity.

This is not to say that your entire chimney was built incorrectly, just to say the the crown could have been built to a better standard.


Sincerely,
A. Bart Ogden, President
Home Safe Hearth & Chimney
Wichita, KS
Voice: 316-265-9828
Fax: 316-265-9890
www.homesafeks.com