June, 2018

Inspection News and Views from the American Society of Home Inspectors

Why Did That Chimney Fail?


Any crumbling or other damage to chimney masonry can easily be attributed to water. Once water enters the brick and mortar, freeze and thaw cycles occur. Water expands when it freezes, breaking apart brick, mortar and even concrete. 

Masonry chimneys are exposed to weather extremes and all five surfaces of the structure take a beating. But why does water enter the structure in the first place? Because someone decided to save a few dollars during construction by installing a cheap mortar wash cap instead of a thick cap that protects the structure from water.

Why This Chimney Failed
Take a look at the details of a damaged chimney in Photo 1. You will see a thin mortar wash cap that has cracked, allowing water into the porous brick and mortar. This poorly designed cap also lacks an overhang to protect the sides of the chimney from water.

Good VS. Poor Design
Now take a look at the illustration that shows one good and one poor chimney design (Illustration F008).

A poorly designed chimney has a thin mortar wash cap. There is no sealant to the flue tile and the cap does not overhang the brick sides. The thin mortar wash is cheap to install and it begins to fail within a few years. No homeowner climbs on the roof to maintain a mortar wash cap, so the damage continues.

The good design for a chimney has a thick, solid cap that overhangs the brick. This protects the top of the structure and the sides of the chimney. Note that the good mortar cap includes a drip edge or groove that prevents water from sticking to the underside of the cap and running inward to the brick.

Photo 2 shows the point of view looking up from under the cast cap—a thick cap and a groove running underneath the cap overhang. This groove, a “capillary break” that is at least 3/8-inch wide, prevents water from running under the cap to the brick. 

The Home Inspector’s Role
Damaged chimneys are easy to identify. You should always report a damaged chimney as an issue. And now that you understand the process behind water damage, you also may want to include notes about an improper mortar wash cap that is likely to cause more damage in the future, as well as a sketch or photo of a proper chimney cap with an overhang and a capillary break.

Tom Feiza has been a professional home inspector since 1992 and has a degree in engineering. Through HowToOperateYourHome.com, he provides high-quality marketing materials that help professional home inspectors boost their business. Copyright © 2018 by Tom Feiza, Mr. Fix-It, Inc. Reproduced with permission.