Perhaps the most important component of a chimney is the Crown (mortar cap) around the flue at the top of the chimney—-and a proper hat on the flue itself.
This post is not about the kinds of damage that can be done to chimneys from the inside by the appliances that are connected to them. Wood, gas and oil appliances ruin chimneys every day. Today’s post is about the kind of damage that happens to chimneys from external forces—-namely WATER! Water can be just as hard on masonry as it can be on wood, and like wood the structure must be properly constructed to protect it the best that it can be from water damage.
The Mortar Cap, or Crown, rarely gets done properly. The mortar cap should hang past the edges of the chimney so that water sheds off the top of the chimney—kind of like the overhang on the roof of the house keeps the roof water from running down the siding—-if there are no gutters. This necessary overhang is not present 99% of the time on chimneys and 99% of those that have overhangs have no drip groove on the underside that prevents water from wrapping around the overhang. Most chimneys suffer from lack of an overhanging cap. A proper overhanging cap can greatly protect sides away from the wind from being impacted by water running off the cap on at least those sides. Without it, water that hits the cap runs down all sides of the chimney.
The basic purpose of the crown (and the flue hat) is to keep water out of the chimney structure. This is especially important in regions where temperatures drop below freezing. Water that freezes inside of chimneys expands and destroys bricks and mortar—-which allows for even more water infiltration.
It should be obvious from this first picture that water WILL enter the chimney structure.
The mortar joints can deteriorate to the extent that holes will become visible all the way through the chimney—-as was the case with this chimney.
Can you see the metal flue liner between the bricks?
Most chimneys have a crown of mortar that is merely a think layer of concrete troweled around the flues and terminated flush with the face of the brick. This creates a very thin area at the brick edges that cracks and loosens easily. It is much better to pour a more monolithic concrete slab that has enough thickness that it is not subject to the expansion and contraction cracking typical of very thin mortar crowns. Ideally this crown will overhang the face of the brick on all sides and have a drip groove on the underside of the overhang to prevent water from wrapping around onto the brick face. In any event the crown must be kept sealed so that moisture cannot get into any cracks that do form in the cap as well as at the connection of the cap and the flue. A large hat on the flue can also aid in protecting this vulnerable area. This next picture shows a poorly made and badly deteriorated mortar crown that no longer protects the chimney structure at all.
This particular chimney (and there are many like them) is constructed of bricks that are hollow. These types of chimneys are particularly vulnerable to damage from water once the water finds its way into the structure.
One particular method of attempting to create a more functional crown is for the mason to overhang the top rows of bricks. While this is “somewhat” better than NO overhang, it still has no drip groove on the underside and the mortar cap is still too thin at the edge of the bricks. If you look closely at this next picture you will see the failing of the thin edge of the mortar cap, the gaps around the flues, and the obvious crack through the mortar cap extending down the center face of the chimney. All of these conditions allow for large amounts of water to enter the chimney structure.
Now if you would like to see how they are “supposed” to be done, check out this schematic from the Masonry Advisory Council.
Someday I hope to have an “actual” picture of a proper cap taken in the field—we will probably stop building masonry chimneys before that happens.
Charles Buell, Real Estate Inspections in Seattle
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