Q&A / 

Chimney Caps

DEAR TIM: This weekend I was up on my roof and I believe I may have discovered why my chimney is so wet when I look at it inside my attic. The cement cap that surrounds the clay flue liners has cracks in it and is in pretty bad shape. Do you think this is the problem? Should I just caulk the cracks and do some mortar repair? What is the best solution to keep my chimney in great shape? Jeff K., Belleville, IL

DEAR JEFF: Your chimney cap is also often called a chimney crown. Just as a crown is the top-most thing on the head of a member of the royal family, a chimney crown is almost always the tallest thing on your home. Because they are up in the air and most people don't regularly climb on their roofs, chimney crowns and caps often are neglected.

You bet the crown might be the cause of the dampness you see inside your attic when you look at the masonry chimney in your attic space. The chimney cap is the roof of your chimney. If it has cracks and holes in it, it will allow copious amounts of water into the inner hidden layers of masonry inside your chimney. This water wants to get out, and it drifts to the sides of the chimney as gravity pulls the water down towards your fireplace.

But water may also be entering your chimney through the actual brick, stone or block. The contact zone between the mortar and the brick, stone or concrete block is a place where water can easily enter a chimney, especially during severe rainstorms where wind is pounding raindrops into the sides of the chimney. You would be shocked how much water can enter a brick chimney or brick wall in this manner, even when you can't see hairline cracks in the mortar.

This cement chimney cap is like most. It is lacking an overhang, and it does not have a flashing beneath the cement mortar. PHOTO CREDIT: Tim Carter

This cement chimney cap is like most. It is lacking an overhang, and it does not have a flashing beneath the cement mortar. PHOTO CREDIT: Tim Carter

Many chimney caps are not built correctly. They should be made, when possible from precast concrete or cut-stone. The openings for the flue liners should be one-inch larger than the actual flue liner so the gap can be caulked. The flue liners expand and contract as they heat up and cool down. This movement will create leak points if cement mortar is troweled up against a clay flue liner.

If your mason insists on pouring or creating a cap in place, it should be steel-reinforced, and have an overhang. The cap should project beyond the face of the brick about 1.5 inches, and there should be a drip groove on the underside of the overhang. This groove prevents water from rolling under the overhang and then down the face of the chimney.

A membrane flashing should also be placed on the last flat level of masonry before the cap is placed. This flashing prevents water from penetrating down into the chimney in the event it makes it past the cap or crown. The flashing should lap up onto the sides of the flue liners and the flashing edge should project out past the face of the brick.

You can purchase some effective restoration products that will allow you to repair and seal your existing chimney cap. Some of these products are a combination of peel-and-stick membrane flashings that cover your existing cap, and then are covered with a special cement-and-sand mixture that bonds well to the membrane. The finished result is attractive, durable and leak-proof if done correctly.

To prevent leaks through the brick face of the chimney, I would suggest applying a silane-siloxane water repellent on the brick. These clear products prevent liquid water from entering the brick and mortar, but allow water vapor to escape. This is very important for those chimneys that experience below-freezing temperatures. If water gets trapped in the brick or mortar, it will expand when freezing weather occurs. This expansion can blast apart well-built chimneys over time.

Be very careful when working on old chimneys. Tall ones, even ones over five feet high, can be very unstable. Years ago I was working on a roof of a home and leaned a ladder against a narrow 8-feet tall chimney. As I climbed the ladder, I thought the ladder was moving. It was, as the old chimney was rocking back and forth. The old mortar between the brick had long ago lost its bond. I carefully backed down the ladder before the chimney tipped over crashing through the roof.

If you are working on a chimney while fuel-burning appliances are in use, be careful of the fumes exiting the flue liners. Toxic carbon monoxide can debilitate you causing you to become ill, lose your balance leading to a fall. If you have little or no experience working on chimneys and roofs, leave this job to a professional chimney sweep. These craftsmen have the tools, nerve and skills to fix your chimney problems. Let them extend the life of your chimney instead of you risking your own for a pile of brick.

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One Response to Chimney Caps

  1. I just purchsed a 75 year old home and the chimney has a concrete cap on its top.It appears to be in good shape but I will have it inspected. I'm making an assumption that even with the cap in place smoke from the fireplace can still escape through the top of the chimney? Your comments are welcome.

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