Ask the Builder
Up on the Rooftop, Where Owners Fear to Tread, Trouble May Brew in Chimney Caps
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Q: DEAR TIM: This weekend, while I was up on my roof, I think I discovered why my chimney is so wet when I look at it from 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, Ill.
A: DEAR JEFF: A chimney cap, often called a chimney crown, is the tallest part of your home. Because the caps are up high and most people don't regularly climb on their roofs, chimney crowns often are neglected.
This could indeed be the cause of the dampness you see on the chimney inside your attic. The chimney cap is the roof of your chimney. If it has cracks and holes, it will allow copious amounts of water into the 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 toward your fireplace.
But water may also be entering your chimney through the brick, stone or concrete block. The contact zone between the mortar and the brick, stone or block is a place where water can easily enter a chimney, especially during severe storms when wind is pounding rain into the sides of the chimney. You would be shocked by how much water can enter a brick chimney or brick wall this way, even when you can't see hairline cracks in the mortar.
Many chimney caps are not built correctly. When possible, they should be made from precast concrete or cut stone. The openings for the flue liners should be an inch larger than the liner so the gap can be caulked. The flue liners expand and contract as they heat and cool. This movement creates leaks if 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 1/2 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 the chimney if it gets past the cap or crown. The flashing should lap up onto the sides of the flue liners, and the flashing edge should project past the face of the brick.
You can purchase some effective restoration products that will allow you to repair and seal your chimney cap. Some of these are peel-and-stick membrane flashings that cover your existing cap, which are then covered with a special cement-and-sand mixture that bonds well to the membrane. The finished result is attractive, durable and leakproof if done correctly.
To prevent leaks through the brick face of the chimney, I recommend applying a silane/siloxane water repellent on the brick. These clear products prevent water from entering the brick and mortar but allow water vapor to escape. This is important for chimneys that experience below-freezing temperatures. If water gets trapped in the brick or mortar, it will expand in freezing weather. This expansion can blast apart well-built chimneys over time.
Tim Carter can be contacted via his Web site, http:/
Copyright 2007 Tribune Media Services
