Q: What is involved in fixing a crumbling chimney?
The top of my chimney is in bad shape, but my wife said she'd kill me if I go up there and work on it myself. Before I hire someone to do the work, can you tell me what needs to be done?
—Joe Medved, Columbiaville, MICH.
Kevin O'Connor replies: Masonry restoration expert John Lambert agrees with your wife and recommends that you turn the work over to a brick mason because "it's important to get the details right." He says your existing flue liners and mortar chimney crown are beyond repair and must be replaced. Here's how Lambert fixes this type of problem:
First, he removes the crown, the top few courses of brick, and the clay flue liners down to their first mortar joint. After the new flue liners and brick have been mortared in place, he builds a form for the new crown using a sheet of galvanized steel as the form's base. "Concrete and brick expand and contract at different rates," he says. The sheet prevents them from bonding. Also, unlike your existing crown, the new crown's edges should extend at least 1½ inches beyond the face of the brick and a have a drip groove molded into the underside. Lambert lays a reinforcing mesh in the form for strength and wraps each liner with cardboard coated with a form- release agent. The cardboard creates a crucial gap that allows the liners to expand and contract.
Lambert fills the form with concrete—"much more durable than mortar," he says—mixed with nylon fibers to eliminate cracking. To encourage drainage, he trowels the top of the crown so that it slopes away from the flues by ¼ inch per foot. After the concrete sets up, he takes the cardboard off each flue and fills the gap with a foam backer rod and high-quality 50-year silicone sealant.
Once the form comes off and all the work has been washed clean, Lambert sprays everything—brick, mortar joints, concrete, and clay flues—with a penetrating, breathable, siloxane water repellent, such as Prosoco's Siloxane PD. "Make sure it's not a sealer," he cautions. "Sealers can trap moisture within the masonry." Finally, Lambert puts a stainless-steel cap over each flue liner to keep out water and animals and to prevent sparks from landing on the roof.
The water repellent needs to be reapplied every 10 years or so as part of your regular chimney maintenance. The silicone around the flues should be checked every couple of years and replaced as necessary.
Shown: The crumbling flue tiles and cracked mortar around them mean that it's time for both to be replaced by a mason skilled in chimney work.