Mortar chimney caps are sometimes vulnerable to cracks along the joints because the mortar shrinks, causing the joints to open. Lime-based mortar decays over time and may crumble. Minor cracks and the joint between the flue and the cap can be sealed with masonry caulking compound.
If the cap is crumbling, you’ll have to chip out the old deteriorating mortar using a ball-peen hammer and a cold chisel. Be sure to wear gloves and goggles. Brush off any loose debris. Then apply a new layer of ready-mix mortar using a trowel. Slope it away from the flue so that rain will drain off properly. Keep the mortar damp for four days to allow it to cure.
To repair mortar around the bricks, remove the old mortar and pack the cracks with new, weather-resistant ready-mix mortar.
Renew flashing seals by chipping out the old mortar. Caulk along the edges of the flashing, and caulk the joints between the flashing and the chimney.
One source of attic-to-home air leaks is the space surrounding your chimney. Chimneys often extend into attics through framed bypasses, and, if these are not properly insulated and sealed, air will escape.
Fill gaps in these areas using unfaced rockwool batts or a similar fireproof material. Seal up where the chimney and the framing meet with furnace cement or a caulk that can withstand high temperatures.
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