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'’l 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-mixed mortar, using a trowel. Slope it away from the flue so 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 mortar; use dry, ready-mixed mortar that’s weather-resistant.
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.
