Last reviewed: 2026
Chimneys cause a disproportionate number of roof leaks because they break the continuity of the covering. The failure is usually cracked cement fillets, tired lead flashings, eroded pointing or failed flaunching around the pots rather than the brickwork itself.
The usual culprits
Cement fillets used in place of lead are the most common problem we find. Cement is rigid, the stack moves and heats, and the fillet cracks — often invisibly — letting water track down inside the roof.
- Cracked or missing cement fillets where lead should be
- Lead step, apron or cover flashings that are split, thin or lifting
- Open joints and eroded mortar in the exposed brickwork
- Cracked flaunching allowing water in around the pots
- A redundant stack left open to the weather with no cap or ventilation
How the leak shows itself
Damp on a chimney breast, staining on the ceiling beside the stack, or salts crystallising on plaster all point to the chimney. Because water travels along timbers, the wet patch inside is often some distance from the entry point — which is why diagnosis matters more than guesswork.
Repair, repoint or rebuild
Sound brickwork with failed detailing needs new leadwork, repointing and fresh flaunching. Where frost has eroded the brick faces or the stack is leaning, rebuilding above the roofline — often reusing the original bricks and pots — is the durable answer.
On an older or traditional building, lime mortar and properly dressed lead are the right materials. Sealant and cement are what create the next generation of leaks.