Last reviewed: 2026
Chimneys cause a disproportionate number of roof leaks because they interrupt the roof covering and rely on flashings, pointing, flaunching and masonry meeting properly. These materials weather at different rates and the stack takes more direct wind, rain and frost than the surrounding slope, so the failure is usually cracked cement fillets, tired lead, eroded pointing or failed flaunching rather than the brickwork itself.
A concentration of junctions in a small area
Almost everywhere else on a roof, the covering runs continuously across a slope. At a chimney, that continuity is broken deliberately, and the gap is closed with a combination of lead flashing, soakers, pointing, flaunching and masonry. Each of these materials behaves differently in weather and ages at a different rate, so the chance that at least one of them needs attention at any given time is higher than on an open slope.
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
Exposure and movement add to the picture
A chimney stands above the roofline and takes wind, rain and frost more directly than the surrounding covering, which accelerates weathering of the pointing and flaunching in particular. Stacks can also move slightly with thermal expansion and settlement over the years, and this movement is often what opens hairline cracks in pointing long before any other part of the roof shows a problem.
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, and why leadwork and masonry are normally checked together rather than as separate jobs.
What we would check
Because these details interact, an inspection looks at the flashings and the masonry together. Repointing without addressing failed flashing, or replacing flashing without addressing cracked flaunching, tends to leave the underlying problem only partly solved.
- Lead flashing and soaker condition on all sides of the stack
- Pointing and flaunching for cracking, erosion or previous poor repairs
- Any signs of stack movement or leaning
- Pots, caps and the flue termination
A note on drone imagery
A drone can photograph a stack from angles that are otherwise difficult to reach safely, which is useful for a first look at flashing lines and pointing from a distance. It cannot reliably show whether flaunching has debonded from the pots beneath the surface, whether timbers around the stack have decayed, or exactly where water is tracking once it is inside the roof space, so close access is still needed before firm recommendations are made.
Repair, repoint or rebuild
Isolated flashing or pointing defects on an otherwise sound stack are usually a straightforward repair: new leadwork, repointing and fresh flaunching where the brickwork itself is sound. Where flaunching and pointing are extensively degraded together, or frost has eroded the brick faces, or the stack is leaning, rebuilding above the roofline — often reusing the original bricks and pots — is the more durable answer than piecemeal patching.
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: they look like a fix for a season or two, then crack or debond as the stack moves again, and the next repair often has to remove them before the proper detail can go in.