Chimney flashing repair in Lewisham
If water is coming in near a chimney, the flashing is the first thing worth checking — and it is the part most often blamed on the stack instead.
Flashing is the weatherproof detail where the chimney passes through the roof. It is a junction between two materials that move differently, sits in the path of everything running down the roof slope, and is the single most common leak point on a pitched roof. Get it right and it lasts decades. Get it wrong and no amount of work on the stack above will stop the water.
How flashing is supposed to work
A properly detailed chimney junction is not one piece of metal. It is a system:
- Soakers — individual pieces interleaved with the tiles or slates on each side, taking water out onto the roof covering.
- Apron flashing at the front, on the up-slope side, throwing water away from the stack base.
- Step flashing up the sides, cut to follow the mortar courses.
- Back gutter behind the stack — on the down-slope side, where water is funnelled around the chimney. This is where the worst leaks usually originate, and the hardest part to detail properly.
- Cover flashing, dressed into a chase cut in the brickwork and wedged and pointed to hold it.
Lead is the traditional material for all of it in London, and it remains the right one for period stacks: it is malleable enough to be dressed to shape, and it lasts a very long time when correctly sized and fixed. If you want to understand how each element is supposed to be formed — and why oversized pieces split — see lead flashing and leadwork.
Why flashing fails
- Cement fillets used instead of lead — a mortar wedge instead of a metal flashing. Cheap, extremely common on older repairs, and it always cracks eventually because brick and roof covering move independently.
- Lead that has slipped out of the chase as the pointing holding it has eroded
- Split or fatigued lead, usually where a piece was made too long and thermal movement has worked it apart
- A missing or poorly formed back gutter behind the stack
- Flashing dressed over the tiles rather than interleaved, so water runs behind it
- Storm damage lifting a section
- Corrosion where a non-lead substitute has been used
Cement fillets are the single most common cause of "chimney leaks" we see in South East London. If someone previously ran a triangular band of mortar around the base of your stack instead of fitting lead, expect it to crack — and expect the leak to return however many times it is patched. The fix is to remove it and detail the junction properly.
Signs the flashing is the problem
The tell-tale pattern is water appearing at or just below ceiling level near the chimney breast, rather than higher up the stack, and appearing after prolonged or driving rain rather than immediately in a shower. Other indicators:
- Staining on the ceiling immediately around the chimney breast
- Damp in the loft on the timbers beside the stack, particularly on the down-slope side
- Visible gaps, lifted metal, or a cracked mortar band at the stack base
- Water tracking down the outside of the stack inside the loft space
- Leaks that stop in dry weather and return reliably with rain from one direction
Damp higher up a chimney breast, or on an upper wall rather than at the ceiling, more often points to the stack itself — see chimney repair and repointing.
Repair or replace?
Localised damage — a single lifted section, pointing that has released the cover flashing from its chase — can often be repaired in place. Where a cement fillet has been used, where lead has been cut too long and fatigued, or where there is no proper back gutter, replacement of that element is the durable answer. Patching a fundamentally wrong detail is money spent to delay the same conversation.
Access
Flashing work is at roof level around the stack, so it needs safe access, usually scaffolding or a tower depending on the position and roof pitch. If a stack needs both flashing and pointing or flaunching work, doing it under one access arrangement is significantly cheaper than two visits.
Water coming in near the chimney?
Tell us where the damp appears, and whether it follows rain from a particular direction. That detail genuinely narrows down where it is getting in.
Flashing questions
Can flashing be sealed with mastic or a roof sealant?
As a very short-term measure to slow water entry before proper repair, sometimes. As a fix, no. Sealants sit on the surface, are exposed to UV and movement, and fail — usually within a season or two, and often invisibly, so the leak returns without any obvious change outside.
How long should lead flashing last?
Correctly sized, fixed and dressed lead is a very long-life material — commonly several decades. Most failures we see are not the lead wearing out but the detailing: pieces made too long so thermal movement fatigues them, or the pointing that retains them eroding away.
The leak only appears sometimes. Is it still the flashing?
Quite possibly. Flashing leaks are often directional — they show up with wind-driven rain from one particular quarter and not with ordinary vertical rainfall. Intermittent leaks are harder to trace, so noting the weather when it happens is genuinely useful information.
Do I need the whole flashing replaced or just part?
It depends what is there. A single damaged element with the rest correctly detailed can be repaired locally. Where cement fillets have been used, or where the back gutter is missing or wrong, that element needs doing properly rather than patching.