A common quick fix is a bead of mastic smeared along a lifted flashing edge from a ladder. It might hold for a season, but mastic degrades with UV exposure and thermal movement far faster than the lead or mortar it's covering, and the same leak reappears — usually at the next spell of driving rain.
A repair that actually stops the staining returning deals with the flashing or flue properly, then addresses the plaster that's already been contaminated.
The sequence of work
Working from the top down matters here — there's no point replastering inside before the water source at roof level is actually closed off, since any remaining leak will contaminate new plaster within one wet spell.
- Access the stack safely and inspect flashings, pointing and flue cap condition together
- Renew flashing correctly — lead dressed and lapped, not sealant smeared over the old detail
- Repoint any open joints in the stack itself with a mortar matched to the surrounding brickwork
- Cap and vent the flue if it's disused, so it's rain-tight but not sealed against condensation
- Strip contaminated plaster back on the affected chimney breast internally
- Replaster using a salt-resistant system where tar and salt staining has soaked into the wall
Why a mastic patch keeps failing
Sealant works by filling a gap, but it doesn't move the way lead does with temperature changes, and it doesn't bond permanently to old, weathered mortar or brick. It cracks at the edges within a year or two in most cases, and because it's applied over an already-failed detail, water finds the same path back in almost immediately once it does.
Inside the room, simply repainting over a tide-mark does nothing about the tar and hygroscopic salts already drawn into the plaster. They'll keep pulling moisture from the air and bleeding through paint indefinitely unless the contaminated plaster is actually removed.
Interfaces with roofing and interior finishes
Flashing repair at a chimney is roofing work and needs proper access equipment, ideally done alongside any other roof maintenance while access is already arranged. Internally, the replastering should wait until the outside is confirmed dry — replastering too early over residual damp just repeats the failure in new material.
How you know the repair has worked
After the next period of wind-driven rain, the breast should show no new staining at all. If a small amount reappears, it's more likely to be residual moisture drying out of thick masonry than a fresh leak — but any staining that keeps growing after several weeks means the roof-level repair needs revisiting.
Common questions
Can the flue be capped without being vented?+
It shouldn't be, if it's disused — a sealed cap with no ventilation traps moist air inside the stack, which condenses and causes the same kind of staining from a different cause. A vented cap keeps rain out while letting the flue breathe.
Do you need to replaster the whole chimney breast or just the stained patch?+
Usually just the affected area, but salts can spread further into the plaster than the visible stain suggests, so we check with a moisture meter before deciding the extent of the strip-back.
Is scaffolding needed for a flashing repair?+
It depends on the height and pitch of the roof — some chimneys are accessible from a properly footed ladder or tower, others need scaffolding for safe access. We assess this on the site visit rather than guessing from the ground.
Book a chimney and flashing repair quote
We fix the leak at the source and repair the plaster it damaged, in one sequenced job with a written itemised quote.