Is a leaking chimney a roofer or a damp proofer?

Start with a roofer if the damp appears after rain and dries between, because that is water coming in from outside and the fix is on the stack. Bring in a damp specialist if the plaster stays damp regardless of weather, feels moist to the touch in dry spells, or has salt crystals on the surface — that is usually hygroscopic salt contamination left behind by an old leak.

Should I take the chimney down instead?

It can be the right answer for a redundant stack in poor condition, but it is not automatically cheaper and it changes the roof, the ridge line and sometimes the structure. Removal may need building regulations approval and, on a terrace or semi, may involve a party wall. Get advice before assuming demolition is the simple option.

Why is the damp only on one side of the chimney breast?

Because rain is driven onto one face of the stack — usually the prevailing weather side, which in most of the UK is the south-west. Water soaks into that face and tracks down inside the masonry, appearing on the corresponding side of the breast indoors.

Will capping the chimney stop the leak?

Only if the water is coming down the flue. Capping an unused stack with an impermeable slab and no ventilation can make things worse, because moisture already in the masonry has no way out. A vented cap or a cowl is the usual approach.

High risk

Updated 30 August 2026

Chimney Leaking When It Rains

Damp appears on or around the chimney breast, in the loft next to the stack, or down the flue, but only after wet weather.

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What's happening?

A chimney stack is the most exposed masonry on a house: it stands clear of the roof, catches wind-driven rain on four faces, and has a hole down the middle. Every part of it is a potential entry point — the pot and its flaunching, the brickwork and its pointing, the flashing where it passes through the roof, and the cavity tray or DPC if it has one. Rain-only damp is the useful clue: if the patch appears within hours of rain and dries between, water is getting in from outside rather than condensing inside. Damp that never dries, or that is worse in winter with the fire lit, points instead at hygroscopic salts in the plaster, which is a damp proofer's job rather than a roofer's.

What usually causes it?

Cracked or missing flaunching — the mortar fillet that beds the pot to the top of the stack
Perished pointing on the stack, so wind-driven rain soaks straight through the mortar joints
Failed lead flashing, apron or back gutter where the stack passes through the roof slope
No cavity tray or DPC in the stack, so water absorbed by the brickwork tracks down inside it
Spalled or frost-damaged bricks on the weather face, which hold water rather than shedding it
Blocked or missing cowl allowing rain straight down an unused flue
Redundant stack capped with an impermeable slab and no ventilation, so the flue never dries

Is it dangerous?

Treat a leaking stack as high priority. Long-term saturation degrades the mortar joints and the brickwork itself, and a tall, slender stack with soft joints is a structural concern in high wind — a leaning or bulging stack needs immediate assessment. Inside, water in a chimney breast wets timber, joists built into the stack, and any electrics in that wall. If the flue serves a working fire or a boiler, water entering the flue also affects how it draws, and any suspicion of a flue problem on a gas appliance needs a Gas Safe registered engineer, not a roofer.

Can I fix it myself?

You can narrow the diagnosis, which genuinely saves money. Note whether the damp appears after rain and dries between, and whether it is worse on one side of the breast — both point to the outside. Photograph the stack from the ground on all visible sides, looking for missing pointing, a gap at the pot, or lead standing away from the brickwork. In the loft, look at the stack where it passes through: staining and salts on the loft-side brickwork place the leak above that point. Do not climb onto the roof, do not lean a ladder on a stack, and do not go up to look at the pot.

When should you call a roofer?

Call a roofer within days. Chimney leaks get worse quickly because the stack is saturated masonry and each rainfall drives water further in. Call immediately if the stack is leaning, bulging, has loose bricks, or if the pot is visibly unstable — that is a falling-masonry risk over a roof and a garden. If the chimney serves a gas appliance and you suspect the flue is affected, contact a Gas Safe registered engineer as well as a roofer.

What will the roofer actually do?

Scaffold the stack — a chimney cannot be properly inspected or repaired from a ladder
Work down the stack in order: pot and cowl, flaunching, brickwork and pointing, then the flashing and back gutter
Renew cracked flaunching and re-bed or replace the pot, fitting a cowl where a flue is unused, then repoint the stack in a mortar appropriate to the brick, replacing spalled bricks rather than skimming over them
Renew lead flashings — front apron, side steps and back gutter — as a set, since patching one leaves the others
Advise on capping and ventilating a redundant stack so the flue can dry rather than being sealed wet
Report on whether the internal staining is active water ingress or salt-contaminated plaster needing a damp specialist

Frequently asked questions

We’re happy to answer all your questions.

Sources & References

Lead Flashing Coming Away From The WallCaution Damp On A Chimney BreastCaution Water Stain On The Ceiling But No Obvious LeakCaution

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