Guide
Roof leaking around the chimney: causes and fixes
Short answer
Most leaks around a chimney come from failed lead flashing, missing soakers, a blocked back gutter or cracked mortar. Water can also soak down through porous stone or a cracked crown. A roofer finds the cause from the roof and the loft, then renews the lead or repoints the stack. Act soon, because hidden timbers stay wet.
Why chimneys leak
A chimney leaks because water finds a gap at the joint with the roof or in the stack itself. The joint is the most common place.
Lead flashings cover the joint where the stack meets the roof. Soakers sit under the slates at the sides. A back gutter takes water around the uphill side of the stack. If any of these splits, lifts or is missing, water gets in.
The stack can also let water in. Cracked mortar, a damaged crown at the top, or an open flue with no cap all let rain reach the inside of the chimney.
On some old stone slate roofs, the joint was sealed with a mortar fillet, not lead. Mortar cracks as the roof moves, and then it leaks.
How to narrow down the cause
Note when the leak happens. The pattern helps the roofer find the cause faster.
- A leak only in heavy, wind-driven rain points to the flashing or the side joints.
- A leak on the uphill side of the stack points to the back gutter.
- Damp high on the chimney breast inside a room can come from the stack or the crown.
- Damp that stays in dry weather may not be a roof leak. It can come from salts in an old flue. Tell the roofer, and ask if a surveyor should look.
- Look at the stack with binoculars for lifted lead, missing mortar, plants and loose pots.
What to do now
Your aim is to catch the water and protect the room until a roofer comes.
Put a container under any drip in the loft. Lift wet insulation off the ceiling so the plaster can dry. Take dated photos inside and out.
Do not climb onto the roof. Do not try to seal the joint with sealant or tape. A sealant does not last, and it can hide the real fault.
When to call a roofer
Call a roofer as soon as you see water near a chimney. Wet timbers around a stack can rot without any sign inside.
Call at once if the stack leans, has loose pots or drops pieces of stone. Keep people away from the area below.
Ask the roofer to check the whole stack, not only the lead. Ask for photos of the top, because you cannot see it from the ground.
On old stone chimneys, ask for lime mortar. Hard cement mortar can trap damp in the stone.
Rules for chimney work
Like-for-like repairs to lead and mortar usually need no consent. Removing, lowering or rebuilding a stack is different.
On a listed building, changing the chimney usually needs listed building consent. Cotswold District Council asks owners to discuss chimney work with its conservation team. A new chimney nearly always needs planning permission and must meet Building Regulations.
Birds often nest in chimney pots. All wild birds and their active nests are protected, so plan the work around the nesting season if you can.
What it may cost
The cost depends mostly on access and scaffolding, the size of the stack and how much lead is renewed.
Our guide Chimney and lead flashing cost explains what changes the price. If the roof around the stack also needs work, see Roof repair costs in the Cotswolds.
Questions
Can a leak around the chimney be condensation?
Sometimes. A blocked or sealed flue with no vent can hold damp air. A roofer or surveyor can tell rain leaks from condensation.
Should I cap a chimney I do not use?
Capping stops rain entering the flue. Fit a cap that lets air through, so the flue stays dry. On a listed building, ask the council first.
How long does lead flashing last?
Well-fitted lead can last for decades. It fails early when it is too thin, badly fixed, or held only by mortar.