Damp on the chimney breast: is it the roof?

1Three causes that leave the same mark

A brown patch on the breast looks the same whatever made it, which is why people spend money on the wrong repair. The first cause is rain entry. Water finds a way into the stack from the top or the sides and runs down the brickwork until it reaches the plaster. The second is condensation: warm air from the room meets the cold flue and leaves its moisture behind, particularly in a chimney that has been closed up. The third is salts. Soot and combustion residues are left in the brick and plaster over years, and these salts pull damp out of the air even when nothing is leaking.

All three can be present together, and a house with a worn stack and a blocked flue often has two. A careful look has to come before any repair, because a guess is expensive when it is wrong.

2How the stain gives it away

Rain entry tends to follow the weather. The patch darkens soon after a wet, windy spell and fades as the weather dries, and it is often worst on the side of the breast that takes the weather or around the ceiling line. If the stain sits in the loft or in a bedroom close to the roof line, look hard at the stack. A patch that appears only after driving rain, and never in a dry cold snap, is the clearest sign of all.

Condensation is seasonal. It builds in cold months, gives a cold, clammy surface and sometimes mould, and often sits low or across the whole breast rather than in one place.

Salts leave a tidemark with a crystalline or powdery edge, and wallpaper or paint blisters again and again after redecorating. A damp meter may read high on salts even when the wall is dry to touch, because the salts hold water from the room air. A reading alone does not prove a leak.

3What we check on the roof and stack

When the stain follows rain, we start from the top. The flaunching, the mortar bed around the pot, may be cracked or have lost its fall. The pot may have a split in it. The pointing on the stack can be open or sandy, and the lead flashing round the foot of the stack may have lifted, or the lead back gutter behind it may be blocked with moss and leaf litter. Replacing a chimney pot and renewing the bed under it is one of the quicker repairs and often solves the problem on its own.

A render coat in poor condition can also let water in behind itself, and in older stacks the brick face may be breaking up. Each fault is photographed on your own stack, and the written quote, one total price, comes once the survey is done. Other work on the same stack is gathered on our chimney repairs page. The aim is to find the way water enters and fix that, not to seal the whole stack and hope.

4Condensation, salts and the flue below

If the roof end is sound, the cause is usually inside. A flue capped solid at the top and blocked at the fireplace holds damp air, and the cure is air movement: a ventilated cap and a vent in the room. We cover this in should you cap a disused chimney.

Salts need the source dealt with first. Once the stack is dry and the flue breathes, the stained plaster can be cut back and replastered with a mix that does not trap moisture. Replastering before the source is found simply brings the stain back through the new finish, often within a season, and a coat of stain-block paint hides it for a while without curing it.

This part of the job often belongs to a plasterer or a damp specialist. If the roof end is sound, we tell you that plainly, rather than offering a roof repair that cannot reach the problem. Letting the wall dry for a few weeks before judging the result is sensible.

5Do not take the breast out to cure the damp

Removing a chimney breast to get rid of a damp problem is a poor trade. Building Regulations approval is always needed, since the breast forms part of the structure, and a party wall notice follows if it stands on a party wall. Very often the damp is a roof or flue fault that a repair would have ended at a fraction of the cost. Before you commit to anything larger, read whether you need permission to remove a chimney stack and think about what you would be taking on.

Fix the cause, let the wall dry, then decorate. Drying can take weeks or months in a solid brick breast, so do not judge the repair by how the wall looks a week later. A moisture meter helps over time: readings that fall steadily show the repair has worked, even while the surface still looks stained.