Chimney breast damp is usually caused by rain getting into the stack or flue, a capped chimney with no ventilation, condensation in the flue, or hygroscopic salts from years of burning fuel that pull moisture from the air. The fix depends on which one it is, so the stack and flue need inspecting as well as the plaster.
Chimney breasts get damp patches all the time, often with brown or yellowish staining that comes back through every coat of paint. On a survey I’m looking at three things: is water getting in, is the flue ventilated, and is the plaster full of salts?
Rain getting in
- Uncapped pots. Rain falls straight down the flue. In a disused chimney it soaks the brickwork inside.
- The stack itself. Eroded pointing, cracked render or a failed crown on top of the stack lets water in.
- Flashings. The lead or mortar where the stack meets the roof can fail and let water down the outside of the stack into the house.
Damp from these usually gets worse after rain and can show on the chimney breast upstairs, in the loft and on ceilings near the stack.
No ventilation
When fireplaces are blocked up, the flue needs a vent at the bottom and a ventilated cap at the top so air can move through and keep it dry. Seal it at both ends with no ventilation and moisture builds up inside the flue and comes through to the room.
Salts
Years of burning coal, wood or other fuels leave salts in the flue lining and brickwork. Some of these salts are hygroscopic, meaning they attract moisture from the air. So even when the chimney is no longer getting wet, the plaster can show damp patches on humid days, often with brown tar staining. This is very common in older Yorkshire terraces that burned coal for decades. Moisture meters read very high on these walls, which is another reason not to rely on a meter alone.
Putting it right
- Get the stack checked: pointing, crown, pots and flashing.
- Cap disused flues with a ventilated cap, and fit a vent in the chimney breast in the room.
- If there are salts, the contaminated plaster usually needs removing and replacing with a salt-resistant system, sometimes with a barrier on the brickwork, to stop the staining coming back through.
- If you’re using the fire, have the flue swept and inspected by a competent person, as a damaged liner can cause problems of its own.
Painting over chimney salts with stain block might work for a while, but the salts will usually find their way back.
When to call a specialist
If your chimney breast has damp patches or staining that keeps coming back, get it surveyed so the stack, flue and plaster can all be checked. Book a survey with All Dry on 01226 287181 or email info@alldrydampproofing.com.
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Frequently asked questions
Should a capped chimney be ventilated?
Yes, with a ventilated cap at the top and a vent in the room to keep air moving.
Why does brown staining come through on my chimney breast?
Usually salts and tar from old fires being carried to the surface by moisture.
Can a damp chimney breast be fixed without removing plaster?
If salts are the cause, the contaminated plaster usually needs replacing for a lasting fix.