Damp on a Chimney Breast: Causes and Fixes

Rung 1: Work out which kind it is

Look at the pattern and the timing.

• Brown stains and powdery salts: moisture carrying soluble salts, often from old soot and flue deposits.
• Damp worse after heavy rain: water getting in at the pot, flaunching, flashing or stack brickwork.
• Damp in the cold months only, with mould on paint: condensation on a cold wall.
• Wet patch low down at the base: check for ground-level or plumbing causes, not just the flue.

Rung 2: Look at the stack from outside

From the ground or, safely, from a ladder or with binoculars, check for cracked pot, loose flaunching (the mortar around the pot), failed lead flashing where the stack meets the roof, and missing or sagging pointing. A chimney pot with no cap lets rain run down the flue; a cap that seals it completely leaves the flue unventilated. The usual remedy is a vented cap.

Rung 3: Ventilate or reopen the flue

If a fireplace has been boarded up, the flue can hold damp air and sweat on cold brickwork. Fit a ventilator in the fireplace opening, or have the flue properly swept and vented. A chimney that is no longer used still needs air, or the damp will show on the breast inside.

Rung 4: Repair, then treat the wall

Repoint and re-flaunch the stack with a mortar that suits old brick, usually lime-based on older buildings. Re-lay lead flashings. Once the water is stopped, let the wall dry before replastering. Do not seal the inside with waterproof paint, because it traps moisture.

If condensation was the cause, a loft-mounted dehumidifier is one option for owner-occupied homes with a loft; it does not fix rain entry or a failed stack.

Mistakes that make chimney damp worse

• Sealing the pot completely with a solid cap and leaving the flue with no ventilation.
• Filling cracks in the stack with hard cement, which can trap water in old brick.
• Replastering over salt-stained plaster before the wall has dried.
• Painting the chimney breast with waterproof or masonry paint inside the house.
• Ignoring the roof junction and treating only the inside face.

In Chatham and the Medway Towns

Medway's older terraces were built in brick under clay tile or slate, and Medway Council's conservation area appraisals note alterations such as render and replaced roof coverings that can change how stacks shed water. Rochester's historic centre has many chimneys on timber-framed and brick buildings; see Rochester, and Brompton for terraces where stacks are shared along the row.

Questions we are asked

Related guides

• Rising, Penetrating or Condensation Damp? How to Tell
• Victorian terraces vs new builds
• Black Mould in Bedroom Corners: Find the Cause
• Leaking Gutters and Downpipes: Damp Walls Explained

Local help

• Damp, mould and condensation help in Rochester
• Damp, mould and condensation help in Brompton
• Damp, mould and condensation help in Gillingham

Sources: Medway Council: Historic Rochester Conservation Area Appraisal (2010); Medway Council: Brompton Lines Conservation Area Appraisal; GOV.UK: Understanding and addressing the health risks of damp and mould in the home; Property Care Association: Condensation in homes.