Fix damp around a window reveal

Diagnose and fix damp around a window reveal, treat mould, reseal externally, and stop it coming back.

DW-01Work out what type of damp you haveDamp around a window reveal is usually condensation, but it can be penetrating damp getting in from outside. Black speckled mould with no obvious wet patch is usually condensation. A stained patch that grows after heavy rain, follows the top of the frame, or feels damp to the touch points to penetrating damp. Check whether marks get worse after rain — if so, start outside, not inside.

1 each · Torch

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DW-02Check the outside of the window firstLook at the brickwork, render and window frame from outside for cracked pointing, damaged render, blocked weep holes, or a leaking gutter dripping onto the wall above. If you need a ladder, stand it on firm level ground, get someone to foot it, and never overreach. A fall from a ladder is one of the most common DIY injuries.

DW-03Work out what's behind the revealSolid brick or block reveals (common pre-1930s) feel cold and are the classic case for condensation, because there's little insulation. Dot-and-dab plasterboard reveals can trap condensation behind the board, so a soft or bulging patch, or mould that keeps returning after cleaning, means moisture is stuck behind the board, not just on the surface. Lath and plaster reveals crumble if they stay damp too long — if the plaster feels soft or sounds hollow when tapped, get a plasterer to check it rather than just painting over it. Timber or metal stud reveals, often found around dormer or loft conversion windows, show damp when insulation or the vapour barrier behind the board has failed, and repeat mould after redecorating usually means the board needs opening up to check behind it.

DW-04Protect yourself and the roomMould spores are unpleasant to breathe in, so open a window, wear disposable gloves, and use an FFP2 dust mask while you clean. Lay a dust sheet under the window to catch drips and scrapings. Keep children and pets out of the room while you work.

1 pack · Disposable gloves (pack)

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1 pack · FFP2 dust mask (pack)

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1 each · Dust sheet

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DW-05Treat the mouldSpray a fungicidal wash directly onto the mould and leave it for the time stated on the bottle, usually around 4 hours. Wipe off with a damp cloth without scrubbing hard, as scrubbing spreads spores. Let the area dry fully before doing anything else — this can take up to 24 hours.

1 bottle · Fungicidal mould wash

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1 each · Cleaning cloth

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DW-06Rake out and renew the external sealantFrom outside, dig out any cracked or missing sealant between the window frame and the brickwork with a scraper. Brush the gap clean and dry so the new sealant sticks properly. Apply a bead of exterior-grade frame sealant with a caulking gun, then smooth it with a wet finger for a neat, weatherproof line.

1 each · Scraper

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1 tube · Exterior-grade frame sealant

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1 each · Caulking gun

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DW-07Fill and repair the plaster reveal insideRake out any crumbling or loose plaster with a filling knife until you reach a firm, solid edge. Fill with multi-purpose filler, slightly overfilling as it shrinks a little while drying. Once dry, sand smooth with fine sandpaper. If the damaged area is larger than a coin, or the plaster keeps crumbling, get a plasterer in rather than pushing on with filler.

1 each · Filling knife

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1 tub · Multi-purpose filler (small tub)

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1 pack · Fine sandpaper

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DW-08Prime the repair with a stain-blocking, anti-mould primerApply a stain-blocking primer over the repaired area and slightly beyond it, so old marks don't bleed through the new paint. Let it dry fully — check the tin, but this is usually 1-2 hours.

1 tin · Stain-blocking anti-mould primer (250ml)

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1 each · Small paintbrush

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DW-09Paint with anti-mould emulsionApply two coats of anti-mould emulsion, letting the first coat dry fully before the second. Cover the whole wall or reveal, not just the patch, so the finish matches and the anti-mould protection is even across the surface.

1 tin · Anti-mould emulsion paint (2.5L)

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1 set · Roller and tray set

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DW-10Reduce condensation long-termWipe condensation off the glass each morning, keep trickle vents open if the window has them, and use an extractor fan when cooking or showering. If you're fitting a new extractor fan wired into the mains, this is notifiable electrical work under Part P of the Building Regulations — use a registered electrician or check with your local council's building control before starting. A dehumidifier in the room can help if condensation keeps coming back despite good ventilation.

RULES

1 each · Bathroom extractor fan (optional)

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1 each · Compact dehumidifier (optional)

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KNOW THE RULES

Fitting a new mains-wired extractor fan is notifiable electrical work under Part P of the Building Regulations. Use a registered competent-person electrician, or get building control sign-off if you're not doing it yourself.

DW-11Tidy up and monitorWash brushes and tools, wipe down surfaces, and air the room out until the paint smell has gone. Check the reveal again after a week and after the first cold snap. If mould returns in the same spot, the moisture problem behind the wall hasn't been fixed and needs further investigation.

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