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Damp and mould in the house: how to find the cause

Damp is always water from somewhere: the ground, the rain, a pipe or the air. Paint and damp sealers applied to the wrong problem waste money and come off again. Find where the water comes from before you treat the wall.

Three kinds of damp

Rising damp

• Where: the bottom of walls, up to about a metre.
• When: all year, worse after rain.
• Signs: a tide mark, peeling paint, white salts, crumbling plaster and rotting skirtings.
• Common causes: soil, paving or a flowerbed piled above the damp-proof course (DPC), a damaged DPC, or a leaking pipe in the floor.

Penetrating damp

• Where: anywhere, often high on walls, on ceilings, around windows and below gutters.
• When: after rain.
• Signs: patches that grow after rain, ceiling stains and bubbling paint.
• Common causes: blocked or leaking gutters, cracked tiles or flashings, cracks in outside plaster and failed window seals.

Condensation

• Where: cold surfaces such as outside walls, ceiling corners and windows, and behind furniture.
• When: in winter, in the mornings, and after showers and cooking.
• Signs: water droplets, black mould spots and a musty smell, with no tide mark.
• Common causes: steam from showers and cooking, washing dried indoors, paraffin and gas heaters, and poor ventilation.

Ask where the water is coming from

• Look outside the wall: soil or paving above floor level, a dripping tap, a downpipe emptying against the wall, cracks, or plants growing against it.
• Look above: gutters, roof tiles, flashings, or a bathroom upstairs.
• Check the water meter for a hidden leak.
• Dry a patch of wall. If the surface is wet again within hours, it is often condensation.

What you can fix yourself

• Lower soil, paving and flowerbeds to at least 150 mm below the DPC, sloping away from the walls.
• Clear the gutters and make sure that downpipes discharge away from the walls.
• Fix dripping taps and move sprinklers away from walls.
• Fill cracks in outside plaster and renew the sealant around window frames.
• Let walls dry before you repaint, and use a breathable paint.

Many cases of "rising damp" are simply soil or paving piled too high. Look outside first.

When to call someone

• Rising damp that persists after the ground has been lowered: a damp-proofing contractor. Get two quotes and a written guarantee.
• Roof leaks and flashings: a roofer or waterproofing contractor.
• Hidden leaks in walls or floors: a plumber.

Cleaning mould safely

Mould can trigger asthma and allergies.

• Wear gloves, safety glasses and a dust mask, and open the window.
• Use a mould treatment as its label directs.
• Never mix bleach with acid or ammonia cleaners. They give off poisonous gas.
• Do not dry-brush mould, because that spreads the spores. Wipe, let the treatment work, rinse and dry.

Preventing condensation and mould

• Open windows daily, even briefly in winter, and after every shower.
• Use extractor fans and cooker hoods that vent outside.
• Put lids on pots and dry washing outside.
• Avoid paraffin and gas heaters in closed rooms.
• Leave a gap of a few centimetres between furniture and outside walls.

This guide is based on Chapter 5 of DIY for the New Homeowner, which also explains how to read, monitor and repair cracks.