Damp, mould or rotten timber? Get a straight answer.

Warm, moist air from cooking, washing and drying clothes settles on the coldest surfaces, and black mould grows where they stay wet.

Usual signs: peeling paint or wallpaper, black mould in corners or behind furniture and windows wet most mornings.

Worth knowing: In the meantime, extractor fans, open trickle vents and drying clothes outside where you can all help.

Worth knowing: Condensation can leave marks low on a cold wall too, so it is worth ruling out before any damp proofing.

Rain getting in from outside: through worn pointing, cracked render, a leaking gutter or round a window. It can show at any height, and often grows after wet weather.

Usual signs: crumbling plaster near the floor, a damp patch worse after rain, damp high up or on a ceiling, worn pointing or cracked render and a leaking gutter or downpipe.

Worth knowing: Damp high on a wall or on a ceiling is rarely rising damp. Water from above, such as a roof, gutter or pipe, is more likely.

Moisture from the ground soaking up the bottom of a wall, past a damp proof course (the barrier that should stop it) that has failed, been bridged, or was never there. It seldom climbs much above a metre.

Usual signs: tide marks low on a wall, crumbling plaster near the floor, white powdery salts on a wall and soil or paving high against a wall.

Worth knowing: Soil or paving above the damp proof course lets moisture straight past it. Lowering it can be the whole fix.

Worth knowing: The white deposits are salts carried up from the ground. They draw moisture from the air, so salty plaster usually needs replacing.

Timber decaying because it has stayed wet, usually from a leak, damp in a wall or poor ventilation under a floor. It stays where the wet is.

Usual signs: a soft or springy floor, dark, soft or crumbly timber and timber cracked into cubes.

Beetle larvae that tunnel through timber for years, then leave by small round holes. Fine, pale dust beneath the holes suggests they may still be active.

Usual signs: small holes in timber, with dust.

A fungus that can spread beyond the wet timber where it started, sending strands through brickwork to reach sound wood. White cotton-wool growth, rust-red dust and timber cracked into cubes are its signs.

Usual signs: a soft or springy floor, timber cracked into cubes, cotton-wool growth on timber and rust-red dust near timber.

A cavity wall is two layers of brick held together by metal ties. As old ties rust they swell, cracking the mortar every few rows, and the wall can start to bulge.

Usual signs: horizontal cracks in the mortar and bulging or leaning brickwork.

Insulation in a wall cavity can carry rain across to the inside wall, especially on walls that take driving rain, or where it has got wet or slumped.

Usual signs: damp cavity wall insulation.

A cellar sits below the ground outside, so ground moisture presses through its walls and floor. Most were never built to keep it out.

Usual signs: a damp or wet cellar.

A smell that lingers usually means damp somewhere out of sight: under a floor, behind fitted units, or in a cellar or roof space.

Usual signs: a musty smell that lingers.