How to Fix Damp: A Homeowner's Guide

What is damp?

Damp simply means unwanted moisture in the fabric of your home — walls, floors, ceilings or timber. Left alone it causes black mould, rotting skirting, peeling wallpaper, crumbling plaster and, eventually, structural damage. It can also affect your health: damp and mould are linked to asthma, chest infections and allergies, especially in children and older people. The key to fixing it is working out where the water is coming from, because the cure depends on the cause.

The three types of damp

Almost every damp problem falls into one of three categories. Each looks different and needs a different fix:

• •Condensation — moisture from cooking, washing and breathing condenses on cold surfaces. By far the most common cause of black mould in UK homes.
• •Penetrating damp — water gets in from outside through a building defect (leaky gutter, cracked render, failed pointing, missing tile). Patches appear after rain.
• •Rising damp — groundwater creeps up through a failed or bridged damp-proof course, leaving a low tide mark on ground-floor walls. The rarest of the three.

How to tell which one you have

Use these clues to diagnose the damp before spending any money on a fix.

• ☐Is there black mould in corners, behind furniture or around window frames? → Condensation.
• ☐Do patches appear only after heavy rain and feel wet to the touch? → Penetrating damp.
• ☐Is there a crisp "tide line" up to about 1m high on a ground-floor wall, with salts on the surface? → Rising damp (but get it confirmed — it is often misdiagnosed).
• ☐Is the damp on an upstairs ceiling under a loft or flat roof? → Penetrating from above.
• ☐Is it on the wall next to a kitchen or bathroom? → Likely condensation or a leaky pipe.

Fixing condensation damp

Condensation is a moisture-and-ventilation problem, not a building defect. The fix is to produce less moisture and let what you do produce escape.

• •Use extractor fans in kitchens and bathrooms — and keep them clean and running for 20 minutes after.
• •Dry clothes outdoors or in a vented/condenser tumble drier, never on radiators.
• •Cover pans when cooking and close kitchen/bathroom doors.
• •Open trickle vents on windows and open a few windows for 10 minutes a day.
• •Keep heating on low and steady rather than blasting it on and off — warm walls don't condense.
• •Keep furniture 50–100mm away from external walls so air can circulate.
• •If the whole house is affected, consider a positive-input ventilation (PIV) unit.

Fixing penetrating damp

Penetrating damp is always fixed from the outside. The damp patch inside is the symptom, not the problem. Get a builder or handyman to find where water is getting in before you treat the inside.

• •Check gutters and downpipes for leaks, cracks and overflow during heavy rain.
• •Look for cracked render, blown pebbledash and failed pointing.
• •Check flashing around chimneys, vents and roof joins.
• •Inspect window sills, seals and door frames for gaps.
• •Check that the ground level outside is not above the damp-proof course.

Fixing rising damp

True rising damp is relatively rare. It happens when the damp-proof course (DPC) has failed or been "bridged" — for example, by a path or floor built too high outside. Before agreeing to expensive injection treatments, get a proper diagnosis from an independent surveyor (not the company selling the treatment). Many "rising damp" cases turn out to be condensation or penetrating damp.

• •Check that external ground levels are at least 150mm below the DPC.
• •Remove bridging (high paths, raised flower beds, plaster touching the floor).
• •Get a second opinion before installing a new chemical DPC.
• •After fixing, replaster with a renovating plaster, not standard gypsum.

How to safely remove black mould

Once the moisture source is fixed, mould can be cleaned off. Wear gloves and a mask, open windows, and wipe down with a fungicidal mould spray (available from DIY stores). Don't dry-scrape mould — it releases spores. Wash affected fabrics at 60°C. If mould returns within a few weeks, the moisture source has not been properly fixed.

When to call a professional

Many damp issues are straightforward, but some need a specialist. Call a tradesperson if:

• •The patch is wet after days of dry weather (suspect a plumbing leak — call a plumber).
• •Mould keeps coming back despite better ventilation (surveyor or specialist).
• •The wall is bulging, plaster is blowing off, or timbers feel soft (builder / surveyor).
• •A diagnosis says "rising damp" and recommends injection (get an independent second opinion).