Quick answer: To stop damp coming through walls, first find the cause: condensation, penetrating damp (water from outside) or rising damp (moisture from the ground). Then fix that cause. Repair gutters, cracks and pointing, improve ventilation, or install a damp proof course. Only then replaster and redecorate. Treating the surface alone will not work. The damp returns.
Written by Micken Patel, CSRT/PCA(QT), Lead Surveyor. Last updated 7 October 2026.
What causes damp to come through a wall?
Three things cause most damp walls in UK homes: condensation, penetrating damp and rising damp. A leaking pipe or a bridged cavity can also be the cause. The fix depends on which one you have, so diagnose first.
| Type | Where it shows | Typical signs | Gets worse |
|---|---|---|---|
| Condensation | Cold external walls, corners, behind furniture, bedrooms | Black mould, wet windows, musty smell, no tide mark | Winter, cold mornings |
| Penetrating damp | Anywhere on an external wall, often below windows or gutters | Patches with defined edges, blistering paint, stains | After heavy rain |
| Rising damp | Ground floor walls, up to about 1 m high | Tide mark, salts, peeling wallpaper, damaged skirting | Constant, slightly worse in wet weather |
| Plumbing leak | Near pipes, radiators, baths, sinks | Localised wet patch, even in dry weather | Whenever the system is in use |
How do you tell which type of damp you have?
Three simple checks narrow it down before you call anyone.
- Foil test: tape a 30 cm square of kitchen foil to the wall and leave it for 24 hours. Moisture on the room side of the foil points to condensation. Moisture under the foil points to damp in the wall (penetrating or rising).
- Rain check: if the patch grows after heavy rain, suspect penetrating damp.
- Height check: a tide mark on a ground floor wall, below about 1 m, suggests rising damp.
Read more in our guide to rising damp vs penetrating damp. A damp survey confirms the cause with calibrated instruments.
How do you stop damp coming through walls? Step by step
Follow these five steps in order. Skipping to repainting or damp proof paint is the most common mistake.
1. Identify the cause
Use the table and tests above. Check outside too: blocked gutters, cracked render, failed pointing, a bridged cavity or soil piled above the damp proof course (DPC).
2. Fix the source
Repair gutters and downpipes, seal cracks, repoint, and clear anything bridging the DPC. For condensation, fix ventilation and humidity. For rising damp, install a new DPC. Water must stop entering the wall before any internal work.
3. Let the wall dry
Solid masonry dries slowly, often over several months. Keep rooms heated and ventilated. Do not seal a wet wall.
4. Treat the internal surface
Remove damaged plaster, treat salt contamination and mould, then replaster with a suitable system. See the internal treatment section below.
5. Redecorate and monitor
Use a breathable paint once the wall is dry. Recheck with a moisture meter after the first heating season.
How do you stop damp coming through external walls?
Stop rain getting into the masonry. Penetrating damp is almost always caused by a defect on the outside, so the repair belongs there.
- Clear and repair gutters, hoppers and downpipes. A leaking joint can soak one patch of wall for years.
- Repoint eroded mortar and fill cracks in brickwork or render.
- Repair or replace cracked render, pebbledash and failed flashings.
- Reseal around window and door frames, and check sills and lintels.
- Keep external ground level at least 150 mm below the DPC, and keep paths and planters off the wall base.
- Cut back ivy and large shrubs close to the wall.
Should you waterproof an exterior wall?
Sometimes. A breathable water repellent (silane or siloxane masonry cream) can help sound, exposed brickwork after repairs. Do not use cement render or acrylic sealants on older solid walls. They trap moisture and make damp worse. Our penetrating damp treatment service covers the surveying and repair. See also what is penetrating damp.
How do you stop rising damp in walls?
Install a new damp proof course, then replaster. Rising damp happens when the original DPC has failed or was never fitted, which is common in pre-1930s homes. It is also over-diagnosed, so confirm it before paying for treatment.
- Chemical DPC injection: a silicone cream is injected into the mortar course to form a barrier.
- Replastering: contaminated plaster is removed, typically to about 1 m, and replaced with a salt-resistant system.
- Ground levels: lower external ground to at least 150 mm below the DPC.
Compare the options in damp proof injection vs membrane, or see our rising damp treatment service.
How do you stop condensation damp on bedroom and internal walls?
Lower the moisture in the air and keep wall surfaces warmer. Condensation is the most common cause of damp on internal walls, especially in bedrooms on cold external walls.
- Aim for 40 to 60% relative humidity, measured with a cheap hygrometer.
- Fit and use extractor fans in kitchens and bathrooms. Run them during use and for 15 to 20 minutes after.
- Dry clothes outdoors or in a vented room. Use a dehumidifier if you must dry indoors.
- Keep furniture 5 to 10 cm off external walls so air can move.
- Heat rooms evenly and keep a low background temperature. Cold rooms collect condensation.
- Consider a positive input ventilation (PIV) unit if condensation is persistent.
Learn more in what causes condensation, or see our condensation control service. For year-round prevention, read how to prevent damp and mould.
How do you treat damp walls internally?
Once the source is fixed and the wall has dried, remove damaged finishes and rebuild with breathable materials. Internal treatment repairs the damage. It does not stop the cause.
- Strip and inspect: remove wallpaper, loose paint and blown plaster. Treat any mould.
- Replaster: use a salt-resistant or renovating plaster system. Ordinary gypsum plaster does not cope with damp and salts.
- Membranes: a cavity drain membrane can manage moisture where the wall cannot be dried, such as in basements.
- Paint last: damp proof paint is a cosmetic finish, not a cure. Never apply it over salt-contaminated or damp plaster.
Painting a damp wall before it is dry causes blistering and peeling. For a detailed method, see how to treat damp walls, damp proofing a wall before plastering and our wall replastering service.
What are the damp proofing methods and what do they cost in the UK?
Costs depend on wall length, access, property type and region. London is usually at the upper end. The figures below are indicative 2026 UK market ranges, excluding VAT. A survey gives an accurate quote.
| Method | Tackles | Typical UK cost (2026) |
|---|---|---|
| Chemical DPC injection | Rising damp | £40 to £120 per linear metre (injection only) |
| Replastering after treatment | Salt-damaged plaster | £40 to £80 per m² |
| Cavity drain membrane (internal) | Persistent moisture, basements | £50 to £80 per m² |
| Extractor fan, installed | Condensation | £100 to £250 per unit |
| Gutter, pointing and render repairs | Penetrating damp | Priced after inspection |
What do moisture meter readings mean on a wall?
A reading above about 20% on a standard damp meter suggests damp. Most handheld meters are calibrated for timber, so masonry readings are indicative only. Salts can push readings up, and different meters use different scales.
| Reading (WME %) | Likely condition | What to do |
|---|---|---|
| Below 16% | Dry | No action |
| 16 to 20% | Borderline | Check ventilation, retest in a week |
| 20 to 28% | Damp | Find the source and treat it |
| Above 28% | Saturated | Book a professional survey |
A single reading never diagnoses damp. A surveyor compares readings with the wall construction, humidity and visible signs. See our full damp meter reading chart.
Can you stop damp coming through walls yourself?
Yes for minor condensation and simple external repairs. No for rising damp, recurring penetrating damp or widespread mould.
- DIY is realistic: clearing gutters, improving ventilation, using a dehumidifier, filling small cracks.
- Call a surveyor: tide marks, salts, damp that returns after repairs, mould in several rooms, or damp affecting health.
Choose a surveyor with a CSRT or CSTDB qualification and look for Property Care Association (PCA) membership. Our surveys are carried out by Micken Patel, CSRT/PCA(QT), with 15+ years’ experience diagnosing damp in London homes. See DIY vs professional damp proofing.
What do UK rules say about damp walls?
Building regulations require a DPC at least 150 mm above external ground level (Approved Document C). See our building regulations guide.
For rented homes in England, government guidance says landlords must address the underlying causes of damp and mould, not just clean the mould. Source: GOV.UK: Damp and mould guidance for rented housing providers.
Serious damp can be a hazard under the Housing Health and Safety Rating System (HHSRS). Awaab’s Law sets fixed investigation and repair timescales for social landlords in England, and is being extended to private landlords. Check current requirements on GOV.UK: Awaab’s Law. Tenants and landlords can read our advice for landlords and tenants.
How to stop damp coming through walls: FAQs
How do you permanently fix damp walls?
Fix the cause, then repair the damage. That means repairing leaks or masonry, improving ventilation, or installing a DPC, then replastering with a suitable system. Surface treatments alone are not permanent.
Why is my wall damp only when it rains?
That points to penetrating damp. Water enters through cracked render, failed pointing, blocked gutters, leaking downpipes or poor window seals.
Does damp proof paint work?
Not as a cure. It can seal a dry, clean wall and cover stains, but it does not stop moisture entering the wall. It fails over salts and damp plaster.
Can you paint over a damp wall?
No. Find and fix the cause first, let the wall dry fully, treat mould and salts, then use a suitable primer and breathable paint.
How much does it cost to stop damp coming through a wall?
Chemical DPC injection is typically £40 to £120 per linear metre in the UK, plus £40 to £80 per m² to replaster. Penetrating damp costs depend on the repair needed. A survey gives an accurate quote.
Can damp be fixed without removing plaster?
Only in mild cases, such as condensation. Plaster contaminated by salts from rising damp normally has to be removed and replaced.
How do you stop damp on bedroom walls?
Reduce humidity, ventilate, heat the room evenly and keep furniture off external walls. Persistent cases may need a PIV unit or insulation of the cold wall.
Is damp coming through walls covered by home insurance?
Usually not for gradual damp or wear and tear. Buildings insurance may cover sudden damage such as a burst pipe. Check your policy wording.
Who is responsible for damp in a rented home?
In England, the landlord is responsible for damp caused by structural defects, leaks and inadequate ventilation. Tenants should not be blamed for such causes.
Is damp in walls dangerous?
It can be. Damp and mould can affect health, particularly for children, older people and anyone with asthma or allergies. It can also damage plaster, timber and decoration.
Stop damp for good
Get the cause diagnosed by a qualified surveyor and a lasting fix, not a quick cover-up.
Call 020 4542 6114