Common problem
How do I know if it's penetrating damp?
Penetrating damp is water tracking horizontally through the wall from outside — via failed pointing, cracked render, blocked cills, faulty flashing, or driving rain on an exposed elevation. It shows on the inside as a defined stain on one small area, worsens 12–48 hours after rain, and — critically — is paired with a specific defect on the exterior of the same wall.
Last updated Reviewed by Vincent Pamenter
What it looks like
Illustrative scenes, not photographs of a real case.
Below-window pattern: often traced back to failed cill or open bed-joint.
- Patch shape follows a horizontal line under the window
- Discolouration worsens 24h after heavy rain
- Wall stays cold long after the room warms up
Driven-rain elevation: south-westerly exposure and worn render leave a signature stain.
- Damp on prevailing-wind elevation only
- Correlates with wind-driven rain, not still rain
- Adjacent room on same wall stays dry
Above-window pattern: think lintel, DPC tray, or faulty flashing above the frame.
- Stain confined above the window head
- No stain below the cill — rules out sill
- Water tracks down from a defined start point
What causes it
Penetrating damp always has a matching exterior defect. Rank the four most common by how often they show up in UK damp surveys.
- #1
Failed pointing or cracked render
Definition. Open bed-joints in brickwork or crazed and detached render allow driven rain into the wall build-up, especially on the prevailing-wind elevation.
How to tell. Externally, visible gaps or crumbling mortar; internally, damp on the same elevation, worsening within 24–48 hours of wind-driven rain.
- #2
Blocked or defective window cill
Definition. A cill without a drip detail, with cracked mortar under it, or overhanging debris, funnels water into the masonry beneath the window rather than clear of the wall.
How to tell. Damp patch appears below the window, roughly the width of the cill; the underside of the cill is dirty-white or crumbling; splash marks on the external face below.
- #3
Faulty flashing or lintel water-shedding
Definition. Missing or corroded lead flashing at chimneys, abutments and window heads lets rainwater into the wall behind the lintel, showing as a stain above the window or beside a chimney breast.
How to tell. Damp is above a specific opening, with water tracking down from a defined point, and the exterior flashing is discoloured, lifted or missing.
- #4
Overflowing gutter, downpipe or gulley
Definition. Rainwater goods that leak, overshoot or block up saturate the wall repeatedly at one point, in the same pattern regardless of wind direction.
How to tell. During heavy rain, a visible cascade or splash at the affected point; a vertical stripe stain externally; a persistent damp patch inside at that height.
Symptom vs likely cause
Every penetrating-damp pattern has an exterior twin. Pair the interior clue with what you find outside on the same wall.
Damp under a window
- Most likely cause
- Cill defect or open bed-joint
- Confidence
- High
Damp above a window or door
- Most likely cause
- Flashing failure at lintel
- Confidence
- High
Damp beside a chimney breast
- Most likely cause
- Chimney flashing or flaunching defect
- Confidence
- High
Vertical damp stripe from ceiling down
- Most likely cause
- Leaking gutter or overshooting downpipe
- Confidence
- High
Whole wall damp on prevailing-wind elevation
- Most likely cause
- Render failure / porous brickwork
- Confidence
- Medium
Damp near external ground level
- Most likely cause
- Bridged DPC or ground-level defect
- Confidence
- Medium
Is it serious?
Penetrating damp is usually gradual, so the risk is cumulative. Left through a UK winter, it saturates plaster, corrodes wall ties and rots timber lintels and floor joists that meet the wall. Where the damp reaches insulation in a cavity or a warm roof, energy performance falls and mould establishes deep in the build-up. There is no medical emergency in the early stages, but the fabric damage compounds fast — a year of untreated penetrating damp typically doubles the cost of the eventual repair.
Health context: Damp and mould in the home (NHS) — NHS.
This is general information, not medical advice. If you're worried about your health, contact your GP or NHS 111.
What to do first
The key with penetrating damp is to look outside on a rainy day. These six actions map the interior patch to its exterior cause without any tools beyond a phone and a torch.
- 1
Photograph the interior stain with measurements
Wide shot for context, a close-up of the edge, and a tape-measure reading of the maximum height and width. Repeat after the next heavy rain — a growing patch is a live defect.
- 2
Photograph the exterior of the same wall
Match the interior patch to its external position. Zoom in on pointing, render, cill, lintel and any flashing above. This paired image set is what makes a survey conclusive.
- 3
Watch a heavy-rain event from outside if possible
Water paths become obvious in the rain: a leaking gutter, an overshooting downpipe, or a cill that funnels water toward the wall. Two minutes of observation replaces an hour of guesswork.
- 4
Check gutters and downpipes for blockage
Debris, moss and displaced brackets cause more penetrating damp than any other single defect. If access is safe, clear or have someone else clear them before commissioning any wall repair.
- 5
Check the DPC line externally
The DPC (usually a slate or plastic band) should sit at least 150mm above external ground. If soil, decking or path have been raised above it, water enters horizontally at that level and is often misdiagnosed as rising damp.
- 6
Get a repair quote before decorating
Never re-plaster or paint over penetrating damp while the source is live. The plaster will fail again in months, and you lose the evidence trail. Fix the exterior first, let the wall dry for weeks, then redecorate.
What NOT to do
Penetrating damp is a fabric problem. Treating it as a decorating problem is the single most common — and most expensive — mistake.
Myth
Waterproof the outside wall with sealant paint.
Why it's wrong. Sealer-paint traps moisture in the masonry, preventing evaporation, and pushes the damp deeper into the wall build-up. Solid-brick and stone walls need to breathe. Fixing pointing and render is the correct route.
Myth
Line the wall internally with foil or plastic.
Why it's wrong. Internal vapour barriers over a damp masonry wall create textbook interstitial condensation — the wet zone moves out of sight and rots hidden timber and plaster.
Myth
It's just weather; leave it and it'll dry.
Why it's wrong. In a UK climate, penetrating damp saturates faster than the wall dries between rain events. The patch grows year on year and quietly damages the structure until it becomes a much bigger repair.
Myth
Cover it up with silicone injection or chemical DPC.
Why it's wrong. Rising-damp treatments do nothing for horizontal water entry. If the diagnosis is penetrating damp, no chemical inside the wall will stop water arriving from outside.
When you need evidence or a survey
Penetrating damp cases are won and lost on paired photos: the interior stain, and the specific exterior defect that caused it. A structured free Case Check brings both sides together in one dated report, quotes the relevant duty (fitness for human habitation, HHSRS, Awaab's Law working days), and gives a landlord, freeholder or insurer a clear route to repair — rather than the usual sequence of unhelpful decorator visits.
This is general information, not legal advice. For advice on your specific situation, contact Shelter, Citizens Advice, or a solicitor.
Common questions people ask
How can I tell penetrating damp from rising damp?
Penetrating damp appears as a defined patch on a specific part of one wall, tied to an external defect above ground, and worsens after rain. Rising damp presents as a horizontal tide mark up to about one metre from the floor, with salt deposits, on a ground-floor wall, consistent through the year. Penetrating is by far the more common of the two in modern housing.
What causes penetrating damp on external walls?
The four most common causes are failed pointing or cracked render, defective window cills, faulty flashing at chimneys or lintels, and blocked or leaking gutters and downpipes. In older solid-wall homes, driven rain on the prevailing-wind elevation is also common.
Does penetrating damp always appear after rain?
Yes, usually within 12–48 hours of wind-driven or heavy rain, and it dries out slowly in between. If damp is present in dry spells and unchanged by weather, penetrating damp is not the most likely cause — check for plumbing leaks or bridged DPCs instead.
Can I paint over penetrating damp with waterproof paint?
Externally, no — waterproof coatings on breathable masonry trap moisture and worsen the problem. Internally, no — painting a damp wall causes the new paint film to blister and lift within a season. Fix the source outside first, let the wall dry, then repaint.
Is penetrating damp expensive to fix?
It depends entirely on the cause. Repointing a small area or resealing a cill is a small job; replacing render, reflashing a chimney or repairing a roof line is a large one. Get written quotes for the specific repair rather than relying on a typical figure. In a rental, the cost of the repair is the landlord's, not yours.
Can penetrating damp affect the value of my house?
Yes, particularly on a survey. Buyers' surveyors record damp readings on the interior of external walls; unresolved penetrating damp can trigger a retention, price reduction or an insurance requirement to obtain a specialist damp report before completion.
Will insurance cover penetrating damp repair?
Not usually — standard buildings insurance excludes gradual damp and maintenance defects. A one-off storm event that damages roof or render may be covered under storm damage; a slow-developing pointing failure is not. Photograph and date the damage from the first sign, in case of dispute.
Found mould? Don't just photograph it.
Upload the right photos, get a clear first assessment, and organise your evidence, report, messages and follow-up in one place.
Free first checkPrivate by defaultIndependent of remediation sales
Keep reading
Related problem
Rising damp signs
Related problem
Leak-related damp
Related problem
Damp patch on the ceiling
Cost guide
Damp survey cost UK