Damp patch on a wall: which of four causes is yours
Photograph it, then check it again after heavy rain and after a dry week. How it changes over time identifies the cause more reliably than how it looks today.
A defined edge that darkens after rain is penetrating damp. A level band low on a ground-floor wall is rising damp or a bridged damp course. Diffuse speckling in a corner is condensation. Constant regardless of weather is a leak.
This is the page to be careful on, because it is where money gets wasted. The damp proofing industry sells one answer, and a large share of the patches it is sold for are condensation — which costs nothing to fix.
Spend an afternoon on the diagnosis. It is mostly observation, you can do it yourself, and it decides whether you are spending nothing or several thousand dollars.
The four causes and how they announce themselves
| What you see | What it means |
|---|---|
| Diffuse, no clear edge, in a corner or behind furniture, worst in winter mornings | Condensation. Free to fix: ventilation, heat and moving furniture. Confirm with the mold risk calculator. |
| Defined edge, at any height, clearly worse during and after rain | Penetrating damp. Go outside and look at that exact spot — cracked render, a failed sealant joint, a blocked gutter, missing flashing. Usually visible from the ground and cheap to repair. |
| Level band under about 3 feet on a ground-floor wall, sometimes with a tide mark and white salt deposits | Rising damp, or a bridged damp course. Check whether a patio, path, flower bed or added render sits above the damp course line — that is far more common and much cheaper to fix than injection. |
| Constant regardless of weather, growing over time, sometimes warm to the touch | A leak. Read your water meter with everything off. A warm patch suggests a heating pipe. |
What to do, in order
Free actions first. Do not buy anything until these have had two weeks.
Today
Photograph the patch with something for scale, and write today's date on the photo.
Read your water meter with every tap and appliance off. Wait an hour and read it again — any movement means a leak, and that changes everything.
Feel the patch. Cold and clammy suggests condensation; warm suggests a heating pipe.
Walk the outside of the wall. Gutters, downspouts, ground level, cracked render, sealant around windows, and the height of any patio or flower bed against the wall.
This week
Photograph it again after the next heavy rain, and again after several dry days. The change over time is your strongest evidence.
Clear the gutters and extend downspouts 4–6 feet from the wall. This is cheap and it solves a surprising share of penetrating damp.
Lower any soil, mulch or paving that sits within 6 inches of the damp course.
If the pattern says condensation, work through ventilation and insulation and heating before spending anything.
If it has not improved
Repair the external defect first if there is one. Render cracks, failed pointing, flashing and sealant are ordinary maintenance and much cheaper than any damp treatment.
For a genuine rising damp diagnosis, pay an independent surveyor — one who does not also sell damp proofing. It is the cheapest insurance available against a five-figure repair for the wrong problem.
Do not repaint until the repair has been through a wet spell. The patch is your only indicator of whether the fix worked.
Expect a wall to take weeks to months to dry after the source is fixed. A masonry wall holds a lot of water.
What it becomes if left
A persistently damp wall grows mold, and that is where the health impact comes from. It also damages plaster, rots joist ends and skirting, and in cold weather the moisture in the masonry substantially reduces the wall's insulating value — so the room costs more to heat and the surface gets colder, which produces more condensation.
The failure mode is a loop that accelerates. Once you have identified the cause, fixing it sooner is meaningfully cheaper than fixing it later.
Common questions
How do I tell rising damp from condensation?
Rising damp stops at a consistent height under about 3 feet, often with a horizontal tide mark and salt deposits, and only on ground-floor walls. Condensation is diffuse, concentrates in corners and behind furniture, is worst on winter mornings, and improves with ventilation and heat.
My damp patch gets worse when it rains — what is it?
Penetrating damp. Water is coming through the wall from outside. Look at the exterior at that exact point for a cracked gutter, blocked downspout, failed sealant, damaged render or missing flashing.
How long does a damp wall take to dry out?
Weeks to several months after the source is fixed, depending on wall thickness and how long it was wet. Masonry holds a lot of water and releases it slowly.
Should I get a free damp survey?
Not from a company that sells damp proofing — the incentive is obvious. Pay for an independent surveyor with no stake in the treatment.