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-proof 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 stucco, 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-proof course. Check whether a patio, path, flower bed or added stucco sits above the damp-proof 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. |
How to be sure it is this
A damp patch is a diagnosis rather than a symptom, and the four candidates need four different repairs costing between fifty and five thousand dollars. Spend an evening on these checks before you spend anything else.
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Does it change with the weather?
Darker for a day or two after heavy rain and fading between means water coming through the wall from outside. Indifferent from the weather and constant all year means a leak. Worst on cold mornings and better in mild weather means condensation.
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Where does it sit on the wall?
A horizontal band from the floor to about three feet, often with a tide line, suggests ground moisture. A patch at head height on a weather-facing elevation suggests penetration. Discoloration spreading downward from a ceiling or along a run suggests plumbing.
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What is directly outside it?
Go outside and stand at the same point. A downspout, a gutter joint, a cracked stucco panel, a missing piece of pointing, a flower bed piled above the damp-proof course, a patio raised over the years. The exterior cause is visible from the garden in a large share of cases.
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The plastic sheet test
Dry a patch of the wall thoroughly, tape a square of clear plastic tightly over it, and leave it overnight. Moisture on the room side of the plastic is condensation. Moisture trapped underneath, against the wall, is water arriving through the masonry.
What is your reading telling you?
- Dew point
- 53 °F
- Mold risk, cold walls
- Moderate
Still inside the accepted range, but this is where cold surfaces begin to sweat.
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 faucet 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 stucco, 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-proof course.
If the pattern says condensation, work through ventilation and insulation and heating before spending anything.
If it has not improved
Repair the exterior 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 happens if you leave it
A patch left alone does not stay the same size. Water spreads through plaster by capillary action, so the affected area grows outward from the source, and the edge of the patch is the honest indicator of whether anything you have done is working. Mark it in pencil with the date.
Within months, decoration fails: paint loses adhesion, wallpaper lifts at the seam, and salts brought out of the masonry crystallize at the surface as a white bloom. Redecorating at this stage without fixing the source produces exactly the same failure in the same place, usually within a year.
Given a year or two of sustained wetting, the damage is structural and the costs change scale. Plaster blows and has to come off back to masonry, timber in contact with the wet area — baseboard, joist ends, a stud wall, a lintel — begins to rot, and any insulation in the wall loses most of its value while it is wet. A gutter repair at two hundred dollars becomes replastering and joinery at several thousand.
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 baseboard, 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 stucco 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.
How long should a wall take to dry once the source is fixed?
Roughly a month per inch of thickness in good conditions, which for a typical solid wall means a season rather than two weeks. Heat and ventilation speed it up substantially; sealing the surface stops it entirely. Do not judge a repair, and do not replaster, until the wall has had a genuine drying period with the room warm and ventilated.
Is a moisture meter worth buying to diagnose this?
A cheap resistance meter is worth having as one input and worth nothing as a verdict. They respond to salts, to foil-backed drywall and to metal fixings as strongly as to water, which is why they routinely read alarmingly high on a dry wall that had damp years ago. The pattern, the weather relationship and what is outside the wall tell you far more than the number does.
Sources
- US EPA — Mold and moisture guidance — the 30–50% range and the 10 sq ft remediation threshold
- ASHRAE Standard 55 — Thermal environmental conditions for human occupancy — indoor comfort and humidity limits
- US DOE 10 CFR 430 — Dehumidifier test procedure — the 2019 move to 65 °F rating conditions