Rising damp stays low, rarely above a metre, follows the whole length of a wall and leaves a tide line of white salts. A hidden leak forms a defined patch, often near a pipe or a radiator, has no salt line and can appear at any height. Run the overnight meter test and check the boiler pressure before paying for a damp-proofing quote.
The wall by the front door has been damp for months. The paint is bubbling low down, the skirting is soft, and a surveyor or a damp company has said “rising damp” and quoted for an injected damp course and replastering. Before you sign it, pause. In old Cornish houses the same symptoms are produced by a leaking supply pipe under the floor, a heating pipe weeping in the screed, or a waste pipe behind the wall, and the treatments are entirely different. Injecting a chemical damp course into a granite wall that is being wetted by a pipe does nothing except cost money and delay the real repair. This guide sets out how rising damp and a hidden leak each behave in the housing stock we see across Cornwall, the clues that separate them, and the tests that settle it. Where the clues point at a pipe, our thermal imaging leak detection shows the shape of the moisture behind the plaster and where it is being fed from.
How a Cornish wall gets wet from the ground
True rising damp is ground moisture drawn up through masonry by capillary action. It needs a wall that can wick water (rubble-filled granite, cob, soft brick or lime mortar all can), no effective damp proof course or one that has been bridged, and a supply of moisture at the base. Older cottages around Cornwall often have one or more of the following:
- No damp proof course at all. Many pre-1900 cottages were built straight off the ground with thick walls that were expected to breathe.
- A slate damp proof course. Common in Victorian and Edwardian terraces in Camborne, Redruth, Penryn and Truro. Slate lasts well but can crack, and it is easily bridged.
- Ground levels raised outside. A path re-laid, a patio added, soil built up against the wall by a century of gardening. Once the outside ground is above the floor inside, water sits against the wall above any damp course.
- Cement render and gypsum plaster on cob or granite. Impermeable finishes trap moisture in the wall and push it to show elsewhere, often low down inside.
- A wall built into a hillside. Terraced cottages on a slope, or a farmhouse dug into a bank, have a wall that is holding back wet ground on one side.
The point is that genuine rising damp is common in Cornwall and is not a myth. The problem is that it gets diagnosed by eye, and a leak looks the same by eye.
How a leak wets the same wall
A cottage of that age usually has its water supply, heating pipes and waste pipes buried in or under a solid floor, chased into the walls, or run in the ground along the outside. Any of them can wet the base of a wall from below or behind in a way that mimics rising damp exactly.
- The supply pipe from the meter enters under the front wall and runs under the floor to the kitchen. A pinhole in old copper or a failed joint wets the ground under the slab and the base of the wall above it.
- Heating pipes in the screed weep at joints and where they were bent round a doorway. The damp is warm, which is a clue.
- A waste pipe or gully outside the kitchen or bathroom wall leaks into the ground against the footing.
- A neighbour’s pipe in a terrace can wet the party wall from their side.
The clues that separate them
| Clue | Points to rising damp | Points to a leak |
|---|---|---|
| Height | Low, with a fairly level top edge, seldom above a metre | Any height, and can be higher in the middle than at the edges |
| Shape | A band along the whole wall or a long stretch of it | A defined patch, often roughly circular or fan shaped |
| Tide line and salts | A white, fluffy salt line at the top of the damp band | Usually no salts, because pipe water is clean |
| Location | Outside walls, worst where outside ground is highest | Near a pipe route, a radiator, a bathroom or the kitchen, and can be on an internal wall |
| Temperature | Cold to the touch | Sometimes warm if it is a heating pipe |
| Season | Worse in winter, eases in a dry summer | Constant, or worse when the heating is on |
| Water meter | Still with everything off | Often moving with everything off |
| Boiler pressure | Steady | Dropping if the leak is on the heating |
| Floor | Floor itself usually dry, wall wet | Floor near the wall wet or warm as well |
No single clue is decisive. A damp patch at the base of a wall next to a radiator, with no salts, in a house where the boiler needs topping up, is a leak until proved otherwise. A level band along the whole front wall with a salt line, in a house with a still meter and steady boiler, is very likely rising damp. Most real cases sit between those two, which is why the tests below matter.
Three tests before anyone quotes for damp-proofing
The meter test
With every tap off, read the meter overnight. If it moves, water is escaping on the mains supply, and the damp is suspect until that leak has been found and repaired. The method is in the overnight meter test guide. A still meter rules out the supply pipe but not the heating or the waste side.
The boiler pressure test
If you have a sealed heating system, note the pressure on the gauge when cold (around 1 to 1.5 bar is typical) and again a week later without topping up. A steady fall means the heating is losing water somewhere. In an old cottage with pipes in the floor, “somewhere” is often the base of the wall you are worried about.
The moisture map
A moisture meter is only useful in a pattern, not as a single reading. Salts in a wall make a surface meter read high whether or not the wall is wet, so a single high number proves little; the guide to a high damp meter reading explains why. What matters is the shape: a band that reads high evenly along a wall behaves like rising damp, a patch that peaks near a pipe and falls away in every direction behaves like a leak.
What thermal imaging adds
Evaporation cools a wet surface, so a thermal camera sees damp plaster as a cooler area even when it looks dry to the eye. It shows the true extent of the moisture, which is nearly always larger than the visible patch, and it shows the shape. Rising damp images as a cool band with a level top. A leak images as a cool plume that is coldest at its source, or, for a heating pipe, as a warm line with a hot spot where the water is escaping. On a cob or granite wall the camera also picks up cold bridging and the line of a buried pipe, which helps rule things in or out without touching the plaster.
Where the camera and the moisture map point at a pipe, acoustic listening or a tracer gas test then confirms which pipe and where. The survey ends with a location and a written report. If the tests all come back clean, the report says so, and that is a useful document to hand to a damp specialist because it tells them the pipes have been eliminated.
Why the order matters
- Injected damp courses do not work on a wall being wetted by a pipe. The moisture simply carries on arriving from the side or below the injection line.
- Replastering over a leak hides it for a season. Waterproof renovating plaster holds the damp back until it finds another way out, usually higher up or in the next room.
- A leak claim needs the leak found. Most buildings policies cover damage from escaping water and include trace and access to locate it. None of them cover rising damp, which is treated as a maintenance matter. Spend the money on a damp course and you have lost the chance to claim if it was a leak all along. The guide to what trace and access covers explains what the policy pays for.
- A cottage needs to breathe. Where it genuinely is ground moisture, the right answer in a granite or cob wall is often lime plaster, lowered ground levels outside and better ventilation rather than chemicals and cement. That is a conversation for a specialist in traditional buildings, once the leak question is closed.
We cover leak detection across Cornwall, and a damp wall in a cottage near Helston or a converted barn on the moor gets the same three-part test before anyone gives an opinion.
Frequently asked questions
Can rising damp go higher than a metre up the wall?
Rarely. Capillary rise in masonry runs out of lift at roughly a metre, and in most walls well below that. Damp well above a metre, damp upstairs, or a patch that is higher in the middle than at the edges is far more likely to be a leak or rain getting in, and should be checked as such.
Does a damp patch with no white salts mean it is a leak?
It is a strong hint. The white, fluffy deposits at the top of a rising damp band are salts carried up from the ground. Water from a pipe is clean and leaves none. A salt-free patch, particularly one near a pipe run or radiator, deserves a meter test and a boiler pressure check before any damp treatment.
Could it be condensation instead of either?
Possibly. Condensation shows as black mould in cold corners, behind furniture and around windows rather than as a wet patch with a tide line, and it is worst in winter in rooms with poor ventilation. Coastal Cornish homes carry high humidity, so it often exists alongside a leak or rising damp rather than instead of them.
Will insurance pay for rising damp?
No. Rising damp is treated as a maintenance issue and is excluded from standard buildings policies. Damage from a leaking pipe is usually covered, and trace and access cover pays for locating it and making good. That difference is the main reason to rule a leak out before treating the wall as damp.
My damp company says it is rising damp. Should I get a leak survey anyway?
If the property has pipes near the wet wall, the meter or boiler pressure has been moving, or there is no salt line, yes. A leak survey is non-invasive and either finds a pipe or gives you a report confirming the plumbing is sound, which makes the damp work a safer decision.
Damp wall, unsure why?
We map the moisture, image the wall and test the pipes before anyone quotes for a damp course. If it is a leak, you get its location and a report your insurer will accept; if it is not, you get that in writing too.
