A surveyor’s damp meter measures electrical conductivity, reported as a wood moisture equivalent, so on plaster, stone and cob the number is a comparison, not a percentage of water. Old Cornish walls often read high without a leak. A leak shows as a pattern centred on a pipe, and it can be proved before you exchange.
The survey has come back and there it is, somewhere near the end: high moisture readings recorded to the ground floor walls, further investigation recommended before exchange. The estate agent shrugs. The seller says the house has always been fine. Your solicitor wants to know whether to hold things up. And you are staring at a number on a page with no idea whether it means a damp course, a leaking pipe or nothing at all.
Here is what that number is actually telling you, why so many older homes in Cornwall trip a meter without anything being wrong, how to tell when a leak really is hiding behind the plaster, and how to get a definite answer in time to act on it. Where a leak is suspected, our thermal imaging leak detection survey is the quickest way to prove it one way or the other without disturbing the seller’s house.
What the surveyor’s meter actually measured
Most surveyors carry a small electrical moisture meter with two pins on the end. Pushed against a wall, it passes a tiny current between the pins and measures how easily it flows. Wet material conducts better than dry, so a higher reading suggests more moisture. Some carry a pinless version instead, with a flat pad that reads a short way below the surface using a radio-frequency field, useful on decorated walls where pinholes are unwelcome.
Both types share a limitation that explains most of the confusion. They are calibrated for timber. On a wooden joist the percentage is a fair estimate of real moisture content. On plaster, brick, granite or cob it is not. The meter reports what the reading would mean if the wall were made of wood, which is why the scale is called wood moisture equivalent, or WME. A wall showing 25% is not a quarter water. It is simply conducting more than the meter expects of dry timber, and there are several reasons a wall might do that apart from being wet.
What the numbers usually mean
Surveyors tend to use broad bands rather than exact thresholds, and the bands are only a starting point. This is the version most of them work from.
| Reading on the WME scale | How surveyors usually treat it | What it can mean in an older Cornish wall |
|---|---|---|
| Below about 17 to 20% | Dry, no comment | Dry |
| Around 20 to 27% | Moderate, recorded and flagged | Often salts or a cold external wall in winter, occasionally the edge of a leak |
| Above about 27% | High, investigation recommended | Salt-laden plaster, a wet wall after weeks of rain, or a genuine leak |
| Off the scale on one spot only | Usually noted as an anomaly | Foil-backed plasterboard, a metal fixing, a cable, or a pipe right behind the surface |
A reading in the high band is a reason to look harder, not a diagnosis. What settles it is the pattern across the wall and the house, which a single figure on a survey cannot give you.
Why old Cornish walls read high without a leak
Much of the housing stock across Cornwall is solid-walled: granite, killas rubble, cob, or a mix of all three under a coat of lime render. Those walls behave nothing like the cavity brickwork the meter bands were written for, and they trip a meter for reasons that have nothing to do with plumbing.
- Salts in the plaster. Decades of moisture moving through a solid wall leave hygroscopic salts behind in the plaster. Those salts pull water out of the air and conduct electricity beautifully, so a wall that is dry to the touch can read high for years after the damp that put the salts there has gone. Chimney breasts are the classic case, because soot salts do the same thing.
- Cob and earth walls. Cob is made of subsoil, and subsoil contains salts and clay minerals. A healthy cob wall reads higher than a brick one, all the time.
- Modern materials in old walls. Cement render and gypsum plaster on a solid wall trap moisture that lime would have let out. The wall is wetter than it should be, but the fix is breathable materials, not a plumber.
- Weather. A survey carried out in a wet February after a coastal storm in St Ives will read higher on every exposed wall than the same house in June. Solid walls absorb rain and dry out over weeks.
- Things behind the surface. Foil-backed plasterboard, metal lath, a cable or a copper pipe directly behind the plaster all send a pin meter off the scale on one spot.
None of this means the survey was wrong. The surveyor’s job is to record the reading and recommend investigation. Working out what the reading means is the next step, and it is yours.
When a high reading really is a leak
A leak has habits that salts and weather do not. The readings are highest at one point and fall away in every direction, and that point is near something that carries water: a pipe run, a radiator, a bathroom above, a kitchen on the other side of the wall. The patch is at an odd height rather than a neat band along the skirting. There are no white powdery salt deposits on the surface. And, if you can get the seller to cooperate, the water meter creeps overnight or the boiler pressure needs topping up.
The distinction matters because the two problems go opposite ways in a negotiation. Salts and a tired render are a known, budgetable job. A live leak behind the plaster is doing damage every day, may have soaked joists or a floor you cannot see, and is exactly the sort of thing a buyer can ask a seller to fix or price in before completion. Our guide on telling rising damp from a hidden leak in an older Cornish home goes through the visual clues in more detail.
How to prove it before you exchange
You do not have to guess, and you do not have to hold the purchase up for weeks. In order of effort:
- Ask the seller two questions. Has the boiler needed topping up lately, and would they read the water meter last thing at night and first thing in the morning with nothing running. A moving meter or a falling gauge changes the picture immediately.
- Ask the surveyor where exactly they took the readings. A sketch of the wall with the numbers on it tells you whether the high spots cluster or scatter. Clustering near plumbing is a leak until proved otherwise.
- Book a specialist leak survey. This is the step that gives you a written answer. We map the moisture across the walls and floors with a meter, then check the high areas with a thermal imaging camera, which shows the cool shape of a wet patch and the warm line of a heating pipe behind the plaster. Where a pipe is implicated we confirm it with acoustic listening or tracer gas, all without opening up the seller’s walls. You get a report stating what was found and what it is not, which is what your solicitor and lender want to see.
A survey like this takes a matter of hours and can usually be fitted in within the week. If you want to know what the visit involves, we have written up what to expect from a leak detection survey, and there is a separate note on how long leak detection takes. We carry out leak detection across the whole of Cornwall, so a house in Bude is no harder to reach than one in Truro.
What to do with the answer
If the report finds no leak, go ahead and treat the readings as a maintenance note: check the render, the ground levels outside, the gutters, and whether the plaster is the right sort for the wall. If it finds one, you have evidence. Most buyers use it to ask the seller to have the leak repaired before completion, or to agree a price reduction that covers the repair and the making good. Either way you are deciding on facts rather than a line in a survey.
Frequently asked questions
What is a normal damp meter reading for a wall?
On the wood moisture equivalent scale most meters use, readings below about 17 to 20% are treated as dry. Between 20 and 27% is usually recorded as moderate and above 27% as high. On solid stone or cob walls the figure is a comparison rather than a true moisture content, so context matters more than the number.
Does a high damp meter reading mean the house has rising damp?
Not on its own. Rising damp leaves a low tide mark, salt deposits and readings that fade out within about a metre of the floor. Salts left in the plaster from old damp, cement render on a solid wall, a wet winter, or a leaking pipe can all produce a high reading with no rising damp at all.
Can a leak be found without damaging the seller’s house?
Yes. Moisture mapping reads through paint and plaster, thermal imaging shows the shape of a wet patch and any warm pipe behind it, and acoustic and tracer gas checks confirm a pipe leak from the surface. Nothing is opened up, so a seller has no reason to refuse access for the survey.
Should I pull out of the purchase over high damp readings?
Usually not before finding out what they mean. Many older Cornish homes read high for harmless reasons. A specialist survey tells you whether there is a leak, and if there is, that becomes something to have repaired or priced in rather than a reason to walk away from a house you want.
Get a definite answer before exchange
If a survey has flagged high readings on a house you are buying, we will map the damp, check it with thermal imaging and tell you in writing whether a leak is behind it, in time for your solicitor and lender.
