No. It cannot. A meter is a useful instrument and we use one on every survey, but a reading is not a diagnosis, and the guidance the industry itself works to says so in terms.
No. A damp meter cannot prove that you have rising damp. It can tell you that one part of a wall is conducting electricity more than another part. It cannot tell you why.
This matters because rising damp is the diagnosis that leads to the most expensive remedial quotation in the trade, and a high reading on a handheld meter is very often the only evidence offered for it.
BRE Digest 245, Rising damp in walls: diagnosis and treatment, states that high readings from a moisture meter alone are not conclusive, and sets out in its Appendix A how to drill samples from a wall to test for moisture content and hygroscopicity instead. It is the reference the industry has worked to for decades.
A pin type moisture meter passes a small current between two pins and measures the resistance between them. Wet material conducts better than dry material, so lower resistance is displayed as a higher reading.
That relationship was established for wood. The scale is calibrated against timber, which is why the reading is properly written as %WME, percentage wood moisture equivalent. It means: this material is conducting as though it were a piece of wood at that moisture content.
A plastered masonry wall is not a piece of wood. So on a wall, %WME is not a moisture content at all. It is a comparative number. It tells you this part of the wall is conducting more than that part. It does not tell you how much water is in either, and it does not tell you where the water came from.
That is not a criticism of the instrument. It is what the instrument is for. The problem arises when a comparative number is presented as a measurement, and then as a diagnosis.
Where moisture has been evaporating out of a wall over a long period, it leaves behind what it was carrying. Nitrates and chlorides accumulate in the plaster and at the surface, and they have two relevant properties.
They conduct electricity. And they are hygroscopic, meaning they pull moisture out of the air around them.
So a wall carrying a load of old salts gives a high reading on a resistance meter even when it is dry to the touch and dry at depth. The meter is not faulty and it is not lying. It reports conductance, faithfully, and conductance is high. It has no way of telling you whether it is reading water or salt.
There is often a clue in the pattern. Water spreads, so genuine moisture fades out gradually at its edges. Salt contamination does not, so you can find a very high reading with a sharp edge to it and material a hand's width away that reads dry. That signature is common at the old thresholds of agricultural buildings and anywhere a wall has been quietly evaporating for a century.
A converted barn, high readings flagged ahead of a sale. The porch threshold read 59.8 %WME, and the same wall 100mm away read 8.2. Every control reading in the house was below the bottom of the meter's range. Read the whole survey, published in full with the client's permission.
This is the part that gets skipped most often.
Before we take a single reading in a problem area, we take a control reading: the same meter, on the same day, on the same construction, somewhere in the building known to be dry and unaffected. That number becomes the benchmark everything else is measured against.
Without it, a reading of 20 on a display means nothing. Twenty against a control of 18 is background. Twenty against a control that reads below the lower limit of the instrument is a real difference that needs explaining.
If you are shown a photograph of a meter reading and there is no control reading anywhere in the report, you have been shown a number, not a finding.
One reading tells you almost nothing. A set taken at fixed heights up the same wall starts to name the mechanism on its own. We take them at four heights on every wall we grid.
| Height | What it is there for |
|---|---|
| 100mm, at the skirting | The bridging and splashback zone |
| 600mm | Where genuine groundwater sits |
| 1200mm | Where groundwater stops. Anything high here did not come up from the ground |
| 200mm below the ceiling | Catches roof, gutter and rainwater ingress from above |
High at 100 and 600 falling away above, with a diffuse upper edge, is the pattern consistent with groundwater. High at 1200 and just below the ceiling, falling as you come down, is water arriving from above. Highest at the coldest surfaces, at reveals and thermal bridges, with the wall dry a few inches away, is condensation, which is the most common finding by a distance and the one most often sold as rising damp.
A meter can read high on salts as well as on moisture, so no single reading tells you very much on its own. The important thing is to take the whole building into account, its history, its layout and how it is actually built, and not get lost in one number.
Rory Thorne, Vigilant SurveysA thermal camera measures surface temperature, not moisture. It does not see through walls and it does not detect damp directly.
What it does very well is show where a surface is colder than those around it, which is where condensation forms, and it shows the shape of an evaporating patch, because evaporation cools a surface. Read alongside moisture readings it is extremely useful. Read on its own it will mislead you, and a coloured picture is unusually persuasive to somebody who has not been told what it is measuring.
Not an electrical instrument. Where it genuinely matters, and where the answer changes what somebody is about to spend, the reference method is the one BRE Digest 245 sets out: a small sample drilled from the wall at depth and analysed for its total moisture content and its hygroscopic moisture content, with the moisture gradient read through the thickness of the wall. Comparing those two figures separates water in the wall from salts in the wall, which is precisely the thing the handheld meter cannot do.
It is mildly destructive, it takes time, and it is not warranted on every job. But where a diagnosis of rising damp is driving a five figure quotation, it is a very small cost to establish whether the diagnosis is right.
The other thing that settles it, and it costs nothing, is time. A great many damp problems are seasonal. If a wall is wet in February and dry in August, that is information, and a survey carried out in dry weather should say so rather than pretending otherwise.
The joint position statement on the investigation of moisture and its effects on traditional buildings, published in 2022 by Historic England, RICS and the Property Care Association, sets out the same approach: establish the root cause rather than treat the symptom, consider every source of moisture including occupant activity, building defects, failed systems and groundwater, and assess on evidence, because the wrong remedy can make a building worse. It is free to download from Historic England and from the PCA, and it is worth reading before you agree to any damp works.
Most damp inspections in this country are carried out free of charge by companies that also carry out the remedial work. The inspection is not the product. The quotation is the product.
That is not necessarily dishonest, and plenty of those firms do good work. But it does mean the person holding the meter has a commercial interest in what the meter appears to say, and a high reading on a resistance meter is a very convenient thing to find.
Ask three questions before you agree to anything.
Rising damp is real, but it is a good deal rarer than the number of quotations for it would suggest. It is worth being sure.
Percentage wood moisture equivalent. The relationship between moisture and electrical resistance was established for timber, so the scale is calibrated against wood. On a plastered masonry wall the number is comparative rather than a measurement of how much water is present.
Nitrates and chlorides left behind by years of evaporation conduct electricity and pull moisture out of the air. A resistance meter reads them as high on a wall that is dry at depth. Water spreads and fades at its edges; salt contamination can give a very high reading with a sharp edge and dry material a hand's width away.
Often, yes. Ask what the control reading was and where it was taken, what else was ruled out and on what evidence, and what would change the answer. A survey carried out in dry weather cannot exclude water entering from outside.
Not an electrical one. BRE Digest 245 sets out drilling samples at depth and testing for moisture content and hygroscopicity, comparing the two to separate water in the wall from salts in the wall.
No payment to book. Tell us what you need and Rory will call you back, usually within a couple of hours, to talk it through and confirm a fixed price.