The short answer
A common “damp meter” (protimeter) measures electrical conductance at the surface, which rises with moisture — but also with hygroscopic salts, foil-backed plaster and metal, so a high reading is an indicator, not proof. RICS warns that surface meters can over-read on walls carrying nitrate and chloride salts, the classic false signal mistaken for rising damp. Reliable diagnosis needs a measurement of the true moisture content of the masonry, not just a surface number.
The handheld meter is the symbol of a damp inspection, and it is genuinely useful — but only if its limits are understood. A high reading does not, on its own, prove rising damp; misreading the meter is one of the main routes to a wrong diagnosis and an unnecessary bill. Here is what the meter measures, the difference between conductance and capacitance types, where they mislead and how a competent surveyor confirms what is really going on.
Damp meters at a glance
- Conductance type Pins measure surface electrical resistance
- Capacitance type Pads sense moisture just below the surface
- Reads high with Moisture — but also salts and metal
- Confirms true damp Calcium-carbide or gravimetric test
- Key risk Salt-contaminated walls reading falsely high
How the two meter types work
There are two main kinds of handheld “damp meter”, and they measure different things. A conductance (resistance) meter — the classic two-pin protimeter — passes a small current between pins pressed into the surface; the wetter the material, the more it conducts, and the higher the reading. A capacitance (dielectric) meter uses a flat pad to sense moisture just below the surface without pinholes, by detecting how moisture changes the material’s electrical capacitance, which is useful for non-destructive screening of finished surfaces and tiles. Both give a quick comparative picture of where moisture concentrates, which is exactly what they are good for — mapping a wall, not diagnosing a building.
Why high readings can mislead
Conductance rises with anything that carries current, not just water, and that is where readings deceive. The classic trap is hygroscopic salts: nitrates and chlorides left in masonry by evaporating ground moisture attract atmospheric humidity and read as “damp” long after the wall has actually dried. This is the false positive most often misread as rising damp, and it is precisely why a meter reading alone cannot confirm a diagnosis that leads to an injected DPC. Meters also over-read on:
- foil-backed plasterboard or other metallic finishes behind the surface,
- embedded pipes, nails, mesh, wiring or radiator backs,
- some paints, adhesives, gypsum plasters and salt-laden renders,
- fresh building work that is still drying out (construction moisture).
RICS guidance is explicit that an electrical moisture meter is a screening tool whose readings must be interpreted in context, never taken as a standalone diagnosis.
How surveyors confirm true moisture
To move from indicator to proof, a competent surveyor confirms the actual moisture content of the masonry, typically by a calcium-carbide (“speedy”) test on a drilled sample taken from depth, or a laboratory gravimetric analysis, which can also distinguish free water from hygroscopic salts by oven-drying the sample. Those readings are then combined with the whole picture — the height and pattern of the damp, external defects, ground levels, ventilation, surface temperature and the building’s construction — before any conclusion is drawn. That is the difference between a number and a diagnosis, and why a full damp survey is worth far more than a meter sweep.
| Method | Measures | Limitation |
|---|---|---|
| Conductance meter | Surface electrical resistance | Over-reads on salts & metal |
| Capacitance meter | Near-surface moisture (no pins) | Affected by density & hidden metal |
| Calcium-carbide test | True moisture in a drilled sample | Slightly destructive |
| Gravimetric analysis | Free + hygroscopic moisture | Needs a lab; takes time |
Why a high reading is not a diagnosis
A meter measures one thing — an electrical property of the surface — and a high number on its own does not tell you the cause, the depth or the cure. Salt contamination is the classic trap: where rising or penetrating damp has dried, hygroscopic salts left in the plaster hold moisture from the air and read “wet” on a conductance meter even when the wall behind is dry. Foil-backed plasterboard, metal pipes, fixings and foil insulation can all give false high readings, and a freshly plastered or recently painted wall can read damp simply because it has not fully cured. This is why a meter is a survey aid, not a verdict. A competent surveyor uses readings to map where moisture is and is not, then combines that pattern with external checks — ground levels, gutters, the damp-proof course, ventilation — and, where it matters, a calcium-carbide or laboratory test of a drilled sample to confirm the true moisture content within the wall.
This page is general information, not a site-specific survey or legal advice. Always have meter findings interpreted by a qualified surveyor. For the difference between damp types, see condensation vs damp.
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Frequently asked questions
What does a damp meter measure?
A conductance meter measures electrical resistance at the surface, while a capacitance meter senses near-surface moisture without pins. Both rise with moisture but are screening tools, not direct measurements of water inside the wall.
Why might a damp meter read high on a dry wall?
Hygroscopic salts left by past moisture attract humidity and conduct current, so a salt-contaminated wall can read “damp” even when dry. Metal, foil-backed plaster and some paints also cause false highs.
Can a meter diagnose rising damp on its own?
No. RICS treats meter readings as indicators that must be interpreted in context. Confirming rising damp needs the true moisture content measured, plus the pattern, external defects and construction considered.
How do surveyors confirm true moisture?
By measuring the moisture content of the masonry itself — usually a calcium-carbide test on a drilled sample or a laboratory gravimetric analysis that separates free water from hygroscopic salts.
Sources & further reading
- RICS — Investigation of moisture and its effects in traditional buildings
- Property Care Association (PCA) — guidance on moisture measurement and diagnosis
- gov.uk — Housing Health and Safety Rating System (HHSRS): damp and mould
This guide is general information, not a site-specific survey, medical advice or legal advice. Damp and mould should be assessed by a qualified surveyor, and health concerns discussed with a GP or the NHS.