Moisture meters: resistance, capacitance, and what each is actually telling you

The moisture meter is the instrument most likely to end up in a complaint. Not because it is unreliable, but because what it measures and what people believe it measures are two different things — and the gap between them usually only becomes visible when somebody disagrees with the report.

Two instruments, two physical properties

Resistance meters (pin meters) pass a small current between two electrodes and measure electrical resistance. Water conducts; dry material does not. The scale is calibrated against the known relationship between resistance and moisture content in timber, and on clean, unsalted timber it is genuinely quantitative — you can report a percentage and defend it.

Point it at plaster, brick or a screed and that calibration no longer applies. The instrument is still measuring conductance; it is simply no longer measuring anything you can express as a moisture content. This matters, because the display carries on showing a percentage regardless.

Capacitance meters (pinless) generate a shallow electromagnetic field and measure the dielectric constant of whatever is inside it. Water has a very high dielectric constant compared with most building materials, so wetter material reads higher. The reading is non-destructive and covers an area rather than two points, which makes it excellent for scanning — and it is affected by density, thickness, embedded metal and the air gap behind a finish, which makes it poor for a number you intend to quote.

The practical division: capacitance to find, resistance to interrogate. Scan a wall to see where the pattern is, then use pins to test the specific points that pattern suggests.

Salts, and why a dry wall reads wet

Hygroscopic salts — chlorides and nitrates in particular — are the single most common reason a resistance meter shows high readings in masonry that is not, in any meaningful sense, damp. Salts are conductive, and they draw moisture from the air. A wall that has previously been wet may carry a salt band long after the source is fixed, and it will keep reading high.

This is the reading most often used to sell a chemical damp-proof course, and the one a surveyor most needs to be careful with. If the pattern suggests salt contamination, that belongs in the report as a possibility to be tested, not as a conclusion either way.

Where a quantitative answer is genuinely needed, the methods are carbide (Speedy) testing on a drilled sample or gravimetric oven-dry testing in a laboratory. Both are destructive to some degree, both need the owner’s written consent, and both are outside the scope of an ordinary Level 2 inspection. Saying so is a legitimate professional answer.

Taking a reading you can defend

• Record the conditions. External temperature, internal temperature, weather in the preceding days, and whether the property is occupied and heated. A reading with no conditions attached is very hard to defend two years later.
• Take a profile, not a point. Readings up the wall at fixed heights, along the elevation at fixed intervals. The shape of the profile carries the diagnosis; a single figure carries nothing.
• Include a control. A reading on an internal wall or an area you expect to be dry gives the rest of the profile meaning.
• Note the instrument and mode. Which meter, pin or pinless, which scale. If the scale is not a timber moisture content, do not present it as a percentage.
• Photograph the position, not the display. A photograph of a number tells the reader nothing; a photograph showing where the reading was taken tells them a great deal.

Wording it in the report

The phrasing that gets surveyors into difficulty is the confident one: “moisture readings of 28% were recorded to the base of the wall.” On plaster, that number does not mean what the reader will take it to mean.

The phrasing that holds describes the instrument, the pattern and the inference separately — what was measured, what pattern it forms, what that pattern is consistent with, and what would be needed to confirm it. The observation, the reasoning and the recommendation stay visibly distinct, which is what makes the conclusion reviewable rather than merely assertive.

Elevated comparative readings are evidence. They are not a diagnosis, and the report should not let them look like one.

What to look for when buying

Most surveyors end up carrying both types, and the sensible order is a decent capacitance meter first for scanning, then a resistance meter for interrogation.

Worth paying for: a clear scale that distinguishes timber moisture content from a comparative scale, so you are not tempted to quote the wrong one; deep-wall probes if you do any investigation work; a calibration check facility, and an actual habit of using it. Not worth paying for: Bluetooth logging you will never open, and a colour screen.

Whatever you carry, check it against a known reference regularly and record that you have. If a reading is ever challenged, the question after “what did it say” is “when was it last checked”.