What a moisture meter reading on a wall actually means

What the instrument is actually doing

A resistance meter passes a small current between two pins and measures what comes back. Wet timber conducts more than dry timber, the relationship is well characterised, and on timber the reading is genuinely a moisture content percentage. That is the instrument working as designed.

Masonry is a different material with a different pore structure, a different density and, critically, a different chemistry. The timber conversion is simply carried across, which is why a reading of 'twenty per cent' on a brick wall does not mean the wall is twenty per cent water by weight. It means the resistance measured corresponded to twenty per cent in a piece of wood.

Pinless or capacitance meters avoid the pinholes but do not avoid the underlying problem. They read a volume rather than a surface, which sounds like an improvement, and they are affected by density changes, by embedded metal, by foil-backed plasterboard and by anything conductive behind the surface. They are useful for comparing one part of a wall to another. They are not an absolute measurement either.

Why a dry wall can read soaking wet

Salt arrives in masonry dissolved in water. The water evaporates at the face and the salt cannot follow it into the air, so it stays — and it accumulates, year after year, long after whatever delivered it has been fixed.

Some of those salts are hygroscopic, which means they pull water vapour straight out of humid air. Sodium chloride goes damp at about 75% relative humidity. In a South East Queensland summer that is most of the time, so a wall carrying chloride genuinely does hold moisture — but it is moisture harvested from the air, not water rising from the ground, and the two need completely different work.

So there are at least three walls that produce a high reading, and they need three different responses: a wall with water actively rising through it, a wall that is dry but salted from a problem cured years ago, and a wall that is neither but is being read through a conductive path the operator has not noticed. The meter returns the same alarming number for all three.

So what reading is normal, and what is too high?

There is no answer to that question on masonry, and anybody who gives one confidently is telling you something the instrument cannot support. Because the scale has no absolute meaning on brick or plaster, there is no threshold above which a wall is wet and below which it is dry.

What the number can support is a comparison. Readings taken across the same wall, at the same height, with the same instrument, on the same day, describe a pattern — and the pattern is diagnostic even when the absolute figure is meaningless. A band of high readings that fades out with height is different from a blotch that maps to a cold corner, which is different again from a stripe that follows a pipe.

The other honest use is time. The same spots, marked in pencil, re-read every few weeks. A falling trend after a downpipe has been fixed is real evidence that the downpipe was the problem. That test costs nothing, requires no expertise, and settles more cases than any single reading ever will.

• Read inside and outside at the same points. A wall wet from the ground, a wall wet from a leak on the far side and a wall wet from condensation do not produce the same picture on both faces.
• Read a wall you have no concerns about, in the same building, as a control. Without one, a high number has nothing to be high against.
• Read at several heights up the same wall. Rising damp fades with height; a leak does not, and condensation does not care about height at all.
• Write the readings down with the date and mark the spots. A single visit produces a number; two visits produce a direction, and the direction is worth more.

The measurement that does have an absolute meaning

Where the answer has to be right rather than merely plausible — a dispute, an insurer, expert evidence, a purchase somebody is arguing over — the method is laboratory work on drilled samples, and it has been standard for decades.

Small samples are drilled from a vertical profile up the wall at known heights and depths, sealed immediately so they cannot dry on the way, and weighed. They are oven-dried and reweighed, and the difference is total moisture content by weight. Separate samples are then re-equilibrated in a controlled 75% relative humidity atmosphere and reweighed again; the mass they regain is hygroscopic moisture content — the water the salt alone will hold out of the air.

Subtract the second from the first and what remains is free capillary water. That is the number that distinguishes a wall with water rising through it from a wall that is simply salty, and it is the only number in this subject that does. The method sits behind BRE Digest 245 and BS 6576, and the Australian heritage guide is plain that it is what separates moisture due to rising damp from moisture held by hygroscopic salts.

Salt analysis usually runs alongside it, because the species points at the source. Nitrates suggest a leaking sewer or an over-fertilised bed; chlorides suggest groundwater, sea spray or an acid clean; magnesium sulfate suggests the sand the mortar was mixed with. More than about 0.5% salt by weight of sample is the working threshold for considering desalination.

Concrete slabs: a surface reading proves nothing at all

On a slab the surface reading is not merely imprecise, it is measuring the wrong thing. What decides whether a floor covering will fail is the humidity deep inside the slab, which migrates upward once an impermeable covering traps it — and a surface meter cannot see it.

The method that governs is in-situ relative humidity testing: probes set into holes drilled to 40% of the slab thickness for a slab drying from one side, left to equilibrate, and then read. That is ASTM F2170, and AS 1884:2021 makes it the normative test for resilient floor coverings in Australia.

This matters commercially as well as technically. Flooring warranties are written against the in-situ figure, not against a surface reading, and the covering's own stated limit may be stricter than the general one. A floor laid on the strength of a pin meter is a floor laid without the test the warranty depends on.

Reading a report that leans on meter readings

A document whose evidence is a set of meter numbers and a photograph of the instrument has not established what it claims to have established. That is not a technicality — it is the difference between a diagnosis and a sales aid.

What a competent document does instead is reason about the building. Where the external ground and paving sit relative to the damp course. Where the downpipes discharge in heavy rain. How high the damp reaches and what shape its upper edge is. Whether salt is present, and where it bands. Which alternatives were considered and on what evidence they were excluded. The meter appears in that document as one observation among several, not as the case.

1. Ask what the reading was taken onPlaster, render, brick or timber. If the instrument was calibrated for timber and the surface was plaster, the number is a relative indication and should be described as one.
2. Ask for the controlWhat did a wall with no suspected problem read, in the same building, on the same day? Without a control there is nothing for a high reading to be high against.
3. Ask whether salt was consideredOn this coast a high reading with no free water is an ordinary finding. A report that does not mention salt has not excluded the most common alternative explanation for its own evidence.
4. Ask what would change the answerAny honest diagnosis can say what observation would overturn it. One that cannot has not been reasoned to.

Questions that actually get asked

What is a normal moisture reading for a brick wall?

There isn't one. A resistance meter is calibrated for timber, so on masonry its scale carries no absolute meaning and there is no figure above which a wall is wet and below which it is dry. Readings are only useful comparatively — across the same wall, against a control wall in the same building, or against the same spots re-read weeks later. A contractor who quotes a specific threshold for brickwork is describing a certainty the instrument cannot provide.

Why did the meter read high when the wall feels completely dry?

Almost always salt. Soluble salts conduct electricity, and the meter measures conductivity. A wall that has been salted by an old damp problem — one that may have been cured years ago — reads high permanently, because the salt never left when the water did. Some of those salts are also hygroscopic and pull real moisture out of humid air at about 75% relative humidity, so the wall genuinely is a little damp on muggy days. That wall needs desalination, not a damp course.

Are pinless moisture meters more accurate on masonry?

More convenient, not more absolute. A capacitance meter reads a volume rather than a surface, which avoids pinholes and gives a better sense of what is behind the finish. It is still affected by density variation, embedded metal, foil-backed plasterboard and anything conductive in the wall, and it still has no calibrated scale for brick or render. Both types are comparative instruments on masonry.

The inspector took one reading and quoted a damp course. Is that enough?

No, and the objection is not a matter of opinion. A single reading cannot distinguish free water from salt-held moisture, has no control to be compared against, and carries no absolute meaning on masonry in the first place. The Australian heritage guide says directly that a meter should never be the sole basis for diagnosing damp. Ask what the ground levels are doing, where the downpipes discharge, how high the damp reaches, what shape its top edge is, and which alternatives were excluded.

Can I just buy a moisture meter and check the wall myself?

Yes, and it is worth doing — provided it is used for what it is good at. Take readings across the wall at several heights, take a control reading on a wall you have no concerns about, mark the spots in pencil with the date, and repeat in a few weeks. The pattern and the trend are genuinely informative. The single number is not, and treating it as one is the same error the trade gets criticised for.

What test actually proves whether a wall has rising damp?

Drilled samples taken up a vertical profile, sealed, weighed, oven-dried and reweighed for total moisture content, with separate samples re-equilibrated at 75% relative humidity for hygroscopic moisture content. Subtracting the second from the first leaves free capillary water — the only measurement that separates water rising through a wall from moisture the salt is holding out of the air. It sits behind BRE Digest 245 and BS 6576. It is slow, invasive and paid, and most jobs are settled long before it is needed.

Does a high reading on a concrete floor mean the same thing?

No, and on a slab a surface reading is measuring the wrong thing entirely. What governs whether a floor covering fails is the humidity deep within the slab, which rises once an impermeable covering traps it. That is measured with in-situ relative humidity probes drilled to 40% of slab depth and left to equilibrate — ASTM F2170, which AS 1884:2021 points at. Flooring warranties are written against that figure, not against a pin meter.

Why do damp companies keep using meters if they prove so little?

Because they are quick, they are cheap, and they produce a number, and a number is persuasive in a way that reasoning about ground levels is not. Used properly — comparatively, with a control, alongside everything else — a meter is a perfectly reasonable screening tool, and competent contractors carry one. The problem is not the instrument. It is a diagnosis that rests on the instrument alone, in front of somebody who has been given no way to check it.