Moisture meter readings show how much water is in a material. For wood, a normal indoor reading is about 6 to 14%. For walls and plaster, meters show a "wood moisture equivalent" (WME), not a true percentage. Under roughly 17 to 20% WME reads as dry, and above 20% suggests excess moisture. Persistent or spreading high readings can mean a hidden leak.
A moisture meter turns an invisible problem into a number, but the number only helps if you know what "normal" looks like. Get it wrong and you either panic over a perfectly dry wall or, worse, ignore a reading that's quietly telling you a pipe is leaking behind the plaster. This guide explains what the readings mean, what counts as normal for wood, walls and concrete, what a genuinely bad reading looks like, and when a high reading is pointing at a hidden leak rather than the weather.
On this page
- What a moisture meter actually measures
- What is a normal moisture reading?
- Reading ranges by material
- Why wall percentages aren't a true moisture content
- What counts as a bad reading?
- When a high reading means a hidden leak
- How to take an accurate reading
- When to call a professional
- Frequently asked questions
What a moisture meter actually measures
A moisture meter estimates the water content of a material as a percentage, the ratio of water to dry material. There are two common types. Pin-type meters push two metal pins into the surface and measure electrical resistance between them; wetter material conducts more easily. Pinless meters press a flat sensor against the surface and read moisture a short way below using an electromagnetic field, which is handy when you don't want to leave pinholes.
The catch is calibration. Most meters are calibrated for wood. On timber the percentage is a fair estimate of true moisture content. On plaster, brick or concrete the same meter gives a relative figure, useful for comparing one spot against another, but not an exact moisture content of the wall. That single fact explains most of the confusion around readings, and we come back to it below.
What is a normal moisture reading?
"Normal" depends on the material, the season and where in the building you're testing. As a working baseline for a typical UK home:
- Wood: indoor joinery and furniture usually sit around 6 to 8%, with general indoor timber up to about 14% depending on humidity. Coastal homes across Cornwall and Devon often run a touch higher.
- Walls and plaster: on the wood moisture equivalent (WME) scale most meters use, below roughly 17 to 20% reads as dry. Readings creeping above 20% are worth a closer look.
- Concrete and screed: notoriously slow to dry and best judged by trend rather than a single figure. A fresh slab can read high for months and still be drying normally.
The single most useful habit is to take a baseline. Measure an area you know is dry, then compare the area you're worried about. A difference of several points between a suspect wall and a known-dry one nearby tells you far more than any one number in isolation.
Reading ranges by material
The table below brings the verified ranges together. Treat the wall and concrete figures as WME (relative) guidance, not absolute moisture content.
| Material | Typically dry / normal | Worth investigating |
|---|---|---|
| Wood & timber (true %MC) | 6 to 14% (joinery 6 to 8%) | Above ~18 to 20% (decay risk) |
| Plaster & masonry walls (%WME) | Below ~17 to 20% | Above 20% (20 to 27% moderate, 27%+ high) |
| Drywall / plasterboard (%WME) | Low single figures | Sustained high vs a dry reference |
| Concrete & screed | Judge by downward trend | Readings rising or not falling over weeks |
For a printable, at-a-glance version of these bands, see our damp meter readings chart, which lays the colour zones out material by material.
Why wall percentages aren't a true moisture content
This is the point most homeowners miss. When you test a wall with a standard meter, the percentage shown is a wood moisture equivalent, what the reading would mean if the material behaved like timber. Because brick, plaster and concrete hold and conduct moisture differently from wood, the same number means very different things. On a manufacturer's reference scale, a reading of around 4% is bone-dry for wood but already "damp" for brick; a reading of 12% is air-dry timber yet effectively saturated brick.
So a wall reading of "20%" is not 20% water by weight. It's a relative signal that the material is wetter than air-dry. That's exactly why a meter is a brilliant screening tool and a poor diagnostic one on its own. It tells you where to look harder; it doesn't, by itself, tell you why the wall is wet. Condensation, rain penetration, rising damp and a plumbing leak can all push the needle into the red.
What counts as a bad reading?
On the WME scale used for walls, a widely used rule of thumb among UK damp surveyors is: below 20% is low, 20 to 27% is moderate, above 27% is high, and readings above roughly 35% are a genuine concern. Most meters mirror this with a traffic-light display: green for safe air-dry (below about 17), amber for borderline (around 17 to 20), and red for damp (above 20).
For wood, the threshold that matters is decay. Timber that stays above about 18 to 20% moisture content over time becomes susceptible to mould and wet rot; most wood-rotting fungi need the timber above roughly 20% to take hold, and conditions become ideal once it climbs toward 30%. The key word is persistent. A brief splash that dries within a day rarely causes harm. It's slow, hidden damp keeping wood wet for weeks or months that turns into rot.
One number on its own rarely settles anything. A wall might read borderline at the surface from a bit of condensation yet be much wetter deeper in from a leak, which is why depth and pattern matter as much as the headline figure.
When a high reading means a hidden leak
A meter can't tell condensation from a leaking pipe. The story is in how the reading behaves over time. Lean towards a hidden leak when:
- The damp patch grows when you mark its edge in pencil and recheck a couple of days later.
- Readings track with water use, higher after baths, washing or heating runs.
- The damp rises from low down or appears away from windows and cold spots where condensation usually forms.
- It's paired with a rising water bill or a warm patch on the floor.
Lean towards condensation or rain ingress instead when readings spike on cold mornings, cluster around windows and cold corners, or jump after heavy rain (often a sign of failed pointing or render rather than plumbing). If you've spotted staining but can't pin down the source, our guide on finding a leak in walls or ceilings walks through the checks, and where damp is coming up through a concrete floor we cover that specific pattern separately. A persistent, spreading high reading with no obvious cause is the classic fingerprint of a leak that needs professional water leak detection to locate without tearing the room apart.
How to take an accurate reading
- Set a baseline. Measure a spot you know is dry first, so you have something to compare against.
- Use the right material setting. If your meter has scales for wood, plaster or masonry, select the correct one, because the same surface reads differently on each.
- Test several points. Take readings at different heights and across the area, not just one. Damp has a shape.
- Check depth where you can. Surface moisture can be condensation; deeper moisture points to penetration. Pin meters of different lengths, or a pinless meter, help you read below the surface.
- Note the conditions. Record the date, the weather and the location. A reading taken after a downpour tells a different story from one in dry spell.
- Re-test over time. One reading is a snapshot; a series tells you whether things are drying out or getting worse.
When to call a professional
A meter is the start of the investigation, not the end of it. It's worth bringing in a specialist when readings stay high or keep climbing despite ventilation and drying, when the damp is spreading, when you suspect a leak behind a solid floor, wall or ceiling, or when you need documentation an insurer will accept. We use non-invasive equipment (thermal imaging, acoustic sensors and tracer gas alongside the meter) so we can pinpoint the source with the least possible disruption, then provide the report your claim needs. Where the damage is being claimed, an insurer-ready trace and access report records how the leak was found and what access was required.
Frequently asked questions
What is a normal moisture meter reading?
For wood, a normal indoor reading is roughly 6 to 14%, with joinery and furniture sitting around 6 to 8%. For walls and plaster, meters show a wood moisture equivalent (WME) rather than a true percentage. Below about 17 to 20% reads as dry, and above 20% suggests excess moisture worth investigating.
What is a bad moisture reading for walls?
On the WME scale most meters use for walls, anything consistently above 20% is treated as damp and worth investigating. UK damp surveyors typically read below 20% WME as low, 20 to 27% as moderate, and above 27% as high. The pattern across the wall matters as much as any single number.
Are moisture meter wall percentages an exact moisture content?
No. On plaster, brick and concrete an electrical meter is calibrated for wood, so the figure is a wood moisture equivalent: a relative guide, not the true moisture content of the wall. The same 20% reading means very different things on timber versus plaster, which is why context and a trained eye matter.
At what reading does mould or decay become a risk?
Timber that stays above about 18 to 20% moisture content over time becomes susceptible to mould and wet rot, and most fungi need wood above roughly 20% to take hold. Brief wettings that dry quickly rarely cause harm. It's persistent, hidden dampness that turns into decay.
Does a high moisture reading always mean a leak?
Not always. Condensation, rain penetration through failed pointing or render, and rising damp can all raise readings. A leak is more likely when the damp patch grows over time, rises rather than falls, or tracks with water use. Persistent or spreading high readings with no obvious cause are worth a professional check.
Can I rely on a cheap DIY moisture meter?
A budget meter is fine for spotting a change, like comparing a damp-looking area against a known-dry one nearby. It's far less reliable for an absolute reading on walls, as most are calibrated only for wood. For diagnosing a suspected leak or supporting an insurance claim, professional detection and documentation are more dependable.
High readings you can't explain? Let's find the source
If your moisture meter keeps showing damp with no obvious cause, we'll trace it across Cornwall & Devon using non-invasive equipment, and give you the insurer-ready report if you need to claim. Fast response, minimal damage.
Call Dickie on 07822 025 911 No Find, No Fee on residential leak detection (subject to terms)