A moisture meter reading is a clue, not a diagnosis. Most handheld damp meters are calibrated for timber, so a percentage on wood means roughly what it says, but a number on plaster, brick or stone is only a relative reading, useful for comparing one spot with another. High readings on a wall can come from salts, foil, metal or surface condensation as easily as from water rising out of the ground.
This guide explains how the meters work, what normal readings look like, and why a “red” reading at the bottom of a wall doesn’t on its own prove rising damp.
How does a moisture meter work?
Handheld meters don’t measure water directly. They measure an electrical property of the material and turn it into a number on a display. There are two common types:
- Pin meters (resistance or conductance meters). Two metal pins are pushed into the surface and the meter measures how easily a small current passes between them. Water conducts electricity, so wetter material gives a lower resistance and a higher reading.
- Pinless meters (capacitance meters). A flat pad or sensor is pressed against the surface and measures how the material affects an electrical field a short way below it. They don’t mark the surface, which is why they’re popular for scanning walls and floors quickly.
Some surveyors also use microwave meters, which read deeper into a wall, but these are less common on a typical house inspection.
The important point is that both everyday types respond to anything that changes the electrical behaviour of the material, not just water. That’s the root of nearly every misleading reading.
Why is a damp meter scale based on wood?
Resistance meters can be properly calibrated for timber, because wood of a given species behaves in a predictable way. That’s why many meters show a percentage labelled as wood moisture equivalent (WME), or simply “% on wood”.
Masonry and plaster are different. Brick, stone, mortar and plaster vary hugely in density, porosity and the salts they contain, so a meter can’t be calibrated to give their true moisture content. When you hold a meter calibrated for timber against a plastered wall, the number tells you what the reading would mean if the wall were wood. It does not tell you the actual moisture content of the plaster.
Many meters also have a “reference” or relative scale (often 0–100, or a green, amber and red band) for use on non-timber materials. Again, that’s a comparison scale, not a measurement. The Property Care Association’s guidance on moisture meters makes the same point: on masonry, electrical meters give qualitative and comparative readings only.
Moisture meter readings chart: what different readings mean
Use this as a rough guide to interpreting readings by material, not as a pass or fail chart.
| Material | What a reading tells you | Caution |
|---|---|---|
| Timber (joists, skirtings, floorboards, roof timbers) | A reasonable estimate of moisture content, if the meter is set for wood | Species, temperature and wet surfaces affect it. Readings persistently above about 20% suggest a decay risk |
| Plaster | Only whether one area reads higher than another | Salts, foil-backed plasterboard and metal beads or pipes can all give high readings on dry plaster |
| Brick, stone and mortar | A pattern across the wall, not a moisture content | Dense stone may read low even when wet; salty or mineral-rich stone may read high when dry |
| Concrete floors and screeds | Little on its own with a basic meter | Needs different methods, such as relative humidity testing |
| Painted or wallpapered surfaces | Mainly the surface, not the wall behind | Condensation or a damp wallpaper paste layer can give a high surface reading |
What is a normal moisture reading for timber?
For timber, the key figure is about 20%. Wood-rotting fungi need wet timber to start and keep growing, and decay will generally stop if timber dries out below about 20% moisture content. The PCA’s guidance note on fungal decay in buildings describes decay as arising because wood has become wet, in excess of 20% moisture content.
In practice:
- Comfortably below 20% — normal for timber inside a dry, heated home. Little risk of decay.
- Around 20% to the mid-20s — damp enough to investigate. Look for a source: a leak, poor sub-floor ventilation, or timber in contact with damp masonry.
- Well above that, and staying there — wet timber that is at real risk of rot if the source isn’t found and fixed.
A single high reading on a skirting board can also be misleading if the skirting is fixed to salty plaster or a wet wall behind it, so readings are always taken alongside what the surveyor can see and smell. Our guide to dry rot vs wet rot explains what wet timber can lead to.
Why does my wall show damp when it isn’t?
These are the most common causes of high readings on walls that are actually fairly dry:
- Hygroscopic salts. Old walls often contain salts left behind by past dampness, chimney flues, or the stone or brick itself. Chlorides and nitrates in particular draw moisture from the air when it’s humid, and salts dissolved in moisture make plaster conduct electricity far more readily. The result is high meter readings even when the wall itself is not wet at depth. The PCA publishes a separate guidance document on hygroscopic salts and rising dampness for exactly this reason.
- Foil-backed plasterboard and dry lining. Foil behind the board, metal fixings and some backing materials conduct, so the meter reads high across the whole wall.
- Metal in or behind the plaster. Corner beads, pipes, cables and metal lath can all push readings up.
- Surface condensation. A cold wall in a humid room can be wet on the surface for hours a day, especially in the morning. The meter picks up surface water rather than moisture coming through the wall. Our humidity and dew point calculator shows whether a surface is likely to reach the temperature where condensation forms.
- Some paints, wallpapers and stones that are naturally conductive or carry minerals.
Readings can also be falsely low. Dense stones, some modern renders and a wall that’s wet deep inside but dry on the surface may not register much at all on a pinless meter.
Is a high reading proof of rising damp?
No. This is the single most important thing to understand about damp meter readings.
A line of high readings at the bottom of a wall is the classic pattern used to “diagnose” rising damp. But the same pattern can come from salts left by damp that stopped years ago, from water splashing off a hard path, from ground levels built up against the wall, from a leaking pipe, from condensation in a cold corner, or from cement render and gypsum plaster trapping moisture in a wall built to breathe. A meter can’t tell these apart, and it can’t tell where the moisture came from.
That’s why the PCA’s guidance warns against diagnosing rising damp from meter readings alone. Our guides to what rising damp actually is and telling rising damp, condensation and penetrating damp apart explain what else needs to be checked.
If you’ve been handed a quote for a chemical damp proof course based mainly on meter readings, it’s worth reading our guide to getting a second opinion on a damp proofing quote.
How surveyors use moisture meters properly
Used well, a meter is a very good mapping tool. A careful surveyor will:
- Take readings in a grid or profile, at several heights and along the wall, to see the shape of the damp. Rising damp, condensation and a leak each tend to leave a different pattern.
- Compare like with like — the same material, inside and outside the suspect area.
- Check the outside for the likely source: gutters, downpipes, ground levels, render, pointing and plumbing.
- Measure air temperature, humidity and surface temperature to judge condensation risk.
- Take samples where the readings are unclear. Drilled masonry samples can be tested by the calcium carbide method, or weighed, dried and reweighed, to find the real moisture content and to separate genuine water from salts.
We explain the full process in what happens in a damp survey.
Yorkshire walls and meter readings
Older homes across Yorkshire and the Pennines are often built of sandstone or gritstone, with solid walls, lime mortars and, in many cases, decades of later cement repairs, gypsum plaster and chimney breasts that carried years of coal smoke. That combination is prime territory for salts, trapped moisture and confusing meter readings. A solid stone wall can read high at low level for reasons that have nothing to do with a failed damp proof course, and plenty of these houses never had one in the first place.
Should you buy a moisture meter?
A cheap meter can be useful for checking timber, for example to see whether a floor or roof timber is drying after a leak has been fixed. On walls, treat it as a comparison tool: is this patch higher than that one, and is it changing over time? Don’t use it to decide whether a wall needs treatment.
If you’re worried about a damp patch, our damp checker helps you narrow down the likely cause before you spend anything.
Getting an independent answer
Our surveyor is a PCA Qualified Technician in damp and timber, and we sell no treatments or repairs, so our advice is based only on what the building needs. An independent damp and timber survey interprets meter readings alongside the outside of the building, the history of the property and, where needed, sampling, and the fee is fixed and quoted for the property. Get in touch with the address and a few photos.
Sources and further reading
- Guidance document: the use of moisture meters to diagnose dampness in buildings — Property Care Association
- Guidance document: hygroscopic salts and rising dampness — Property Care Association
- Guidance note: fungal decay in buildings — Property Care Association
- Measuring moisture in masonry — Building Conservation
- Damp in historic buildings — Historic England