What does a damp meter actually measure, and can I trust the reading?

Almost every damp survey in the country involves someone walking a wall with a two-pin meter and reading numbers off it. The instrument is a genuinely useful tool used the way a torch is useful — it shows you where to look. Where it goes wrong is when the number on the dial is presented as a measurement of how much water is in your wall, because on masonry it is not measuring that at all.

Step by step

1. Know what the two types do.A pin meter passes a tiny current between two electrodes and reads resistance. A capacitance or 'search' meter reads the dielectric properties of the material just under the surface without pins. Both infer moisture; neither weighs it.
2. Understand where the scale is honest.Pin meters are calibrated against the moisture content of timber, usually softwood. On timber, a reading is meaningful: below about 18 per cent is safe, above about 20 per cent sustained will support decay fungi. On anything else the scale is arbitrary.
3. Know the false positives.Hygroscopic salts, foil-backed plasterboard, expanded metal lath, plaster containing carbon black, some emulsions, damp-proof membranes, wallpaper adhesive residues and embedded cables all read high on a dry wall.
4. Know the false negatives too.A dense cement render can be soaking behind and read low at the surface, and a capacitance meter reads only the first few millimetres. A low reading does not prove a wall is dry.
5. Use it comparatively.Take a reading on a wall you know is dry — an internal partition away from any suspect area — and use it as your baseline. Then work along the suspect wall and note where readings change, not what they are.
6. Map, do not measure.Sketch the wall and mark the readings. A pattern that stops at a level line, a pattern that follows a downpipe, a pattern that is highest behind a wardrobe — the shape tells you far more than any single number.
7. Test the timber, not the plaster, when it matters.If the question is whether a joist end or a floorboard is at risk, put the pins in the timber. That is the one reading the instrument is designed to give.
8. Confirm anything important by a different method.A carbide meter test or a laboratory gravimetric test on a drilled sample tells you how much water is actually in the masonry. That is what a genuine dispute needs, and it is what an independent surveyor can arrange.

Tips

• A cheap meter and an expensive one work the same way and share the same limitations on masonry. Spending more does not buy you a valid reading on plaster.
• If a wall reads high, is not cold, has no mould, and is highest in warm humid weather, suspect salts rather than water.
• Never let anybody drill a wall for a reading without agreeing where and how many holes first — it is invasive and the holes are yours to fill.

Common mistakes

• Quoting a plaster reading as a moisture percentage — The percentage scale is a wood moisture content scale. Applying it to plaster produces a number with no physical meaning, which is then used to justify expensive work.
• Testing through wallpaper or paint — The coating changes the reading. Foil-backed and vinyl papers in particular can produce very high readings over a completely dry wall.

Questions people ask