Damp meter readings explained: what is normal for walls?

How damp meters work

Pin meters have two metal pins you press into the surface. They measure how easily a small current flows between them. Water conducts electricity, so wetter material gives a higher reading. They are calibrated for timber, giving a moisture content figure that is accurate for some woods.

Pinless meters use a pad pressed flat against the surface and measure changes in an electrical field a short depth into the material. They do not mark the wall and are quick for scanning large areas, but they read only the outer layer and are affected by density.

On plaster, brick and concrete, both types give a relative reading. Many professional meters show this as wood moisture equivalent, meaning the moisture a piece of timber would have if it were in contact with that material.

What the numbers mean

Meter makers usually colour-code the scale. On a common WME scale, readings below about 17 % are shown as dry, 17 to 20 % as at risk or borderline, and above 20 % as damp. Cheap meters often show a percentage without saying what scale it is, so use them for comparisons rather than as absolute values.

The pattern matters more than any single figure. Rising damp tends to give high readings at the bottom of a wall that fall off sharply above a certain height. Condensation gives high surface readings in cold corners and behind furniture. Penetrating damp gives a patch that matches an outside fault, such as a leaking gutter.

Why readings can mislead

Salts left in plaster by past damp, or from the ground, conduct electricity even when dry, so the meter reads high. Foil-backed plasterboard, metal beads and some wallpapers and paints do the same. A wall can therefore show a high reading for years after the original damp has gone. Surveyors use other methods, such as drilling samples, when they need to know the true moisture content of masonry.

Equally, a meter reads only the surface or a short depth. A wall that reads dry on the surface can be wet deeper in, which is why plaster after a leak needs time before decorating.

How to take useful readings

First, find a dry reference: an inside wall in a heated room. Take readings there. Then take readings on the problem wall in a grid, for example every 20 to 30 cm across and up from the floor. Mark them on a sketch. Repeat in the same spots over days or weeks to see whether things are improving.

Take readings at the same time of day, because condensation varies between morning and evening. Note the room's humidity on a hygrometer too.

1. 1

   Read a known dry inside wall as a reference

2. 2

   Read the problem wall in a grid

3. 3

   Sketch the readings on a plan

4. 4

   Repeat in the same spots over time

5. 5

   Note room humidity and time of day

See dehumidifiers for condensation →

Using readings to decide what to do

High readings only near cold corners and behind furniture, with condensation on windows, point to humidity: ventilate, heat steadily and use a dehumidifier. High readings from the floor up to around a metre on a ground floor wall need a closer look at the damp proof course and ground levels; see what a damp proof course is. A localised patch under a window or near a gutter points to penetrating damp. Our guides on signs of damp walls and how to treat damp walls cover each.

Be cautious if a company uses a meter reading alone to recommend expensive treatment. Ask for an independent survey from someone who does not sell the remedy, such as a surveyor with a recognised qualification.

Frequently asked questions

What is a normal damp reading on a wall? On a typical WME scale, below about 17 % is usually treated as dry. Compare with a known dry inside wall in your home.

Are damp meters accurate on walls? They are accurate on timber. On plaster and masonry they give relative readings, useful for comparisons and patterns but not exact moisture content.

Why does my wall still read damp after it has dried? Salts left behind by past damp, foil-backed plasterboard or metal in the wall can all give high readings when the wall is dry.