Find the cause of damp, not just the stain.
Moisture, temperature and humidity readings that point you to the real problem. Tap a problem to jump to it.
Damp walls
Why it happens
Water gets into walls in four main ways: condensation, penetrating damp (gutters, roofs, render, window seals), leaks from pipes ("traumatic" damp) and rising damp. More than one can happen at once.
How to tell it apart
A tide mark or salt band starting at floor level and spreading upwards suggests rising damp, which the Westmorland and Furness Council guide calls the rarest form. Patches around windows, chimneys or below gutters point to penetrating damp. Mould in corners with dry wall readings points to condensation (see below).
How to test
- Take readings on a grid, for example every 100 mm up from the skirting and across the patch.
- Compare them with a wall you know is dry in the same room.
- Mark the pattern on a sketch: rising from the floor, around an opening, or isolated.
On plaster and masonry an electrical meter gives a relative reading, so use it to map where it is wetter, not as an exact moisture figure. A pinless meter such as the QT700I is quick for mapping.
What next
Fix the source you find (gutter, seal, leak, ventilation) before redecorating. For suspected rising damp, the council guide advises getting professional advice.
Common mistakes
Treating one high reading as proof of rising damp (salts and foil-backed finishes also read high); not comparing with a dry wall; redecorating before the source is fixed.
Timber at risk
Why it happens
Skirting, joists and window frames next to a damp wall or leak stay wet, and wood-rotting fungi need that moisture.
How to tell it apart
Dark wood that cracks into cubes and crumbles is a sign of rot that has been going on for a long time. Paint that lifts to show soft wood behind can mean wet rot.
How to test
- Set the meter's species setting and take at least 3 readings per piece, including the end nearest the wall.
- Compare with the same timber away from the damp area.
- Re-test after the source of the damp has been fixed.
Good and bad readings
| Timber moisture content | What it means |
|---|---|
| Below 20% | Rot is unlikely to develop; existing decay stops |
| 20% or above | At risk of fungal decay |
Source: Historic Environment Scotland, Engine Shed: Timber ("Decay will cease if the level of moisture in the timber falls below 20%").
What next
Remove the source of moisture first. Weakened timber needs replacing; ask a surveyor who is independent of any treatment contractor.
Common mistakes
Measuring only the painted face; treating with chemicals without fixing the moisture source.
Condensation & mould
Why it happens
Cooking, showers and drying clothes add moisture to the air. When it meets a cold surface it condenses, and repeated condensation leads to mould.
How to tell it apart
Black spots in corners, behind furniture or along ceiling edges, with dry readings inside the wall, usually mean condensation rather than a building leak.
How to test
- Log the room's relative humidity for several days; a single reading can mislead.
- Note when it peaks (after showers, cooking or overnight).
- Check the surface temperature of the cold wall (see Cold spots).
A monitor such as the ZN-MT13 logs humidity over time.
Good and bad readings
| Indoor relative humidity | What it means |
|---|---|
| 40%–65% average | The recommended range |
| 70%–80% | Mould spores multiply |
Source: Westmorland and Furness Council, Damp and mould guide, which also notes that temperatures below 16°C encourage mould growth.
What next
Ventilate (extractor fans, trickle vents, opening windows), heat rooms evenly and cut indoor moisture. For rented homes, the GOV.UK damp and mould guidance says landlords must find and fix the underlying cause.
Common mistakes
One snapshot reading; wiping mould without fixing the cause; blocking vents to stop draughts.
Cold spots & heat loss
Why it happens
Missing insulation and cold bridges (lintels, wall–ceiling junctions) make some surfaces much colder than the room. That is where condensation forms first.
How to tell it apart
A dark line of mould along a wall–ceiling junction or around a window reveal usually marks a cold bridge rather than a leak.
How to test
- Scan when it is clearly colder outside than inside.
- Look for cold patches, lines and corners on the thermal image.
- Note the surface temperature at each spot and compare with the room.
Thermal imaging multimeters such as the ZT-R01 or TDM-120P show cold spots quickly.
What next
Insulation and ventilation fixes. Recheck after work.
Common mistakes
Scanning shiny surfaces, which reflect heat; scanning on a mild day when there is little temperature difference.
Studs & cables
Why it happens
Opening up a wall, fixing a sensor or drilling for a survey means going through whatever is behind the plaster.
How to tell it apart
A detector signal can be a cable, a pipe, a stud or a nail. Cables usually run straight up, down or sideways from sockets and switches.
How to test
- Calibrate the detector on a clear part of the wall.
- Scan slowly in two directions and mark the edges of each signal.
- Never drill in line with a socket or switch.
A wire and stud detector such as the WD-01 does this.
What next
If in doubt, isolate the circuits feeding that wall before drilling, and use a depth stop.
Common mistakes
Moving the detector too fast; trusting it alone where cables may be in metal conduit or deep in the wall.
Instrument for this step
WD-01 Wall Stud and Wire Detector£34.99
FAQs
Can a moisture meter prove rising damp?
No. On masonry it gives a relative reading. Use it to map the pattern, then look for the cause.
Do I need a thermal camera for damp?
It helps you find cold spots and hidden leaks quickly. Always confirm with a moisture reading.
Can I get a VAT invoice?
Yes. All prices include VAT and VAT invoices are available.
Need help choosing?
Contact us and tell us what you need to measure.
Photo credits: Timber at risk: photo by Mätes II., CC BY-SA 3.0, Wikimedia Commons, Braunfaeule Holz FI20070208.jpg (https://commons.wikimedia.org/wiki/File:Braunfaeule_Holz_FI20070208.jpg); Hidden leaks: photo by Raimond Spekking, CC BY-SA 4.0, Wikimedia Commons, Wasserschaden Kölnisches Stadtmuseum Juni 2017-5560.jpg (https://commons.wikimedia.org/wiki/File:Wasserschaden_K%C3%B6lnisches_Stadtmuseum_Juni_2017-5560.jpg); Damp walls: photo by Patrick Oberdörfer, CC BY-SA 4.0, Wikimedia Commons, Feutigkeitsschaden innen.jpg (https://commons.wikimedia.org/wiki/File:Feutigkeitsschaden_innen.jpg); Condensation & mould: photo by NordhornerII, CC BY-SA 4.0, Wikimedia Commons, Black mold in a building.JPG (https://commons.wikimedia.org/wiki/File:Black_mold_in_a_building.JPG)