Damp is one of the most commonly flagged issues in UK house surveys — and also one of the most misunderstood. Here's exactly how surveyors detect moisture, what the different types of damp mean, and how seriously to take a high reading.
The primary tool is a damp meter (often a Protimeter Surveymaster). This device uses electrical resistance — damp materials conduct electricity better than dry ones. The meter is pressed against walls, floors, and skirting boards, and gives a reading indicating moisture level:
| Reading | Interpretation | Colour (LED) |
|---|---|---|
| Below 15% | Dry — no action needed | 🟢 Green |
| 15–20% | Borderline — monitor | 🟡 Amber |
| Above 20% | Damp — further investigation recommended | 🔴 Red |
Important: High readings don't always mean active damp. New plasterwork, recently painted walls, metal behind plaster (pipes, lintels), foil-backed insulation, and salts in old plaster can all cause false high readings. A good surveyor will assess context, not just the number.
Surveyors look for visible signs of damp: tide marks (horizontal staining), black mould, peeling wallpaper, blistering paint, efflorescence (white salt deposits), and rotting timber at floor level. The pattern and height of staining helps distinguish rising damp from penetrating damp.
Some Level 3 surveys and specialist damp surveys use thermal imaging cameras. Cold bridges and areas of moisture show as temperature differentials on the thermal image, helping identify the source and extent of damp problems that aren't visible to the naked eye.
What it is: Groundwater rising up through walls via capillary action, usually when the damp-proof course (DPC) has failed, is absent, or has been bridged.
Signs: Tide marks up to about 1m high, salt deposits (efflorescence), damaged skirting boards, bubbling plaster at low level.
Treatment cost: £2,000–£5,000+ depending on the extent. Involves injecting a chemical DPC and replastering. Note: true rising damp is less common than many damp companies claim.
What it is: Water entering through the building fabric from outside — failed pointing, cracked render, leaking gutters, window surround gaps, or porous brickwork.
Signs: Damp patches at any height (not just low level), often worse after heavy rain. Can appear on any wall, including upper floors.
Treatment cost: Varies widely — £200–£500 for minor pointing repairs; £2,000–£10,000+ for new render or roof repairs if the source is major.
What it is: Warm, moist air hitting cold surfaces (walls, windows) and condensing. Most common in kitchens, bathrooms, and poorly ventilated rooms.
Signs: Black mould spots (especially in corners, around windows), steamed-up windows, musty smell. Often worst in winter.
Treatment cost: Often very cheap — improved ventilation (extractor fans, trickle vents), better insulation, or a positive pressure ventilation (PPV) unit (£300–£700). This is the most common cause of damp in modern homes.
Many damp treatment companies offer "free damp surveys" — but these aren't independent. They almost always recommend treatment (their product). Always use an independent RICS-qualified specialist or a building surveyor for an unbiased second opinion on damp.
Compare CIOB, RICS and RPSA accredited surveyors — thorough damp checks included in every Level 2 and Level 3 survey.
Get Free Survey Quotes →