Damp diagnosis tool
Three quarters of the damp treated in British homes is treated as the wrong kind. Six questions will get you a lot closer than a moisture meter will.
Last updated July 2026 · built by working UK builders
The four kinds of damp, and how often each one actually occurs
In British housing, condensation is by far the most common, penetrating damp second, internal leaks third, and true rising damp a distant fourth. The damp-proofing industry's diagnostic order tends to run the other way, which is worth knowing before anyone quotes you for injection.
Condensation
A family of four puts several litres of water into the air every day through breathing, cooking, showering and drying clothes. If it cannot get out, it lands on the coldest surfaces — window reveals, external corners, behind wardrobes — and mould follows. The fix is ventilation, gentle background heat and insulation, in that order.
Penetrating damp
Water getting through the outside of the building. There is always a source and it is usually visible from the ground with a pair of binoculars on a wet day. Gutters and downpipes cause more penetrating damp than every other defect combined.
Internal leaks
Waste pipes, shower trays, radiator valves and heating pipes under floors. Constant, localised and completely indifferent to the weather. The boiler pressure gauge and the water meter will often confirm it in an afternoon.
Rising damp
Real, but uncommon, and much more often mimicked by a bridged damp-proof course. Before accepting a diagnosis, check that the external ground level is at least 150mm below the DPC and that render, paths, decking and flower beds are not carrying moisture across it.
Before you accept any damp quote
- Ask what caused the damp, not just what treats it. If there is no answer, walk away.
- Use an independent damp surveyor who does not carry out remedial work.
- Be wary of a "free damp survey" from a company that sells treatment — the incentive is not subtle.
- Never replaster before the wall is dry. A month per 25mm of thickness, and longer in winter.
- On any pre-1919 solid wall, insist on breathable lime plaster and breathable paint. Cement render and plastic paint on a solid wall cause the problem they are sold to solve.
Frequently asked questions
How do I tell rising damp from penetrating damp?
Rising damp stops at a fairly level line under about a metre, comes with a salt tide mark, and is present all year. Penetrating damp appears at any height, follows heavy rain, and usually has an identifiable external cause above or beside it — a gutter, a crack, a failed sill.
Is black mould dangerous?
It is a respiratory irritant and matters most for children, older people and anyone with asthma. It is also almost always a symptom of condensation rather than a building defect, which means it is fixable with ventilation, heating and insulation rather than expensive works.
Do I need a damp proof course injection?
Far less often than the industry suggests. Chemical DPC injection treats one specific cause — a failed damp-proof course — and does nothing for bridging, penetrating damp or condensation, which between them account for most damp in UK homes. Always get a diagnosis from someone who is not also selling the treatment.
My survey says damp was recorded by a moisture meter. Is that reliable?
Only partly. Protimeter-style meters measure electrical conductivity, and hygroscopic salts left in old plaster from historic damp conduct extremely well even when the wall is bone dry. A high reading is a prompt to investigate, not a diagnosis.
How long does a wall take to dry out?
Roughly a month per 25mm of wall thickness once the source is fixed, and longer in winter. A solid nine-inch brick wall can take six months. Replastering before it has dried simply traps the moisture and the problem reappears.