Damp and mould investigation: finding the root cause
The main types of damp
| Type | Typical signs | Common causes |
|---|---|---|
| Condensation | Mould on cold walls, corners, windows and behind furniture; worse in cold weather | Moist air meeting cold surfaces: poor ventilation, weak or unaffordable heating, little insulation, moisture from cooking, washing and drying |
| Penetrating damp | Damp patches that follow rain, stains on ceilings or walls | Defective roofs, gutters, pointing, windows, or cracked render |
| Leaks | Local damp near pipes, baths, sinks or radiators | Failed joints, seals or waste pipes, overflowing or leaking appliances |
| Rising damp | Damp near the base of ground-floor walls, tide marks | Damp-proof course failure or bridging (less common than people think, so confirm before treating) |
Find the cause, not just the mould
- Take the report seriously and record when and how it was made.
- Inspect the home and photograph the affected areas, with measurements where you can.
- Check ventilation: extractor fans, vents and windows that open.
- Check heating and insulation: cold surfaces, draughts, heating that works and can be afforded.
- Check the building fabric: roof, gutters, walls, windows and pointing.
- Check plumbing and appliances for leaks.
- Talk to the resident, listen, and avoid blaming how they live. Look for causes you can fix in the home and the process.
Root cause groups for housing
- Ventilation: missing, broken or blocked.
- Heating and insulation: surfaces too cold or heating not working.
- Building fabric: defects that let water in.
- Leaks and plumbing.
- Earlier repairs: a fix that treated the symptom.
- Process: how reports are triaged, repairs closed and follow-ups made.
Worked 5 Whys (illustrative)
| Step | Answer |
|---|---|
| Problem | Black mould keeps returning on a bedroom external wall every winter. |
| Why 1 | The wall surface stays cold and damp. |
| Why 2 | Warm moist air condenses on the cold wall. |
| Why 3 | The bathroom extractor fan is not working, so moist air spreads. |
| Why 4 | The broken fan was reported earlier, but the job was closed without checking it ran. |
| Why 5 | The repair closure process had no check that the fix worked. |
| Root cause | No verification step when repair jobs are closed. |
Actions: repair the fan, treat and clean the mould, add a check on repair closure, and re-inspect after a few weeks.
Keep a clear record
- Date and time of the report, and how it was reported.
- Photos, measurements and the inspection date and inspector.
- The root cause, in plain words.
- Actions with an owner and a due date.
- What was told to the resident, and when.
- The follow-up check date and its result.
This guide describes a method. It is not legal advice, so check the current government guidance and your own policies for your duties.
Frequently asked questions
What causes damp and mould in homes?
Most often condensation from moist air meeting cold surfaces, but also leaks, building defects that let rain in, and sometimes rising damp. The cause decides the fix, so confirm it before treating.
How is condensation different from penetrating damp?
Condensation forms inside the home when moist air meets cold surfaces and is worse in cold weather. Penetrating damp comes from outside through defects and often follows rain.
Who is responsible for fixing damp and mould?
It depends on the cause, the type of tenancy and the law that applies. Landlords generally have duties to keep homes safe and in repair, so check current guidance or take advice for your situation.
Describe the failure or add a photo or file, and RapidRCA builds the 5 Whys, fishbone, actions and report, using the ROOTS method.
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Last updated 14 October 2026. These guides explain a method and give examples. Always follow your equipment manufacturer's manual, your site procedures and applicable regulations.