A leak gets fixed, everything looks dry, then black spots appear along the skirting three weeks later. This is one of the most common calls we get, and it has a simple cause: the wall is still wet inside.
Why it happens
Masonry and screed hold litres of water per square metre. Surface-dry paint can be hiding a saturated core that keeps humidity high in the room, exactly what mould spores need. Airflow-shadowed spots (behind furniture, in corners) show it first.
What to do
- Confirm the leak is actually fixed: a partial repair or a second leak is common. Detection rules it out.
- Measure, don’t guess: moisture mapping shows how far the wetness extends and tracks genuine drying over time.
- Treat the mould professionally: treatment removes the growth safely; the moisture evidence stops it returning.
- Document for insurance: if the damage is part of a claim, an evidence report ties the mould to the leak event properly.
Not sure whether what you’re seeing is leak-related or condensation? A mould inspection answers it with readings, not guesses.
Think this is happening in your home? 📞 Call free: 0800 054 1524 · 💬 WhatsApp: 07733 019492 · or send us a message. We cover London & Surrey, every inspection is quoted based on the job and the price is confirmed before we attend.
Last reviewed: 27 October 2026. Independent investigation and written reports. Repairs are quoted separately.