Mould after a water leak — why does it happen?

Mould spores are present in every building, all the time. They only become a problem when they land somewhere with enough moisture, a surface they can feed on, and reasonable warmth. After a leak, the moisture arrives in quantity and the other two conditions are usually already there.

Plasterboard paper, timber, wallpaper paste, dust, carpet backing and insulation all provide something for mould to grow on. Remove the moisture and growth stops; leave the moisture and it continues, whatever you clean.

The single biggest factor is how long materials stay wet. A property dried promptly after an escape of water often shows no mould at all. The same property left for a fortnight — while an insurer is contacted, a plumber is arranged and everyone waits — frequently does.

This is one practical reason to start assessment and drying early, even while the wider claim or repair is still being sorted out.

When mould appears weeks after everything looked fine, the moisture is nearly always somewhere out of sight:

• Insulation in a ceiling void or wall cavity that soaked up water and never dried
• The back face of plasterboard, where air never reaches
• Floor voids and joists under a re-laid floor covering
• Carpet underlay that was dried on top but not underneath
• Screed or concrete that still holds moisture below a new floor finish
• Behind fitted furniture, where there is no air movement at all

After water damage, a property often holds far more moisture in the air than usual. When that moist air meets cold surfaces — external walls, window reveals, corners, behind wardrobes — it condenses, and mould appears in places that were never touched by the original leak.

Steady warmth and sensible ventilation help, but during an active drying programme follow the guidance for that setup. Opening windows while dehumidifiers are running works against the equipment.

Wiping mould off a wall with a household product removes what you can see. If the plaster behind it is still damp, growth returns — often within weeks, sometimes in a slightly different spot. Painting over it with mould-resistant paint has the same limitation: it addresses the surface, not the cause.

The order that works is: find the moisture, stop it, dry the materials properly, then deal with the affected surfaces and finishes.

1. 1Identify the moisture. Establish which materials are wet and how far the water travelled.
2. 2Resolve the source. Drying cannot succeed while water is still getting in.
3. 3Remove what will not dry. Saturated insulation and some soft materials are better removed than dried in place.
4. 4Dry the structure. Controlled dehumidification and air movement, monitored with readings.
5. 5Treat affected surfaces. Deal with growth appropriately once the substrate is dry.
6. 6Reinstate. Redecorate or replace finishes only after readings support it.

We are drying specialists, not medical professionals, and we are not going to make claims about health outcomes. What we can say plainly is that damp indoor environments are generally undesirable, that some people are more sensitive to mould than others, and that if anyone in the property is concerned about symptoms, that is a conversation for a GP. The practical action either way is the same: remove the moisture.