A tenant phones the lettings office about black spots in the bathroom corner. By the time the call happens, the mould has been growing for somewhere between four and twelve weeks, the affected wallboard is compromised, and the cleanup is no longer a wipe-down — it is a remediation.
Every line of that call costs you money.
Mould grows on conditions, not on calendars.
The recipe is straightforward. Surface temperature within 2 °C of dewpoint, sustained, for roughly two weeks. Moisture-loving spores (Cladosporium, Penicillium, Aspergillus) need nothing more. They are airborne everywhere already; they wait for surfaces to invite them.
The leading indicator is humidity, not damage.
- Bathroom relative humidity above 65% for more than four hours after a shower, day after day, is the warning.
- A bedroom that runs above 60% RH overnight, every night, is the warning.
- A kitchen with no extraction running during cooking is the warning.
You see the warning two to four weeks before the tenant sees the spots.
What landlords typically do.
- Wait for the call. Pay £200–£900 to remediate. Repaint. Carry the void cost.
- Add a tenant-facing "open the windows" leaflet. Hope.
What a measured fleet does.
- A humidity sensor in every bathroom and bedroom of the property — a one-time hardware cost.
- Threshold alerts to the property manager, not the tenant. The tenant did nothing wrong; they need a humidistat extractor or a trickle vent, not a lecture.
- A 14-day humidity trace as part of the deposit return paperwork. The data protects both sides.
The mould call is the most expensive piece of correspondence in the property business. Almost all of it is preventable, and the prevention is a notification, not a leaflet.