Why Suspended Timber Floors Need Airflow
A suspended timber ground floor sits above a void, with airbricks in the external wall letting air pass underneath it. That constant airflow keeps the timber joists and boards dry enough that decay can’t take hold.
Block that airflow, and moisture in the sub-floor void has nowhere to go – which is exactly the environment timber-decaying fungi need.
Wet Rot vs Dry Rot
Wet rot needs consistently damp timber to survive, and generally stays confined to the wet timber itself. Dry rot (Serpula lacrymans) is a different, more serious problem.
“Spread through a building from one timber to another.” — RICS
That’s what makes dry rot particularly dangerous: it can travel across masonry and other materials with no nutritional value to it at all, reaching new, previously unaffected timber elsewhere in the building. Wet rot can’t do this.
What Lets Decay Take Hold
Several of these are simple, everyday things – which is exactly why sub-floor ventilation gets overlooked until timber has already started to suffer.
How It’s Diagnosed & Treated
Confirming which type of decay is present matters, since dry rot needs a more thorough response – removing affected timber with a safety margin beyond the visible damage, because it can have already spread further than it looks.
Either way, treating the timber without restoring proper sub-floor ventilation only buys time. The moisture source – usually lost airflow – has to be addressed, or the conditions for decay simply return.
The Bottom Line
Sub-floor ventilation isn’t a minor detail – it’s what keeps a suspended timber floor dry in the first place. If airbricks are blocked, buried or missing, that’s worth fixing before timber decay becomes a much bigger, more expensive problem.
Frequently Asked Questions
How do I know if my airbricks are working properly?
Check they’re not blocked by plant growth, debris, paint or raised ground levels outside, and that nothing has been placed over them internally. If in doubt, a proper inspection can confirm whether sub-floor airflow is adequate.
Is dry rot really more serious than wet rot?
Yes. Wet rot generally stays confined to the timber that’s actually damp, while dry rot can spread across masonry to reach other timber elsewhere in the building, which is why it needs a more thorough response.
Can I insulate under my suspended floor without causing problems?
Yes, but the airflow underneath needs to be maintained around and beneath the insulation, not blocked by it. This is a common way well-intentioned insulation work accidentally creates a decay risk.
Does treating the timber chemically fix the problem?
Chemical treatment can deal with existing fungal growth, but it doesn’t address why the timber got damp in the first place. Without restoring proper ventilation or fixing the moisture source, decay can return.
Worried About A Suspended Floor?
Get in touch and we’ll check the sub-floor ventilation and the timber itself before recommending anything.