Minimum
England
Ventilated void depth under a suspended timber floor
75 mm to the underside of the wall plate and 150 mm to the underside of the joists
Suspended timber ground floor over a ventilated void
Clear depth required between the ground surface and the timber of a suspended ground floor.
Basis in regulation
How to check it on site
Measure at the deepest point of the oversite before the floor is decked. Check the void is clear of spoil, offcuts and mortar droppings that reduce the depth or block cross-flow.
Common failure
Oversite left high after backfill, or the void part-filled with site debris so air cannot move across it.
Last updated 23 August 2026. This is our editorial summary of publicly available regulation, not building control approval or a warranty determination.