Sub-floor ventilation opening area

1500 mm² per metre run of external wall, or 500 mm² per square metre of floor area — whichever is the greater

Suspended ground floor with a ventilated void

Every figure for this topic is on one page: Roof void ventilation, access and fire compartmentation

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The minimum ventilation provided to a suspended ground floor void.

Basis in regulation

Part C
Approved Document C
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How to check it on site

Count and measure airbricks on every elevation, then check the ducts through the cavity are clear and not blocked by mortar droppings or insulation.

Common failure

Airbricks correct in number but ducted through with sleeves that are blocked, crushed or bedded solid.

Effective from 20 August 2026. Last updated 23 August 2026. This is our editorial summary of publicly available regulation, not building control approval or a warranty determination.

Related dimensions at this stage

Minimum

Minimum thickness for ground-bearing floors

100 mm

Ground-bearing floor slabs, where applicable, must have a minimum thickness of 100mm

Minimum

Minimum thickness of a polythene damp-proof membrane

0.3 mm (1200 gauge)

Polythene sheet used as a damp-proof membrane below or above a ground-bearing slab.

Minimum

Ground-supported floor slab thickness

100 mm

Ground-supported concrete floor slab on hardcore

exact

Ground-bearing floors over deep fill or sulphate-rich ground

600 mm

When a concrete floor is constructed over deep fill exceeding 600mm, it should be designed as a suspended floor rather than ground-bearing

Maximum

Maximum depth of a hardcore bed under a slab

600 mm

Ground-supported concrete floor slab on a hardcore bed

Minimum

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