Ground-supported floor slab thickness

100 mm

Ground-supported concrete floor slab on hardcore

Minimum thickness of in-situ concrete in a ground-supported ground floor slab.

Basis in regulation

Part C
Approved Document C
Read the source

How to check it on site

Check depth pegs or level markers across the pour, not just at the perimeter, and confirm the membrane has not been pushed up into the slab depth.

Common failure

Slab thinned in the middle of the bay where the hardcore has been over-filled.

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.

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

Minimum

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

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