Maximum depth of a hardcore bed under a slab

600 mm

Ground-supported concrete floor slab on a hardcore bed

Greatest depth of well-compacted hardcore that may be used under a ground-supported floor slab without a specific engineering design.

Basis in regulation

Part C
Approved Document C
Read the source

How to check it on site

Check the reduced-level dig against the finished slab level before hardcore is placed. Deeper fill needs a designed solution rather than more hardcore.

Common failure

Over-dig made up with additional hardcore, giving long-term settlement under the slab.

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

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

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