Quality.
    Minimum
    England

    Minimum thickness for ground-bearing floors

    100 mm

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

    Ground-bearing floor slabs, where applicable, must have a minimum thickness of 100mm. This measurement includes any float finish applied to the surface.

    Basis in regulation

    How to check it on site

    Check the thickness of ground-bearing slabs during construction to ensure it meets the 100mm minimum requirement.

    Common failure

    Slabs are cast too thin, leading to reduced strength and increased risk of cracking under load.

    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 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

    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

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