What Is the Minimum Concrete Thickness for an Insulated Raft Foundation?

Q: What Is the Minimum Concrete Thickness for an Insulated Raft Foundation?

200mm for most houses. 250mm for taller buildings. If a supplier is showing you anything less, ask them to explain why. Structural calculations alone might suggest a thinner slab is adequate, but Building Regulations require a minimum concrete depth for fire resistance reasons — and those requirements apply across England, Scotland, Wales, and Northern Ireland.

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200mm for most houses. 250mm for taller buildings. If a supplier is showing you anything less, ask them to explain why. Structural calculations alone might suggest a thinner slab is adequate, but Building Regulations require a minimum concrete depth for fire resistance reasons — and those requirements apply across England, Scotland, Wales, and Northern Ireland.

Comparison of 100mm, 150mm and 200mm insulated raft slab thicknesses and their fire-resistance performance — Comparison of slab depths and the illustrated effect on fire-resistance performance.

The Complete Answer Why the ground floor is involved at all — and why the concrete depth matters more than it first appears. The fire performance of building structures is governed by Requirement B3 of the Building Regulations, which states that a building must be designed and constructed so that, in the event of fire, its structural stability will be maintained for a reasonable period. For most two-storey domestic buildings, the required period is 30 minutes, expressed as REI 30. REI 30...

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