What is a raft foundation?

A raft foundation is a large concrete slab that covers the entire footprint of a building, spreading the load over a large area. Raft foundations are used on poor ground (clay, made ground, areas near trees) where strip foundations would need to be excessively deep. They typically consist of a 200-300mm slab with thickened edge beams (450mm+) and reinforcement mesh top and bottom.

How thick should a raft foundation be?

A typical domestic raft slab is 200-250mm thick with edge beams 450-600mm deep and 450mm wide. The edge beams are thickened to resist loads from the external walls. Internal thickenings may also be needed under load-bearing walls. The exact dimensions depend on soil conditions and building loads — a structural engineer must design the raft.

What reinforcement goes in a raft?

Raft foundations typically have two layers of mesh reinforcement — top and bottom. A252 (8mm @ 200mm) is common for domestic rafts; A393 (10mm @ 200mm) for heavier loads. Edge beams usually have additional rebar (e.g., 4 x T16 bars with T8 links). The top mesh prevents surface cracking and the bottom mesh resists bending under load.

Do I need blinding under a raft?

Yes, a 50mm blinding layer of lean-mix concrete (C10/GEN 0) is laid on the compacted formation before the raft. It provides a clean, level working surface for placing reinforcement and helps protect the damp-proof membrane. Some engineers allow a sand blinding layer (25-50mm) as an alternative on firm, dry ground.

How much does a raft foundation cost?

A raft foundation for a typical UK house (80-100 m²) costs approximately £12,000-25,000 including excavation, blinding, reinforcement, concrete and insulation. The concrete alone (C30, 200mm slab with edge beams) for a 100 m² raft is typically 25-35 m³ at £130-150/m³ = £3,500-5,000. Raft foundations are more expensive than strips but cheaper than piling.

TradeCalculator

Raft Foundation Calculator — Slab, Edge Beam & Mesh

Calculate raft foundation concrete volume, edge beam depth, top and bottom mesh reinforcement and blinding.

Raft Length

Raft Width

Slab Depth

200mm typical domestic

Edge Beam Depth

450mm typical domestic

Edge Beam Width

Mesh Type

Concrete Price

How We Calculate This

This calculator determines the concrete volume, mesh reinforcement and blinding layer for a raft foundation with thickened edge beams.

Concrete volume

Slab Concrete = Raft Area x Slab Depth
Edge Beam Concrete = Perimeter x Beam Width x Extra Depth
Total = Slab + Edge Beam concrete

Mesh

Mesh Sheets = (Raft Area x Layers x (1 + Overlap%)) / Sheet Area
Two layers (top + bottom) with 15% overlap allowance. Sheet size: 4.8m x 2.4m.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.