Raft Foundation Calculator — Slab, Edge Beam & Mesh

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. See our Foundation Calculator to compare raft with strip and trench fill options.

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.

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.

Yes, a 50mm blinding layer of lean-mix concrete (typically GEN1 / C8/10, or GEN0 / C6/8 for lighter duty) is laid on the compacted formation before the raft. Per BS 8500-1, GEN1 = C8/10 is the standard blinding grade and GEN0 = C6/8 is the weakest non-structural mix. The blinding gives 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.

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.