What is a raft foundation?

A raft acts as one stiff slab so that the building moves as a unit rather than settling unevenly. Rafts are typically reinforced throughout and thickened at the edges and under load-bearing lines. Because the load is spread across the whole footprint, the pressure on the soil is lower than under a strip foundation.

Rafts are common where ground is soft or variable, where fill is present, or where a shallow solution is preferred to avoid deep excavation. They are also used for insulated foundation systems that combine the slab, insulation and underfloor heating in one build-up.