What is a raft foundation?

A raft foundation is a reinforced concrete slab that extends across the entire footprint of a building (or beyond) to distribute the building loads over a large area of ground. Unlike strip foundations, which concentrate loads under walls, a raft spreads them across the whole base - reducing the bearing pressure per square metre and making rafts suitable where the ground has low or variable bearing capacity. Raft foundations are also used on filled ground, made ground, and where mining subsidence is a risk.

When would you use a raft foundation instead of strip foundations?

Raft foundations are used when: the ground has low bearing capacity (such as soft clay, peat, or made-up fill); the ground conditions vary significantly across the site; the building is on filled or contaminated land; there is a risk of mining subsidence or differential settlement; or strip foundation excavations would be so wide that they effectively form a raft anyway. They are also used where the oversite slab can be combined with the foundation into a single structural element.

How thick is a raft foundation?

For a domestic dwelling, a raft foundation is typically 200-300mm thick with reinforcement top and bottom. Edge beams are often thickened to 450-600mm to carry the higher loads from external walls. The exact dimensions depend on the soil bearing capacity, building loads, and span between walls, and must be designed by a structural engineer. Concrete is typically C30 or C32/40 with A393 mesh reinforcement plus designed bar reinforcement in the edges and at openings.

Does a raft foundation need Building Regulations approval?

Yes - raft foundations require Building Regulations approval under Part A (Structure). Unlike simple strip foundations, rafts must be designed by a structural engineer and the design submitted as part of the Building Regulations application. Building Control will inspect the excavation, formwork, reinforcement, and concrete placement. The engineer's design drawings and calculations must be provided to Building Control before work starts.

Groundworks noun

Raft Foundation

/ rɑːft faʊnˈdeɪʃ(ə)n /

Also known as: raft slab, mat foundation, floating slab

Definition

A raft foundation is a reinforced concrete slab that spans the entire footprint of a building, distributing the structure's loads across the whole ground area rather than concentrating them beneath individual walls or columns. Raft foundations are used where strip foundations are unsuitable - typically on ground with low or variable bearing capacity, filled ground, or where differential settlement is a risk.

In practice

A domestic raft typically consists of a reinforced concrete slab 200-300mm thick with thickened edge beams (450-600mm) to carry the higher wall loads. The slab contains top and bottom reinforcement mesh (typically A393) plus designed bar reinforcement at edges, openings, and re-entrant corners. The raft bears on compacted hardcore and blinding and incorporates the DPM and any under-slab insulation within the build-up.

The main advantage of a raft is that any differential settlement of the ground beneath it causes the whole slab to deflect as a unit, limiting cracking in the structure above. This makes rafts particularly suitable on soft clays, filled ground, and areas with a history of mining subsidence. The disadvantage is cost - more concrete and reinforcement than a strip foundation, plus the need for engineer design and inspection of the reinforcement before pouring.

Building Regulations

Relevant PartPart A - Structure

Design requirementStructural engineer required - no prescriptive tables

Typical slab thickness200-300 mm (plus thickened edge beams)

Concrete gradeC30 minimum (C32/40 common)

Raft foundations fall outside the prescriptive solutions in Approved Document A - they must be designed by a structural engineer to Eurocode 7 (geotechnical design) and Eurocode 2 (concrete structure). The design drawings and calculations must be submitted to Building Control before work begins. Reinforcement must be inspected and approved before concrete is poured - Building Control will typically require this inspection.

Full Building Regulations guidance

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Raft Foundation

Just so you know...