What is a raft foundation?
A raft foundation, also called a mat foundation, supports the entire building on one continuous reinforced concrete slab. Instead of transferring loads at points through strip footings or pads, it spreads them across the full footprint. Ground pressure drops sharply as a result: the larger the area, the lower the load per square metre.
When to use one
- On moderately bearing ground where strip footings would settle too much but piling is unnecessary or uneconomic.
- For light structures on soft ground — an extension, garage, garden room or timber frame house.
- Where the water table is high, so the slab doubles as a watertight basin (a tanked “concrete bathtub”).
- Under evenly distributed loads. A raft is at its best when loading is roughly uniform across the plan.
Build-up
From bottom to top, a modern raft usually looks like this:
- A compacted sand or crushed stone layer on the undisturbed ground, placed and compacted in layers.
- A capillary break, such as coarse gravel or crushed stone, to stop rising damp.
- Load-bearing insulation, usually EPS or XPS, beneath the entire slab.
- A membrane acting as a damp-proof and separating layer.
- The reinforced slab itself, with reinforcement top and bottom, because the slab bends both in the span and over loaded points. Beneath load-bearing walls and at the perimeter it is often thickened into an edge beam or rib.
In housing, slab thickness typically falls in the range of 25 to 50 cm, depending on loading, the span between supports and the ground conditions. The exact thickness and reinforcement always follow from calculation.
Points to watch
- Ground investigation is still needed. A raft forgives small variations in bearing capacity, but not a soft pocket or a peat lens under part of the building. Uneven ground beneath a stiff slab causes cracking.
- Insulate beneath the whole slab, not only at the edges, or the slab itself becomes one large thermal bridge.
- Plan the services first. Drainage, ducts and penetrations must be exactly in position before the pour; drilling or breaking out a reinforced raft afterwards is an expensive operation.
- A raft is not a waterproof basement. Under groundwater pressure you need more: waterproof concrete, carefully designed joints and movement details, and a contractor experienced in that work.
- Cost. A raft uses more concrete and reinforcement than strip footings but saves excavation, and is often cheaper than piling. Which option wins only becomes clear after the ground investigation — let that decide, not a standard specification.
Related terms
- Shallow foundation
- Strip foundation
- Piled foundation
- Bearing capacity
- Settlement
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