Raft foundation design
A raft spreads the whole building over one stiffened concrete slab, so no single point overloads weak or variable ground. We design ground-bearing and stiffened rafts to suit the soil and loads, for clients across the UK.
When should I use a raft instead of strip foundations?
A raft suits poor, made or variable ground and lightly loaded buildings, where spreading the load over the whole footprint keeps soil pressures very low and lets the slab ride over soft spots. It is most economic when it avoids the deep excavation or piling a strip would otherwise need. We compare the options on cost before recommending one.
When a raft is the right answer
A raft foundation makes sense when the ground is too weak or too variable to carry sensible strip or pad pressures, or when the loads are light enough that spreading them over the full footprint keeps the soil pressure low. Made ground, soft or normally consolidated clay, filled sites, areas of shallow mine workings and ground prone to differential movement all point towards a raft, because one connected slab moves as a unit and rides over local soft spots rather than settling into them.
The other driver is heave and variable support. On shrinkable clay a stiffened raft can be designed to tolerate the ground swelling or shrinking beneath it, and on old fill it distributes load so widely that the bearing pressure drops to a few tens of kilonewtons per square metre. We compare the raft against deeper strips and piles on cost, programme and buildability before recommending it, because a raft is only economic when it removes deep excavation or piling that would otherwise be needed.
How a raft is designed
A raft is a reinforced slab that has to work in bending in two directions, spanning between the lines of load from the walls and columns above while the ground pushes up underneath. We model the slab on the ground stiffness, find the bending moments and shear forces from the pattern of loads, and size the slab thickness and reinforcement to Eurocode 2. Concentrated column loads also have to be checked for punching shear, where the column tries to push a cone of concrete straight through the slab, which often sets the slab depth locally.
Most domestic rafts are stiffened at the edges and under internal load lines with downstand beams, giving the slab the stiffness to keep differential deflection small without making the whole slab thick. We detail the edge beams to carry the perimeter walls and to found below any soft surface crust, tie the top and bottom reinforcement through the beams, and set the ground-bearing slab thickness for both the loads and the crack control the finished floor needs.
Ground works, membranes and detailing
A raft is only as good as what it sits on. We specify the reduced dig, the compacted granular sub-base and the blinding that give the slab uniform support, because a raft bearing on a soft patch under one corner will still tilt. Under the slab we detail the damp-proof membrane and, where the site needs it, gas or radon protection, all lapped and sealed so the barrier is continuous through service penetrations and up the edge upstands.
Detailing the reinforcement is where a raft is won or lost. We set the cover for the exposure and ground aggressiveness, detail the laps and the top and bottom mats so the slab has continuity over the beams, and coordinate drainage and service entries so nothing is cast in the wrong place or cut through a beam later. The calculations and drawings are prepared to Eurocode 2 and 7 and Approved Document A, delivered to clients anywhere in the UK or Europe.
What we check.
The points our calculations resolve for a project like this.
- Bearing pressure spread over the full footprint on weak or made ground
- Slab bending in two directions between wall and column load lines
- Punching shear under concentrated column loads
- Edge and internal stiffening beams to control differential deflection
- Sub-base, blinding and uniform support beneath the slab
- Damp-proof, gas or radon membranes detailed continuous through the raft
Issued drawings, redacted.
From enquiry to sign-off.
Enquiry
Send drawings or describe the problem. We confirm the scope, the deliverables and a target timescale.
Information
We agree the survey, drawings or data we need and any site access required.
Engineering
Design, calculation or assessment to the relevant Eurocodes and UK National Annex.
Issue
A clear, defensible report or set of calculations, with assumptions and limitations stated.
Common questions
When should I use a raft instead of strip foundations?
A raft suits poor, made or variable ground and lightly loaded buildings, where spreading the load over the whole footprint keeps soil pressures very low and lets the slab ride over soft spots. It is most economic when it avoids the deep excavation or piling a strip would otherwise need. We compare the options on cost before recommending one.
How thick is a raft foundation?
A domestic ground-bearing raft is often a 200 to 300mm slab stiffened with deeper edge and internal beams, but the thickness depends on the loads, the span between load lines and the punching shear at column positions. We calculate it to Eurocode 2 rather than adopting a standard depth.
Can a raft be used on shrinkable clay?
Yes. A stiffened raft can be designed to tolerate clay shrinking and swelling beneath it by being stiff enough to bridge the movement, or by being isolated from the heaving zone. The design depends on the plasticity of the clay and any nearby trees, which we assess from the ground investigation.
Do rafts need reinforcement?
Always. A raft works in bending in both directions and carries concentrated loads, so it needs top and bottom reinforcement mats and often additional bars in the stiffening beams. We size and detail all the steel to Eurocode 2 with cover suited to the ground conditions.
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