If you're planning a new build in South Wales — or working with a structural engineer who's recommended a raft foundation — you may have questions about what's involved, why it's the right choice for your plot, and what to expect during the build.
This guide explains everything clearly, from ground conditions to concrete pours.
What Is a Raft Foundation?
A raft foundation (also called a raft slab or mat foundation) is a thick reinforced concrete slab that extends across the full footprint of a building and beyond. Unlike strip foundations, which carry load along lines under walls, a raft spreads the building's weight across the entire ground area beneath it.
Think of it as the building sitting on a concrete "raft" that floats on the ground rather than sinking into it at specific points.
When Is a Raft Foundation Used?
Raft foundations are typically recommended or required when:
Ground conditions are poor or variable
If the soil beneath your plot is weak, soft, waterlogged, or contains filled ground (made ground), a raft distributes load more evenly and reduces the risk of differential settlement — where one part of the building sinks at a different rate to another.
Strip foundations aren't viable
On some plots, the soil bearing capacity is too low for conventional strip footings to work economically. Digging to find competent ground could mean going very deep, at which point a raft becomes more cost-effective.
The ground has been previously disturbed
Building on or near old landfill, demolished buildings, former mining areas, or made-up land often calls for a raft. South Wales has extensive areas of former industrial and mining activity, and raft foundations are particularly common across Caerphilly, Merthyr, Ebbw Vale and the Valleys.
There is a risk of heave or subsidence
In shrinkable clay soils, trees nearby can draw moisture and cause ground movement. A raft can be engineered to resist this more effectively than strip foundations.
How Is a Raft Foundation Built?
While every raft design is different (your structural engineer specifies the exact details), the general process follows these steps:
Step 1: Setting Out
The plot is set out using surveying equipment to establish exact levels and dimensions. Getting this right at the start prevents expensive problems later.
Step 2: Reduced Dig / Excavation
The topsoil and any unsuitable material is stripped back to the correct formation level. On poor ground, this may involve removing weak or contaminated material and replacing it with engineered fill.
Step 3: Sub-base and Blinding
A compacted sub-base layer (typically MOT Type 1 aggregate) is laid and compacted. A thin layer of "blinding" concrete is then poured to create a clean, level surface for the reinforcement.
Step 4: Waterproof Membrane
A heavy-duty DPC (damp proof course) membrane is laid over the blinding to prevent ground moisture passing up through the slab.
Step 5: Insulation (if required)
For energy-efficient builds, rigid insulation boards are placed on top of the membrane before the reinforcement. This is standard for modern building regulations compliance and is almost universal in new builds.
Step 6: Reinforcement
Steel reinforcement bars (rebar) or mesh are laid to the structural engineer's specification. This gives the concrete slab its tensile strength. Edge beams — thickened sections at the perimeter and under load-bearing walls — are formed and reinforced at this stage.
Step 7: Concrete Pour
The concrete is poured in one continuous operation (or in planned pours depending on slab size), screeded level, and cured. The concrete specification — grade, mix, admixtures — is determined by your structural engineer.
Step 8: Building Control Inspection
At various stages, your building control inspector will check the raft construction. This typically includes inspections at sub-base, reinforcement, and post-pour stages.
Raft Foundation vs Strip Foundation — Which Is Right for Your Plot?
| Factor | Strip Foundation | Raft Foundation |
|---|---|---|
| Good bearing ground | Preferred choice | Possible, but usually unnecessary |
| Poor/soft/variable ground | May not be viable | Recommended |
| Former mining or industrial land | Often unsuitable | Common choice |
| Tree proximity / clay soils | Consider carefully | Often preferred |
| Typical depth | 600mm–1m+ | 300–600mm slab depth |
| Build speed | Fast once at depth | Potentially faster overall |
| Cost comparison | Generally cheaper on good ground | More competitive on difficult ground |
Your structural engineer will specify which foundation type is appropriate based on a ground investigation survey (trial pits or boreholes). Do not skip the ground investigation — it's the most important information in the foundation design process.
Raft Foundation Costs in South Wales
Raft foundation costs vary considerably based on:
- Slab dimensions and thickness
- Depth and complexity of the excavation
- Structural engineer specification (reinforcement density, concrete grade)
- Ground conditions (made ground removal adds cost)
- Whether insulation is included
- Access to site
For a typical new build house in South Wales with a straightforward raft, you might expect the groundworks and foundation phase to represent 10–15% of the total build cost. A structural engineer and a groundworks contractor can give you a much more accurate figure once ground investigation results and drawings are available.
At Jac Jones Construction, we provide itemised quotations broken down by earthworks, sub-base, concrete, reinforcement, and associated works — so you know exactly where the money is going.
Common Questions About Raft Foundations
Does a raft foundation require a ground investigation?
Yes — always. You cannot properly design a raft without understanding what's underneath it. A desktop study followed by trial pits or boreholes is standard practice.
How long does it take to build a raft foundation?
For a typical 3–4 bedroom house plot, the groundworks and raft phase usually takes 4–8 weeks depending on ground conditions, programme, and weather.
Can a raft foundation be used on sloped ground?
Yes, but it requires careful design. Stepped rafts or combinations of raft and retaining structures are used on sloping plots.
Who designs the raft foundation?
Your structural engineer. They will design the raft based on ground investigation results, the structural loads from the building, and building regulations requirements. The groundworks contractor builds to their specification.
Working with an Experienced Groundworks Contractor
A well-designed raft foundation can only perform as intended if it's built correctly. Errors in setting out, sub-base preparation, reinforcement placement, or concrete specification can compromise the whole structure.
At Jac Jones Construction, we've been building raft foundations across South Wales since 2013. We work closely with structural engineers and building control from the outset, and our team uses setting-out equipment on every project to ensure accuracy.
If you're planning a new build and have been advised that a raft foundation is required, we're happy to review your drawings and discuss the groundworks programme.
Call 07446 832296 or send us your project details.
Published by Jac Jones Construction Limited — groundworks and new build contractors in South Wales since 2013. FMB accredited | TrustMark registered | £10m public liability and £5m employers' liability insurance.