Strip, trench-fill, raft or piled. What each one is, why the ground chooses between them, and why nobody honest answers “how deep?” over the phone.
A foundation has one job: to carry the weight of your house down to ground that will hold it still. Not ground that will hold it up, because most ground manages that for a while, but ground that will not move with the seasons. Movement is what cracks plaster, sticks doors and opens stepped cracks through brickwork. The weight itself is rarely the problem. A two-storey house is light compared with what most soils can bear. The trouble starts when the soil underneath changes its mind.
Why the ground moves
The main culprit in southern England is shrinkable clay. Clay swells when wet and shrinks when it dries, and a foundation sitting in the zone where that happens is lifted and dropped a little every year. Enough, over time, to pull a house apart slowly. Trees make it worse. A mature oak or poplar can dry clay out several metres down and a surprising distance sideways, so a foundation near a big tree has to reach below the roots’ influence or bypass it altogether.
The ground round here is a patchwork. Chalk, gravel and clay-with-flints can all turn up within a few streets of one another, which is one reason no two jobs in this corner of the county are quite alike. The water table matters too. Dig below it and the trench floods and slumps, and sometimes the honest answer is a different foundation type rather than a long fight with the water.
The four families
Nearly every domestic foundation is a version of one of these. The list runs roughly from simplest to most involved, and it’s the ground, not the budget, that decides which one you need. Worth knowing before you read on: no single contractor carries out all four with its own machines. Piling in particular is a trade of its own, with its own rigs and crews, let as a separate package and run to the engineer’s design. The groundworks contractor’s part of a piled job starts at the ground beams and the slab above them. The point of the list isn’t what any one firm offers. It is so you can follow what your engineer is choosing between, and why.
Strip
A trench with a layer of concrete in the bottom, often a few hundred millimetres thick, and masonry built up off it to damp-proof course. The traditional choice where the ground is sound and the dig is shallow. Its weakness is labour: someone has to lay blocks at the bottom of a trench.
Trench-fill
The same trench, filled with concrete to within a few courses of ground level. More concrete, far less bricklaying, and nobody working down the hole, which is why it has become the default on a great many house jobs. On clay near trees, one face of the trench often gets a compressible board, so swelling ground pushes against foam instead of your concrete.
Raft
A reinforced concrete slab under the whole footprint, usually thickened at the edges, spreading the load of the house across all of it at once. Used where the ground is uniformly poor or gently mobile. Soft spots, made-up ground, some infill plots. A raft is an engineered structure, not just a thick floor slab.
Piled
Concrete columns bored or driven down to firm ground, tied together at the top by reinforced ground beams that carry the walls. It’s the design an engineer reaches for when firm ground is a long way down, or when clay and trees would demand a trench too deep to dig safely. As a rule of thumb, once a trench wants to be much beyond two and a half metres, piling starts to compete on cost and wins outright on safety. The piling itself is specialist work by a specialist firm. So if your design comes back piled, expect a separate piling contractor on site, and expect the design to have come from the engineer rather than from whoever is digging.
Why “how deep?” has no standard answer
You will see figures quoted. Three-quarters of a metre to a metre is a common rule of thumb in shrinkable clay, less in chalk or gravel, with something like 450mm the absolute floor anywhere simply to get below frost. Treat all of them as starting points, not answers. Your engineer sets the real number. The depth that matters is the depth at which the ground stops moving and can carry the load, and that turns on a handful of things:
- What the soil actually is, layer by layer, which is why trial holes should be dug before anyone prices the concrete.
- Trees: species, size and distance, including trees recently removed, because clay a tree kept dry will swell back for years after it has gone. Engineers call that heave, and it pushes upwards.
- The water table, and whether the sides of a deep trench will stand.
- Drains and sewers nearby. Building over or close to one can drive the foundation deeper, and you may need a build-over agreement from the water company. As a rule of thumb it is worth asking once anything, foundations included, comes within about three metres of a public sewer, but the trigger and the process vary by water company, so ask yours early. Check before anyone designs a thing.
The final say happens on site. Building control looks into the open excavation before any concrete is poured, down at the formation, the trade’s name for the cleaned-off floor of the dig. If the inspector sees roots or soft ground down there, the trench goes deeper. That day. It’s normal and it’s the system working. It is also why a foundation price fixed to the penny should prompt one question: what happens when the ground disagrees?
The honest answer to “how deep?” is “as deep as the ground demands”, and nobody knows that from the kerb.
The engineer, the inspector and the digger
Three parties, three jobs. The structural engineer designs the foundation, whether from a proper ground investigation on a new build, or from trial holes and local knowledge on an extension. Building control, either your council’s own team or a private one, now known in England as a registered building control approver, checks that the design meets the Building Regulations, then inspects the hole before the pour. The groundworker digs to the drawing, keeps the excavation clean and dry, and books that inspection in, never pouring ahead of it. Anyone who offers to pour first and sort the paperwork after is asking you to carry the risk for the rest of the time you own the house.
ASK YOUR CONTRACTOR
- Who designed this foundation, and were trial holes dug first?
- What’s the rate if we have to go deeper than drawn, and who decides on the day?
- Is heave protection allowed for if we’re on clay near trees?
- Where is the spoil going, and how many lorry loads has the price allowed for?
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