Foundations & Concrete Works

Concrete is the one part of a house you never see again and can never easily put right. What goes into the ground in the first few weeks decides how the building behaves for the rest of its life.

What a foundation is actually doing

A foundation carries a building down to ground that will not move. Carrying the load is the easy half. A two-storey house is light against what most soils can bear. The hard half is stillness: soil swells, shrinks and settles with the seasons, and a building that rides those movements unevenly cracks.

Concrete works carry the same idea outwards. The slab your floor sits on, the base under a garage, the footings under a boundary wall, the structural concrete holding a change in level. Our patch is Gerrards Cross, Beaconsfield and the villages around them. See where we cover. The ground round here refuses to be one thing: chalk, gravel, clay-with-flints and London Clay, all within a few streets.

The families, and who chooses between them

Nearly every domestic foundation is a version of one of four ideas. The ground picks, not the budget, and the choice belongs to your structural engineer.

What sets the depth

Nobody honest gives a depth over the phone. Approved Document A carries figures for simple strip foundations in England. As a rule of thumb 450mm to clear frost, rising to around 0.75m, 0.9m and 1.0m on low, medium and high shrinkage clay. Treat those as a floor, not an answer. The real number turns on:

• The soil, layer by layer, which is why trial holes come before a concrete price.
• Clay shrinkage: clay swells wet and shrinks dry, lifting and dropping whatever sits in that zone.
• Trees. Species, height, distance. Felled ones too: clay a thirsty tree kept dry swells back for years, and that heave pushes upwards.
• The water table, and whether the sides of a deep trench will stand long enough to pour into.
• Made ground. Old fill, buried foundations, a garden built up over decades. Looks like soil, behaves like rubble.
• Drains and sewers. Building near a public sewer can drive a foundation deeper and may need a build-over agreement, so ask your water company early.

Our guide to foundations goes further, and what happens below your home covers the ground investigation.

Setting out, and the inspection nobody should skip

Setting out is the quietest skill on site and the most expensive to get wrong: lines pulled off the drawing rather than off a fence, diagonals checked so the box is genuinely a rectangle, everything tied to one level datum. Thirty millimetres of error at the first corner is thirty millimetres the bricklayer inherits, and then the roofer. Then the formation, the cleaned-off floor of the dig, is made ready. Spoil out, water out, no soft patches under the pour. Building control looks into the open excavation before any concrete goes in. If the inspection finds roots, soft ground or standing water, the trench goes deeper. That day.

Nobody ever admires a foundation. They only ever notice one. And by then it is under a hundred tonnes of house.

Why the mix matters

Concrete is not one material. British practice runs on the designated mixes in BS 8500. GEN for unreinforced work such as trench fill, RC where there is steel in it, PAV where paving must survive frost and de-icing salt, FND where a ground investigation has found sulfates that attack concrete. That last risk is invisible: the wrong mix in aggressive ground looks perfect on the day and softens for years.

Reinforcement earns its keep by being in the right position, not by being present. Bars and mesh sit on spacers so concrete wraps them completely, as a rule of thumb around 75mm of cover where it is cast straight against soil. And concrete gains strength by chemical reaction, not by drying, so every extra litre added at the truck to loosen a stiff load takes strength out of it for good.

Slabs, oversite and the floor you walk on

Above the foundation sits the oversite, the build-up between ground and finished floor. Hardcore compacted in layers, not tipped in one go and driven over. Clean, inert, free of the sulfates that would attack the concrete above it, and as a rule of thumb no deeper than about 600mm. Then a damp-proof membrane, typically 1200-gauge polythene, joints sealed and lapped into the damp-proof course, continuous rather than nearly continuous. Then insulation, then the slab, which Approved Document C puts at 100mm or more, thicker and reinforced where it is structural. Where fill is deep, a suspended beam-and-block floor is often sounder.

Drainage and service ducts go in while the ground is open. Retro-fitting a pipe through a finished slab costs many times what laying it in the right week does. More in our drainage guide.

Weather decides the last of it. Fresh concrete wants holding above about 5°C until it has hardened, and must never be poured onto frozen ground. In heat and wind the problem reverses: the surface dries faster than the body beneath, and a slab left uncovered crazes. Curing is work, not padding on the programme.

Two things a homeowner can watch for, neither of them technical. Ask to see the delivery tickets. Every load carries one showing the mix and the time it was batched. And notice whether anyone is in a hurry to pour before somebody official has seen the hole. That hurry is the tell.

Foundations and concrete are the part of a build with no second chance and no visible reward. Everything after stands on decisions made in a fortnight of mud.

What the ground does here changes street by street. There is more on it village by village.

• Gerrards Cross
• Beaconsfield
• Chalfont St Peter
• Stoke Poges
• Northwood