Ground floor slab

Frequently asked questions

Does a ground slab carry the walls?

Sometimes, and it is the first thing to establish because everything else follows from it. In the common British arrangement the walls stand on their own strip or trench-fill foundations and the slab spans nothing — it carries the floor finish, the furniture and the people, bearing directly on compacted hardcore. In a RAFT, an Australian stiffened slab, or an American monolithic slab, the slab and its thickened edges ARE the foundation: the walls sit on them, and the reinforcement, the edge geometry and the ground preparation are all designed for that. The two look similar on a site photograph and are not interchangeable. Loading a ground-bearing slab as though it were a raft is the failure that follows, and it is silent at first: the slab cracks along the line of the load, and the wall it is carrying settles differentially with it.

Why does the membrane go under the slab and not over it?

Because concrete is porous and the vapour is coming up. A damp-proof membrane placed on top of the slab keeps ground moisture out of the floor finish but leaves the slab itself permanently wet from below, which in turn keeps any adhesive, timber or screed above it at risk and stops the slab ever drying. Placed UNDER the slab, on sand blinding over the hardcore, it stops moisture entering the concrete at all, and the slab dries upward as it cures. There is a second reason specific to the American practice of pouring directly onto the retarder: a slab sitting on an impermeable sheet can only bleed water upward, which affects finishing and curing and has been argued about for decades — hence the standard specifically for under-slab vapour retarders and the arguments about a blotter layer over it. What both markets agree on is that the membrane belongs under the concrete and must be lapped and sealed into the wall's damp-proof course, because an unconnected membrane is a tray with an open edge.

Where does the insulation go, and does the edge matter more than the middle?

The edge matters far more, and this is the least intuitive thing about a ground floor. Heat leaving the middle of a slab travels a long way through soil before it reaches anywhere cold, and soil at depth stays close to the annual average temperature — so the centre of a large floor loses heat slowly whatever is under it. At the PERIMETER the path is short: a few centimetres of concrete to the outside air, through an edge that is also a structural junction. The result is that heat loss through a ground floor is concentrated in a band around its edge, that the loss per square metre falls as the floor gets bigger, and that edge or perimeter insulation buys most of the benefit of a fully insulated floor for a fraction of the material. It is also why the slab edge is one of the worst thermal bridges in a building and why the detail there — insulation turned down the edge, or a thermally broken upstand — decides the floor's real performance more than the thickness under the middle.