Slab, Suspended Timber or Beam and Block

A ground floor has to do two things that pull apart: carry what stands on it, and deal with the ground immediately beneath it. One construction lays the floor onto ground that has been prepared to receive it. The other two hold the floor clear of the ground and span across the gap, and they differ in what does the spanning.

That is the whole shape of the decision. Once you know whether the design intends to bear on the ground or span over it, and if it spans, what material is doing the spanning, the rest of the conversation becomes legible.

A ground-bearing floor slab lets the lowest floor rest on ground that has been prepared and consolidated. That ground takes the floor loading directly, and the layers above exist to interrupt moisture and, where investigation calls for it, soil gas, to place the floor at a chosen thermal position, and to give the finish something flat and stable to bear on.

What distinguishes the assembly from its ingredients is that the layers are not interchangeable in position. Concrete, insulation and a barrier sheet each mean something different depending on where in the stack they sit, and the same materials in a different order produce a floor with different behaviour, different edge details and different limits on what may be laid over it.

A suspended timber ground floor holds the deck clear of the ground on joists bearing on the perimeter walls and, where the span asks for it, on low intermediate walls beneath. The gap left below is not waste space. Air moves across it from one side of the building to the other and carries away moisture that would otherwise collect against the timber.

The library is explicit that this ventilation exists for the sake of the floor itself. The structure is organic and has to be kept dry, so the openings serving the void are part of the floor even though they sit in the walls. Blocking them is not a cosmetic decision, and it is the intervention this floor is least able to absorb.

A beam and block floor spans the same kind of void, but the spanning is done by precast concrete beams with blocks dropped between them and a grout, levelling or structural topping worked over the surface. The consequence of the material change is direct: a concrete deck is indifferent to the humidity of the void beneath it, so the ventilation there exists for the ground rather than for the floor.

The topping is the component most often mistaken for a finish, and the entry says why the mistake matters. In some designs it only fills the joints; in others it is a reinforced layer that joins the separate units into a plate the walls beneath rely on. The two look alike once laid, and which one a design intended is a question for the structural engineer.

The library settles the timber-against-concrete question with an observation rather than an argument: look into the void and see what spans it. Timber joists here, precast beams with block infill there. Everything else follows from that, including whether the ventilation is protecting the structure or the ground.

A raft and a ground-bearing slab look identical once cast, and the library settles them with one question: what bears on the element. If the load-bearing walls, columns or piers stand on it and it takes them into the ground, it is a raft and belongs with the foundations. If those elements bear elsewhere and the element carries only floor loading, it is a floor.

The answer is not academic. It changes the reinforcement, the deepened zones underneath that carry line and point loads into the plate, whether wall lines have to sit over those zones, and what cutting an opening through the element means years later. A drawing labelled as a slab that carries walls is describing a raft whatever the title block says.

You are not being asked to choose. You are being asked to recognise which question is being answered, so that a later alteration — a new drain, a raised patio, an extension across a ventilated wall — is discussed against the floor that was actually built rather than the one you assumed was there.