What is it?
A concrete slab is the structural floor deck a storey is built up from. At ground level it's usually a ground-bearing slab, poured on a compacted hardcore sub-base. Between storeys it's a suspended slab, cast in-situ or precast, spanning between beams or walls and carrying the floor above on structure rather than on the ground.
- 1Concrete Slab (curing)
- 2DPM / DPC
- 3Blockwork
What does it do?
Carrying the floor loads The slab bears on the ground or spans between supports, and carries everything above it (finishes, partitions, furniture, people) down to the ground or the structure below.
The base everything else builds up from Insulation, screed and floor finish all build up from the slab's top surface, and a partition's base track fixes straight down into it. So the slab's level and its ability to take a fixing matter to everything that sits on it.
Fire and acoustic separation between storeys A suspended slab is usually the main fire and acoustic separating element between one storey and the next, which is why its thickness and make-up are specified as part of a tested floor system, not on structural grounds alone.
Where is it used?
At ground level as a ground-bearing slab, and at every floor between storeys as a suspended slab. A stud partition on an upper floor sits on and fixes into a suspended slab the same way it would into a ground-bearing one.
Types
How a slab is built depends mainly on whether it bears on the ground directly or spans between structure.
Ground-bearing slab
Cast on a prepared, compacted hardcore sub-base at ground level, with a damp-proof membrane beneath or within it. Usually reinforced with steel mesh to control cracking.
Suspended in-situ concrete
Reinforced concrete cast on site, spanning between beams or load-bearing walls. Used where the slab has to span a distance the ground can't be relied on to support.
Precast concrete plank
Factory-cast concrete units (hollowcore or solid plank) craned into place and often topped with an in-situ structural screed. Quicker to put up than casting in-situ, but it needs crane access.
Beam and block
Precast concrete beams laid between supports with infill blocks between them, usually topped with a structural screed. A common suspended ground floor in low-rise housing.
Typical sizes
A domestic ground-bearing slab is typically 100–150 mm thick, reinforced with steel mesh to control cracking. Suspended slabs are usually thicker (around 150–250 mm), but thickness and reinforcement are always set by the span and the loads the slab carries.
How to specify
Slab type, depth and reinforcement are a Structural Engineer's calculation, set by the span, the loads it carries, the ground conditions (for a ground-bearing slab) and, for a suspended slab, the fire and acoustic performance the floor as a whole needs. The slab also has to be coordinated with the insulation strategy (insulation above or below the slab changes its detailing), with the finished floor level, and with any partition that fixes into it.
Other considerations
- A ground-bearing slab needs a damp-proof membrane beneath or within it to stop ground moisture rising through the floor. Where it meets an external wall, the insulation and DPC need to run continuously across the junction, a common weak point in the waterproofing.
- A partition's base track fixing has to suit the slab: a precast plank takes fixings differently from an in-situ slab.
- Sound can travel sideways through a slab at a partition junction even when the wall itself is well detailed, so an acoustic break may be needed where flanking matters.
- A suspended slab deflects slightly under load over time, which is exactly what a deflection head at the top of a partition is there to take up.
Interacting elements
- Hardcore: the compacted sub-base a ground-bearing slab sits on
- DPC: links to the damp-proof membrane so the damp-proof line is unbroken
- Rigid insulation: sits either below or above the slab, depending on the floor build-up
- Screed: the levelling layer poured on top of the slab so floor finishes go down on a flat surface
- Floor finishes: build up from the screed above the slab
- Metal studs: a partition's base track fixes directly into the slab
- Deflection head: the detail at the top of a partition that takes up the slab's movement