Ground-Bearing Slab: Thickness and Reinforcement Specification

A ground-bearing slab is the concrete floor slab that sits directly on the prepared ground, without any void beneath it. Getting the thickness and reinforcement right ensures it performs across the full design life of the building without cracking, settling or allowing moisture ingress.

Standard PorthouseDean Specification

For domestic extensions and new builds, PorthouseDean’s standard slab specification is:

– Thickness: 150 mm
– Bottom reinforcement: A252 mesh (bottom of slab)
– Top reinforcement: A142 mesh (top of slab)
– Concrete grade: C28/35 (sometimes specified as C35 when also acting as part of a pad or column foundation system)
– Cover: 25 mm to top mesh; 50 mm to bottom mesh (from underside of slab)

Why the Different Mesh Layers?

The two mesh layers serve different purposes:

A252 mesh at the bottom provides structural capacity. It allows the slab to span across soft spots in the ground — if a section of sub-base is weaker than the rest, the reinforced slab can bridge over a soft patch of up to approximately 1.5 metres without cracking. This protects against the inevitable minor variability in ground conditions.

A142 mesh at the top provides crack control. Concrete shrinks as it cures and heats and cools in service. The top mesh restrains that shrinkage cracking, keeping any cracks tight and controlling their distribution so they do not become structurally significant.

Can the Slab Be Unreinforced?

In some cases — particularly where the ground has been investigated and is very uniform, or where the BCO is comfortable with unreinforced construction for a light-loaded outbuilding — an unreinforced slab is acceptable. PorthouseDean has no objection to this in principle, but it should only be done with the BCO’s agreement and when ground conditions justify it.

If you are considering omitting reinforcement to save cost, check with your Building Control Officer before proceeding.

Perimeter Edge Details

At the perimeter of the slab, the edge typically thickens to form an integral strip footing beneath the wall line. The dimensions of this thickening are shown on the structural drawings. Where the slab meets a block and beam floor or suspended floor, a movement joint may be needed — check with your structural engineer before bridging between slab types.

Get Your Slab Specified

PorthouseDean includes ground-bearing slab specifications in structural packages for domestic extensions. Get a quote to start your project.