What is an oversite slab?

An oversite slab (or oversite concrete) is the concrete ground floor within a building, cast directly onto the prepared ground inside the foundation walls. It forms the structural floor of the ground level and must resist moisture rising from the ground, support the intended floor loads, and provide a level base for floor finishes. The typical build-up is: compact sub-grade, hardcore, blinding, DPM, insulation, reinforcement (if needed), and concrete slab.

What is the standard thickness of an oversite slab?

The minimum thickness for a domestic oversite concrete slab under Approved Document C is 100mm, typically unreinforced or with a single layer of A142 or A193 mesh. In areas subject to heavier point loads (under internal walls, partition loads, or industrial use), the slab may be thickened to 150mm or reinforced to suit. The concrete grade is typically C25 or C30, with a sulfate-resistant mix where required by ground conditions.

Does an oversite slab need a DPM?

Yes - a damp proof membrane (DPM) is required under the oversite slab to prevent ground moisture rising through the concrete into the building. The DPM is typically 1200 gauge (300 micron) polythene sheeting, lapped at least 300mm at joints and turned up at the edges to lap with the wall DPC, forming a continuous moisture barrier. Without a properly installed and lapped DPM, ground moisture will rise through the slab by capillary action, causing damp floors.

Should insulation go above or below the oversite slab?

In most modern UK construction, floor insulation is placed between the DPM and the concrete slab (under-slab insulation) using rigid PIR or EPS boards. Alternatively, insulation can be placed above the slab (over-slab), under a screed. Under-slab insulation keeps the thermal mass of the slab above the insulation layer, which helps with solar gain. Over-slab insulation is easier to add retrospectively. Both positions are acceptable under Part L, subject to achieving the required U-value.

Groundworks noun

Oversite

/ ˈəʊvəsaɪt /

Also known as: oversite concrete, oversite slab, ground-bearing slab, ground floor slab

Definition

An oversite (or oversite slab) is the concrete ground floor cast within the foundation walls of a building, bearing directly on the prepared ground below. It must resist moisture rising from the ground, support floor loads, and provide a level base for floor finishes. The standard build-up from bottom to top is: compacted sub-grade, compacted hardcore, blinding, DPM, insulation, and structural concrete slab.

In practice

For a typical domestic ground floor, the full build-up is: 150mm compacted Type 1 hardcore; 50mm sand or C10 concrete blinding; 1200 gauge polythene DPM with 300mm laps; 100mm PIR or EPS insulation boards (tightly butted); A142 mesh reinforcement on spacers at 35mm cover; and 100mm C25 or C30 concrete slab. The DPM laps up at the perimeter to meet the wall DPC, ensuring a continuous moisture barrier at the junction.

Before placing the oversite, all vegetation, topsoil, and organic material must be removed from within the foundation walls - any organic material left in will decompose and allow ground air to enter the building. The concrete must be placed in a single operation if possible, with construction joints only where unavoidable. Curing (keeping the concrete moist or covered) for at least 3 days after placement reduces shrinkage cracking.

Building Regulations

Relevant PartsPart C (moisture), Part L (energy)

Minimum slab thickness100 mm concrete

DPM1200 gauge polythene - must connect to wall DPC

Floor U-value (new build)0.13 W/m²K notional (AD L 2021)

Approved Document C requires ground floors to resist moisture rising from the ground via a continuous DPM connecting to the wall DPC. Approved Document C Table 1 sets out the build-up options including the solid concrete floor construction. Approved Document L sets the U-value target for ground floors in new dwellings at 0.13 W/m²K notional and 0.18 W/m²K limiting, requiring significant insulation within the floor build-up.

Full Building Regulations guidance

Related Calculators

Concrete Volume Calculator

Oversite

Just so you know...