What is a ground-bearing slab?

A ground-bearing slab (or slab-on-grade) is a concrete floor slab that rests directly on prepared ground rather than being supported by the foundations at its perimeter. The standard build-up is: excavated and compacted subgrade; hardcore (compacted granular material, typically crushed stone or recycled concrete); blinding (thin sand or concrete layer to blind the hardcore surface); damp-proof membrane (1200-gauge polythene); rigid insulation (PIR or EPS); structural concrete slab (typically C25 grade, 100mm minimum thickness, often mesh-reinforced); and finished floor (screed, tiles, or direct finish). It is the most common ground-floor construction for modern domestic buildings.

When is a ground-bearing slab NOT suitable?

A ground-bearing slab is not suitable where: ground conditions are poor (soft clay, fill, peat, or other compressible soils) and significant settlement is likely; there are large trees close to the slab that will cause soil heave on clay as the trees are removed (roots dry out the clay, removal allows it to swell); the site is in a high-risk radon area requiring a sub-slab depressurisation system (a suspended floor with ventilated void is preferred); or where the floor needs to be raised above ground level to avoid flood inundation. In these cases, a suspended floor (beam-and-block, or suspended concrete slab on pile caps) is preferred.

How thick should a ground-bearing slab be?

For standard domestic floors with normal loading, a 100mm ground-bearing slab in C25 concrete with A142 mesh reinforcement (6mm bars at 200mm centres, 142mm2/m area) is standard. Heavier loads (garages for HGVs, warehouse racking) require thicker slabs in stronger concrete - a structural engineer should calculate the required slab depth, reinforcement, and concrete specification. The slab must be at least 150mm below the DPC level in the walls to prevent bridging of the damp-proof course. A minimum 150mm concrete slab is recommended in garage floors where vehicles will be parked.

What is the difference between a ground-bearing slab and a raft foundation?

A ground-bearing slab is a floor-only element - it transfers its own weight and imposed floor loads to the ground but is NOT the foundation. The walls above sit on separate strip foundations that bear independently. A raft foundation combines the floor slab and the foundation into a single reinforced concrete element that spreads the entire building load (walls and floors) across a wide area of ground. Rafts are thicker (200-400mm+), more heavily reinforced, and structurally designed to resist differential settlement. Ground-bearing slabs are cheaper but require good, uniform ground conditions; rafts are used on poorer or more variable ground.

Foundations noun

Ground-Bearing Slab

/ ɡraʊnd ˈbɛərɪŋ slæb /

Also known as: ground slab, slab-on-grade, oversite concrete, concrete floor slab

Definition

A ground-bearing slab is a concrete floor slab that rests directly on prepared ground, transferring floor loads to the soil below rather than to the perimeter foundations. The standard build-up is: compacted hardcore; blinding; damp-proof membrane; rigid insulation; reinforced concrete slab (typically 100mm, C25, A142 mesh). Walls bear on separate strip foundations - the slab is a floor-only element, not a foundation. It is the standard ground-floor construction for modern domestic extensions and new builds on good ground.

In practice

The hardcore sub-base (typically 150-200mm of MOT Type 1 or recycled concrete aggregate) must be well compacted in layers using a plate compactor or roller to prevent settlement. The damp-proof membrane (1200-gauge polythene minimum) must be continuous and lapped at joints, with upstands at the perimeter lapping into the cavity and connecting to the DPC in the wall. The membrane position (above or below insulation) affects condensation risk - below the insulation is preferred as it keeps the slab warmer. Insulation (typically 70-100mm PIR) sits below the slab to meet Part L U-value requirements for ground floors (0.13-0.18 W/m2K depending on specification route).

The concrete slab must be cast level (or at falls for wetrooms) and allowed to cure fully before the screed or direct floor finish is applied. Curing time is typically 28 days before screeding on a concrete slab. Underfloor heating pipes or electric mats are cast into the screed or slab. Service entries (gas, water, electricity) pass through the slab in ducts or sleeves with proper sealing at the membrane level.

Building Regulations

Thermal (Part L)Ground floor U-value target 0.13-0.18 W/m2K (route dependent)

Moisture (Part C)DPM required, continuous, lapped to wall DPC

Radon (Part C)Radon-resistant DPM required in high-risk areas

Not suitableAvoid on fill, soft clay, or where trees may cause heave

Approved Document C requires the ground floor to resist moisture from the ground - a continuous DPM lapped to the DPC in the walls achieves this. Approved Document L requires ground floors to meet U-value targets - the slab edge (where the concrete meets the wall foundation) is a significant thermal bridge that must be addressed with perimeter insulation. Approved Document C also covers radon protection - in radon-affected areas (mapped by the UK Health Security Agency) a radon-resistant membrane and/or sub-slab ventilation pipe may be required.

Full Building Regulations guidance

Ground-Bearing Slab

Just so you know...