Summary

The oversite concrete slab is the ground floor of most modern domestic buildings — the concrete platform on which the floor finish (tiles, screed, or timber) is laid. It must be: strong enough to carry imposed floor loads without cracking, impermeable to rising damp (via the DPM), and insulated to meet Part L thermal performance requirements.

Ground-bearing slabs are cost-effective for most domestic construction where the ground conditions are adequate. Where there is significant fill, contamination, high water tables, or poor ground conditions, a suspended ground floor (beam and block, or precast plank) is a better choice.

The slab construction sequence — compacted subgrade, sub-base, blinding, DPM, insulation, reinforcement, concrete — is standard but every element must be executed correctly. The DPM is critical: a poorly lapped or perforated DPM allows moisture ingress that will damage any floor finish and may cause structural issues in timber floors above.

Key Facts

  • Minimum slab thickness — 100mm for domestic residential; 150mm for commercial or higher loads
  • Concrete grade — minimum C25/30 (RC25/30) for most domestic; higher for contaminated or aggressive ground
  • Reinforcement — A142 welded steel fabric mesh (one layer) for most domestic; engineer specifies for heavier loads or variable ground
  • DPM — 1200 gauge (300 micron) polythene minimum; typically placed between blinding and insulation (or on top of insulation)
  • DPM laps — minimum 300mm; all laps must be taped with polythene DPM tape to prevent moisture ingress
  • DPC link — DPM must be linked (lapped, sealed) to the DPC in all walls at the junction between floor and wall
  • Insulation — EPS (expanded polystyrene) or PIR board placed below or above the DPM; thickness to achieve Building Regs Part L U-value (typically 0.25 W/m²K for ground floor)
  • Blinding concrete — minimum 50mm ST2 (lean mix) on compacted sub-base; provides clean level base for DPM and insulation
  • Approved Document C — Building Regs guidance on moisture resistance; covers DPM requirements
  • Approved Document L — Building Regs thermal performance; ground floor U-value target
  • Radon — in radon-affected areas (parts of Cornwall, Devon, Derbyshire, Northamptonshire, etc.), a radon barrier is required under the slab, and a radon sump may be needed
  • A142 mesh — 6mm bar at 200mm × 200mm centres; weight 2.22 kg/m²; standard floor mesh

Quick Reference Table: Oversite Slab Construction Layers

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Layer Specification Notes
Subgrade Compacted, topsoil removed, soft spots treated Assessment per soil investigation trial pits
Geotextile 200 g/m² non-woven, on prepared subgrade Optional but recommended on clay or variable ground
Sub-base Type 1 MOT, 150mm compacted minimum See hardcore sub base compaction
Blinding 50mm ST2 lean concrete Clean level surface for DPM
DPM 1200 gauge polythene, 300mm lapped and taped Below or above insulation depending on design
Insulation EPS or PIR board; thickness to Part L target 70–100mm typical for 0.25 W/m²K
Reinforcement A142 mesh (75mm cover minimum) Centralised in slab depth
Concrete slab C25/30, minimum 100mm Cure for 28 days minimum for full strength

Detailed Guidance

DPM Position: Above or Below Insulation?

There are two standard arrangements:

DPM below insulation (Type 1 construction per BS 8102):

  • Blinding → DPM (lapped 300mm, taped) → Insulation → Mesh → Concrete
  • DPM protects the insulation from ground moisture
  • Any gap between insulation boards risks a cold bridge; insulation must be tight-fitted
  • Most common in new-build domestic construction

DPM above insulation (sandwich construction):

  • Blinding → Insulation → DPM → Mesh → Concrete
  • DPM protects the slab from any moisture vapour transmission through insulation
  • Used where insulation is hydrophobic PIR board that needs protection from the fresh concrete
  • DPM above insulation is better at waterproofing the slab but insulation may take up water if DPM is damaged

For domestic construction, DPM below insulation is most common. Follow the insulation manufacturer's guidance for their specific product.

DPM Installation

Critical points:

Coverage: DPM must cover the entire slab footprint with no gaps. It must extend up the inner face of all surrounding walls to link with the DPC. The link between DPM and DPC must be continuous — otherwise moisture from the ground can bypass the DPM by wicking up the wall and across the DPC gap.

Laps: Minimum 300mm at all joints. Tape all laps with compatible DPM tape (double-sided tape rated for polythene DPM; not standard masking tape). Do not use staples or nails through the DPM.

Penetrations: Where services penetrate the DPM (drainage, electricity, gas), use proprietary pipe sleeves and collar seals specifically designed for DPM. A service pipe simply pushed through an unprotected hole creates a direct moisture path.

Damage: Any tear or hole must be patched before concrete is poured. Use DPM tape on a patch of DPM material that overlaps the damage by minimum 150mm on all sides. Walk the DPM surface before laying insulation to check for damage.

Reinforcement Placement

The purpose of reinforcement in a ground-bearing slab is to control cracking, not primarily to carry structural load (the ground carries the load). A142 welded fabric mesh (6mm bars at 200mm centres) is standard.

Cover: The mesh must have a minimum of 75mm of concrete below it (bottom cover) and remain at least 75mm from the top surface. With a 100mm slab, this means the mesh is roughly centred. Use proprietary bar chairs (plastic spacers) at 600mm centres to maintain cover.

Laps: Mesh sheets must be lapped by one full mesh opening (200mm) plus 75mm at all joints — minimum 275mm lap. Do not butt-joint without a lap; a gap between sheets creates a crack line.

Position at edges: Pull the mesh back from slab edges by minimum 100mm to avoid the mesh being exposed at edge cracks.

Concrete Grade and Pour

Grade: C25/30 (characteristic compressive strength 25 N/mm² at 28 days) is the standard for domestic ground-bearing slabs. In aggressive ground conditions (sulphates, contaminated ground), the engineer will specify a higher-grade mix with sulphate-resisting cement.

Placement:

  • Pour from one end to the other; do not pour in isolated areas and then fill in
  • Avoid displacing the mesh as you pour; keep it approximately centred in the slab depth
  • Vibrate the concrete thoroughly around service penetrations and at slab edges
  • Screed off level to the required finished level

Curing: The concrete must be kept damp and protected from:

  • Frost — don't pour in temperatures below 5°C unless specific cold weather protection (insulating blankets, accelerated mixes) is used; ground frost can penetrate the fresh slab
  • Rapid drying — cover with polythene sheeting or use a curing compound immediately after surface finishing; maintain for minimum 7 days, preferably 14

Construction joints: If the slab is too large to pour in one continuous operation (typically >25m in any direction), construction joints are required. These must be specifically located and formed — a random stopping point creates a weak joint. Engineer's advice on joint location and detailing for large slabs.

Building Regs Part L: Insulation Requirements

The ground floor U-value target under Part L is typically:

  • New dwellings (Part L1A): U ≤ 0.25 W/m²K for ground floor
  • Extensions (Part L1B): U ≤ 0.25 W/m²K unless constrained by existing construction

Typical insulation thicknesses to achieve 0.25 W/m²K:

  • EPS (expanded polystyrene, λ = 0.04 W/mK): 75–100mm depending on floor area-to-perimeter ratio
  • PIR board (λ = 0.022–0.023 W/mK): 50–60mm
  • Phenolic board (λ = 0.018–0.020 W/mK): 40–50mm

Use the floor area/perimeter ratio (A/P ratio) in the calculation — larger floor areas relative to perimeter have a higher effective insulation value. Use the BRE SAP calculation methodology or a U-value calculator with the specific product lambda values.

Radon Protection

In areas of the UK with significant radon risk (parts of Cornwall, Devon, Northamptonshire, Derbyshire, Lincolnshire, and parts of Scotland), additional radon protection is required:

  • Radon barrier — gas-resistant membrane below the slab; higher specification than a standard DPM; purpose-made radon membranes include Visqueen Radon Gas Membrane and similar
  • Radon sump — in high-risk areas (>3% or >10% of homes above radon action level, depending on local authority requirements), a radon sump (sealed void below the slab connected to an extraction point at the external wall) is required

Check the UK Health Security Agency's radon risk map to determine if the site is in an affected area.

Frequently Asked Questions

Does a ground-bearing slab need Building Regulations approval?

Yes. A ground-bearing slab for a new dwelling or extension requires building control approval. It is a structural element and must comply with Approved Document A (Structure), Approved Document C (moisture resistance), and Approved Document L (thermal performance). The building control officer will inspect the sub-base and reinforcement before the concrete is poured.

Can I use A193 mesh instead of A142?

Yes. A193 mesh (7mm bars at 200mm centres) is heavier than A142 and provides more reinforcement. It is acceptable and in many cases preferred where ground conditions are variable or where the slab may carry heavier loads. A142 is the minimum standard mesh for domestic ground-bearing slabs; A193 or higher-specification meshes can always be substituted.

What is the minimum cover to DPM above sub-base before insulation?

There is no strict minimum for blinding thickness below DPM, but 50mm is standard. The purpose of the blinding is to provide a clean, level, dry surface for the DPM. Without blinding, the DPM rests directly on the sub-base, which can puncture it and makes it difficult to achieve a flat, well-sealed DPM.

Can I pour a slab in cold weather?

Not without protection. Fresh concrete must not freeze during the first 48 hours (and ideally the first 7 days). In temperatures below 5°C, protect with insulating blankets, use accelerated mix (with reduced water:cement ratio or admixtures), or delay pouring. Frost damage to fresh concrete is irreversible and will result in weak, friable concrete that will not achieve its design strength.

Regulations & Standards

  • Building Regulations 2010 Approved Document C — Site preparation and resistance to contaminants and moisture; DPM requirements

  • Building Regulations 2010 Approved Document L — Conservation of fuel and power; ground floor U-value requirements

  • Building Regulations 2010 Approved Document A — Structure; slab thickness and reinforcement

  • BS 8204-1:2003+A1:2009 — Screeds, bases and in situ floorings; concrete bases

  • BS 8102:2009 — Code of practice for protection against water ingress below ground; moisture control

  • BS 4483:2005 — Steel fabric for the reinforcement of concrete; mesh specification

  • Approved Document C — GOV.UK — DPM and moisture requirements

  • Approved Document L — GOV.UK — insulation U-value requirements

  • UK Health Security Agency Radon Map — radon risk area mapping

  • hardcore sub base compaction — sub-base layer below the slab

  • strip foundation design — foundation design coordinated with slab level

  • raft foundation guide — when to use a raft instead of a ground-bearing slab