Concrete Floors for Extensions: Ground Bearing Slabs, Beam and Block, and Insulation

A ground-bearing slab is a concrete floor slab that rests directly on the prepared ground below — rather than on beams, piles, or joists. It is the standard specification for single-storey residential extension floors on stable ground.

• **When a ground-bearing slab is appropriate**:
• •Site ground conditions are stable and competent — typical made-up ground, firm clay, gravel, or chalk
• •No significant tree roots within the zone of influence of the slab (roots can cause heave as they grow, or shrinkage as clay dries when trees are removed)
• •No significant variation in bearing capacity across the slab footprint
• •The extension is single-storey and loads are modest (standard residential occupation)

• **When a ground-bearing slab is NOT appropriate (use beam and block or piled solution instead)**:
• •Tree roots present within the zone of influence (typically within 1–2 tree heights of the slab) — a suspended floor (beam and block, or insulated concrete formwork) eliminates the heave risk
• •Unstable or soft ground (fill material, former made ground, soft clays)
• •Significant variation in bearing capacity across the footprint
• •Very high loads (plant rooms, significant masonry loads transferred to the floor)
• •A tree has been recently removed — the clay that the tree was drawing moisture from will swell as it re-saturates (this can take years), causing uplift on any slab resting on it

**Standard ground-bearing slab construction sequence**: 1. Excavation to formation level (typically 450–600mm below finished floor level) 2. Remove all organic material and topsoil — organic material compresses and decays, causing settlement 3. Sub-base of compacted hardcore or Type 1 sub-base material (150–200mm typically) — provides a stable, well-compacted platform 4. Blinding layer (50mm lean mix concrete or sand blinding) — provides a clean, level surface for the DPM and insulation 5. DPM (1200 gauge polythene, 300 micron minimum) — lapped and taped at all joints, upstand to edges 6. Insulation (PIR board at 100–150mm typically, or EPS if budget constrained) — see insulation section below 7. UFH pipes if specified (laid on mesh reinforcement or clipped to insulation boards before concrete pour) 8. Reinforced concrete slab — typically 100–150mm thick (150mm recommended when UFH is included), C30 concrete, A393 mesh reinforcement (or fibre-reinforced if structural engineer confirms adequate) 9. Power float finish if tiles or vinyl will be applied direct; or wood float for levelling compound or screed over

**Settlement and movement joints**: Expansion joints (perimeter edge strip) should be installed around the entire perimeter of the slab, between the slab and the foundation walls. This allows the slab to move independently of the wall structure — preventing cracking at the wall-floor junction as the new slab settles.

Part L1B of the Building Regulations requires the ground floor of a new extension to achieve a maximum U-value of 0.22 W/m²K (the 'notional building' target used for compliance calculations). The insulation specification determines whether this is achieved.

**Insulation types for ground floors**:

*PIR (Polyisocyanurate) rigid foam boards*: The most thermally efficient rigid board insulation — thermal conductivity (lambda value) approximately 0.022–0.023 W/mK. The most compact (thinnest) specification for a given U-value. Brands: Celotex GA4000, Kingspan Thermafloor TF70, Recticel Eurofloor.

*EPS (Expanded Polystyrene)*: Lower thermal performance (lambda approximately 0.032–0.038 W/mK) — requires more thickness to achieve the same U-value as PIR. However, EPS is significantly lower in cost. Also better at handling moisture than PIR in a ground floor application.

*XPS (Extruded Polystyrene)*: Better moisture resistance than EPS, moderate thermal performance (lambda approximately 0.030–0.035 W/mK). Sometimes specified in flood-risk areas or where soil conditions are persistently damp.

• **Calculating the required thickness for Part L compliance**:
• For a 0.22 W/m²K U-value (Part L1B new extension ground floor), typical insulation thickness required:
• •PIR at 0.023 W/mK: approximately 90–100mm
• •EPS at 0.036 W/mK: approximately 140–150mm
• •XPS at 0.033 W/mK: approximately 130mm

Note: These are approximate and depend on the specific floor construction, concrete slab thickness, and screed thickness — the SAP/U-value calculator used by the Building Control officer will confirm the required thickness for the specific specification.

• **Floor level implications**:
• The total floor build-up depth determines the finished floor level relative to the external ground level and relative to the existing house floor. A typical full specification:
• •Sub-base: 150mm
• •Blinding: 50mm
• •DPM: nominal (polythene sheet)
• •Insulation (PIR): 100mm
• •Concrete slab: 150mm
• •Screed or adhesive bed: 50–75mm
• •Floor finish: 10–20mm
• **Total from formation: approximately 510–545mm**

This means the ground must be excavated to approximately 510–545mm below the intended finished floor level — which itself must be at least 150mm above external ground level (to comply with Part C — damp-proofing requirements for ground floor levels). Getting the floor level right requires careful coordination between the structural engineer, the groundwork team, and the architect at the design stage.

A beam and block floor is a suspended precast concrete floor system — a series of prestressed concrete T-beams laid at regular spacing, with concrete infill blocks laid between them. The finished surface is then covered with a screed.

• **Advantages over ground-bearing slab**:
• •Suspended above the ground — trees, fill, or variable ground conditions below do not affect the floor
• •Eliminates the need for excavation and removal of problematic material
• •Can be installed faster than a ground-bearing slab in complex ground conditions
• •Provides a void below the floor — can allow radon mitigation systems, drainage maintenance access, or future services routing

• **Disadvantages**:
• •Higher material and installation cost than a ground-bearing slab (typically +20–40%)
• •Requires adequate bearing onto the extension foundation walls — beam spans and bearing lengths must be coordinated by a structural engineer
• •The void below the floor must be ventilated (Part C) — requires air bricks at perimeter to prevent moisture accumulation in the sub-floor void
• •Underfloor heating is installed in the screed above the beam and block, not integrated into the structural system — more straightforward than ground-bearing UFH integration, but still requires screed of adequate depth

• **When beam and block is typically specified**:
• •Presence of trees or recent tree removal within the zone of influence
• •Made-up ground or fill that cannot be guaranteed to be stable under a slab
• •Sites where excavation to formation level would be disproportionately deep or expensive
• •Radon-affected areas (where a void with ventilation is part of the radon mitigation strategy)