Ground-Bearing Concrete Floor Slabs for London Extensions: Specification, Insulation, and Part L Compliance

**Excavation and ground preparation**:

Before any slab is poured, the ground must be prepared to provide a stable, load-bearing formation for the slab and insulation. The sequence:

• *1. Excavation to formation level*:
• The ground within the extension footprint is excavated to the required formation level — the depth at which the slab's underside will sit. For a typical London single-storey extension with an internal floor-to-ceiling height of 2.4m and an external ground level at 0mm, the formation level is calculated as:
• •Structural slab: 100–150mm
• •Insulation under slab: 75–150mm (PIR or EPS)
• •Sand blinding: 50mm
• •Hardcore: 150mm minimum (compacted)
• •Formation level: typically 375–500mm below the external ground level

This calculation is important: if the slab formation level is higher than the bottom of the external foundation strip or trench fill, the slab can be poured on the natural ground; if it is significantly lower than the extension foundations, there is a risk of undermining the foundations when excavating.

*2. Removing organic material*: All topsoil, organic material, roots, and vegetation must be removed from the formation level — any organic material left under the slab will decompose, causing settlement of the slab above. In London rear gardens, topsoil is typically 150–300mm deep; in made ground areas it can be deeper. The formation level should be clean, undisturbed London Clay (firm and stiff to the heel) or clean granular material.

• *3. Hardcore (compacted granular fill)*:
• A layer of compacted granular hardcore is laid on the formation to provide a stable, level, well-drained working surface for the DPM and insulation above. Specification:
• •Material: Type 1 granular fill (crushed concrete or limestone; graded aggregate); MOT Type 1 from a concrete recycling or quarrying supplier; NOT recycled mixed demolition waste (which can contain sulphates, organic material, or heavy metals that could contaminate the ground or attack the concrete)
• •Thickness: minimum 150mm compacted (before compaction, lay approximately 180–200mm to allow for 15–20% compaction settlement)
• •Compaction: plate compactor or roller in 100mm layers; check compliance with level
• •Blinding: after compaction, a 50mm layer of sharp sand blinding is spread and levelled over the top of the hardcore — this fills the voids in the surface of the hardcore and provides a smooth, level bed for the DPM without puncture risk

*4. Damp proof membrane (DPM)*: The DPM prevents ground moisture from rising through the slab into the building. It also prevents aggressive ground water or sulphates (present in some London Clay soils) from attacking the concrete slab.

• Specification:
• •Material: minimum 1200 gauge (300 micron) polythene sheet; 1200 gauge is the minimum and is widely used; 1500 gauge is preferred for London Clay sites where ground moisture is higher
• •Layout: sheet laid flat across the full slab area; lapped minimum 300mm at joints; lapped up the inside of the foundation walls around the perimeter; sealed at laps with DPM sealing tape
• •Position: below the insulation (the 'sandwich' arrangement — hardcore → blinding → DPM → insulation → slab — is standard in UK residential construction)

Alternative — DPM above insulation: Where the insulation is not moisture-resistant (e.g., standard polystyrene EPS without moisture protection), the DPM can be laid above the insulation and below the slab. This arrangement places the insulation at risk of moisture ingress from the ground — check that the insulation specification is appropriate for the sub-DPM position (EPS70 and PIR boards can be laid below the DPM if sealed against moisture ingress at the edges).

**Part L requirement for ground floor U-value**:

Building Regulations Approved Document L1B (Conservation of fuel and power in existing dwellings) requires that the ground floor of a new extension achieves a U-value of 0.25 W/m²K or better. The insulation under (or within) the slab is the primary means of achieving this.

**Under-slab insulation options**:

• *Option 1 — PIR board (polyisocyanurate) under the slab — the premium choice*:
• •Product examples: Celotex GA4000; Kingspan Kooltherm K3 Floor; Recticel Eurofloor
• •Lambda value: approximately 0.022–0.023 W/mK
• •75mm PIR: thermal resistance (R) = 75/0.022 = 3.41 m²K/W; combined with ground resistance and concrete slab → U-value approximately 0.22–0.25 W/m²K (borderline)
• •100mm PIR: R = 4.55 m²K/W; U-value approximately 0.17–0.20 W/m²K (well within Part L)
• •PIR boards are resistant to moisture (closed-cell structure) and can be laid below the DPM (below the DPM they are exposed to ground moisture — specification should be confirmed with the manufacturer)
• •Boards are rigid and stable — they do not compress significantly under the concrete slab load
• •PIR is the most cost-effective insulation per unit of thermal resistance — thinner boards achieve the same U-value as much thicker EPS

• *Option 2 — EPS (expanded polystyrene) under the slab*:
• •Product examples: Jablite EP70; Jablite EP100; Kingspan GreenGuard Floor; Celotex XR5000
• •Lambda value: approximately 0.033–0.038 W/mK (significantly higher than PIR — less insulating per mm thickness)
• •100mm EPS70: R = 100/0.036 = 2.78 m²K/W; U-value approximately 0.28 W/m²K (marginally above the Part L limit of 0.25 — typically not sufficient at 100mm; increase to 150mm)
• •150mm EPS70: R = 150/0.036 = 4.17 m²K/W; U-value approximately 0.20–0.22 W/m²K (compliant)
• •EPS is cheaper per m² of board than PIR but requires greater thickness to achieve the same U-value — the saving is partially offset by the additional depth required in the floor construction and the additional hardcore excavation to accommodate the thicker insulation layer
• •EPS must be laid above the DPM (not below — EPS absorbs moisture if in long-term contact with ground water)

*Option 3 — Insulated screed or composite slab system*: For projects where the total floor build-up height is severely constrained (low ceiling height; level matching requirements), specialist composite insulated floor systems (e.g., Jackon Thermowand; Cemfloor Thermal) combine the insulation and slab/screed into a single proprietary product. These systems are more expensive than board + slab but may be the only option where floor depth is limited.

**Perimeter (edge) insulation — addressing the floor edge cold bridge**:

The thermal calculation for a ground floor slab includes the effect of the floor edge — where the slab meets the external wall. The concrete slab is in thermal contact with the external masonry wall at this junction, creating a cold bridge that reduces the effective thermal performance of the floor. Building Regulations guidance requires that this perimeter cold bridge is addressed:

• •Specification: 50mm EPS or PIR rigid insulation strip, continuous around the full perimeter of the slab, positioned vertically between the concrete slab and the outer leaf of the masonry wall (or cavity fill)
• •This insulation strip interrupts the thermal path from the slab to the outside through the masonry
• •Without edge insulation, the floor-to-wall junction is a condensation risk in cold weather — moisture condenses on the skirting board and adjacent floor surface at this location
• •In concrete-to-masonry contact situations (traditional practice without edge insulation), visible condensation at skirting level on the inside of the external wall of an extension is extremely common in London Victorian-era additions built before edge insulation became standard practice

**Reinforcement mesh — A142 or A193**:

• Ground-bearing slabs in UK residential construction are typically lightly reinforced with welded steel mesh:
• •A142 mesh: 6mm diameter bars at 200mm centres both ways; weight 2.22 kg/m²; suitable for slabs up to 150mm thick carrying standard residential loading
• •A193 mesh: 7mm diameter bars at 200mm centres both ways; weight 3.02 kg/m²; for 150mm+ slabs or elevated loading
• •The mesh is placed at the mid-height of the slab on 'chairs' (plastic or steel spacers, typically 50mm high for a 100mm slab; 75mm high for 150mm slab) to maintain the correct cover
• •Mesh is not structural reinforcement for spanning — the slab is not designed to span and the mesh does not contribute to spanning capacity. The mesh controls shrinkage cracking (concrete shrinks as it cures; the mesh holds the curing shrinkage cracks closed and distributes them into many small, harmless cracks rather than a few large cracks)

**Concrete specification for a London extension floor slab**:

The concrete for a ground-bearing slab must have adequate compressive strength, durability, and (in some conditions) sulphate resistance:

• *Standard mix for residential extensions on London Clay (low sulphate ground)*:
• •C25/30 (also designated RC25 or as GEN3 in the NSCS descriptive specification): minimum 28-day cube strength of 25 N/mm²
• •This is the standard specification for residential slabs in London on normal London Clay ground
• •Slump: S3 (100mm) for pump delivery; S2 (75mm) for skip delivery — a wetter mix is easier to place and level but has marginally lower final strength; specify slump to suit placement method

• *Sulphate-bearing ground*:
• London Clay in some areas has naturally elevated sulphate content — particularly in areas where there is also groundwater. Sulphates attack Portland cement concrete over time, causing expansion, cracking, and eventual disintegration of the slab. For sites with suspected or confirmed sulphate conditions (check the BRE Special Digest 1 DS Class for the site):
• •DS-2 to DS-3 conditions: use C32/40 concrete with a sulphate-resistant cement (SRPC) or blended cement (30–65% GGBS content)
• •DS-4 conditions: specialist concrete specification required

• *Concrete thickness*:
• For a standard residential ground-bearing slab:
• •Minimum 100mm concrete slab thickness
• •150mm for areas with heavier loading (utility room with washing machine/dishwasher; kitchen with heavy island unit)
• •For extensions with underfloor heating (wet/hydronic UFH pipes cast within the slab): minimum 100mm concrete cover above the pipes

• *Ready-mix delivery*:
• Ready-mix concrete from a batching plant is the standard delivery method for London extension slabs. Typical order size for a 25m² extension floor: 25m² × 0.150m thickness = 3.75m³ — a small 4m³ half-truck load is the minimum delivery from most London ready-mix plants. Delivery within inner London involves the following practical constraints:
• •Access for the delivery truck (minimum 2.5m wide access; minimum 3.5m height clearance)
• •Pour distance from truck to slab location (standard chute reach approximately 3–4m; pump hire required for longer distances or restricted access)
• •Pour speed matching (concrete must be poured and levelled before initial set — approximately 90 minutes from batching plant departure in hot weather, 120+ minutes in cooler conditions)

**Floor screed — the finished layer above the structural slab**:

For most London extensions, the structural concrete slab (100–150mm) is followed by a screed layer (40–75mm) that provides the finished, level surface for floor finishes. The screed may be:

• *Traditional sand-cement screed (50–75mm)*:
• •Mixed on site (1 part cement : 3–4 parts sharp sand; water added to achieve a 'firm damp' consistency — the screed should form a ball when squeezed but not exude water)
• •Applied by a screeding gang and levelled with screeding battens or a powered screed rail system
• •Curing time: foot traffic at 24–48 hours; full loading at 28 days; ceramic tile laying typically at 21+ days (check manufacturer recommendations for tile adhesive/grout compatibility)
• •Cost: typically £15–£25/m² labour and materials

• *Liquid anhydrite screed (flowing screed — Gyvlon, Cemfloor, or similar)*:
• •Pump-applied: the screed is pumped from a pump on the road through the building to the slab area; self-levels under gravity
• •Produced a significantly flatter, more level surface than traditional sand-cement screed (because it is liquid when poured and self-levels)
• •Faster: a pump crew can lay 500m²/day of liquid screed vs. 60–80m²/day for traditional screed gangs
• •Important: liquid anhydrite screed is calcium sulphate based — it must NOT be used with embedded UFH pipes that use aluminium components (anhydrite attacks aluminium); it requires a priming coat under tile adhesive; it is not suitable for wet areas (showers, wet rooms) without waterproofing
• •Curing: foot traffic at 24–48 hours; liquid screed must be 'laitance removed' (light sanding or sweeping) before floor finish application; allowed to dry thoroughly (typically 1mm per day drying time up to 40mm depth; 80mm liquid screed = 80 days drying at 1mm/day — which is why liquid screed is typically specified at 50mm maximum for tight programmes)
• •Cost: typically £18–£30/m² pump-applied, including pump hire allocation

**Common mistakes on London extension concrete slab installations**:

*1. Insufficient compaction of hardcore*: Uncompacted hardcore settles after the slab is poured, causing the slab to crack and sink. All hardcore must be compacted in maximum 100mm layers with a plate compactor. In London garden extensions where access for a petrol plate compactor is restricted (rear garden gate), hand tamping is sometimes substituted — which is not adequate for the lower layers of a deep hardcore bed. Specify a plate compactor and confirm it can be brought through the access route before works start.

*2. Concrete poured directly on soil without hardcore (to save depth)*: Occasionally a contractor attempts to omit the hardcore layer and pour directly on the compacted soil formation to reduce the required excavation depth. This is not acceptable — the soil surface is uneven, the concrete will have variable thickness, the DPM will be damaged by sharp stones, and the clay may expand with moisture introduced during the pour. Always include at least 150mm compacted hardcore and 50mm sand blinding.

*3. DPM punctured during insulation or mesh installation*: The DPM is thin polythene — it is easily punctured by the edges of PIR insulation boards, by the feet of workers walking on the insulation before the slab is poured, or by the wire ends of the mesh reinforcement. Punctures allow ground moisture to wick up through the slab. Mitigation: walk on the insulation on purpose-made walkboards distributed across the insulation; use steel fixings on mesh rather than wire ties where possible; inspect the DPM for damage before pouring.

**Total costs for a ground-bearing concrete slab in a London extension (2025, supply and install)**:

For a 25m² single-storey rear extension floor area:

| Element | Cost range | |---|---| | Excavation to formation level (300mm) | £800–£1,500 | | Hardcore supply + lay + compact (150mm Type 1) | £400–£700 | | Sand blinding (50mm) | £100–£200 | | DPM (1200 gauge polythene) | £80–£150 | | PIR insulation 100mm (25m²) | £600–£1,000 | | Perimeter edge insulation | £100–£200 | | A142 mesh + chairs | £120–£200 | | Ready-mix C25 concrete (4m³) | £400–£700 | | Concrete pour labour + float | £300–£500 | | Sand-cement screed (50mm, 25m²) | £400–£700 | | **Total** | **£3,300–£5,850** | | **Per m²** | **£130–£234/m²** |

Note: the higher end of this range reflects inner-London logistics premiums (restricted access; small ready-mix deliveries; premium hardcore prices from local suppliers). The lower end applies to more accessible sites with easier delivery and disposal arrangements.