Beam & Block vs Concrete Slab Ground Floors

The choice of ground floor system affects programme, insulation strategy, cost and long-term durability. Three options dominate UK domestic construction: beam and block (B&B), ground-bearing concrete slab, and suspended timber. Each has a distinct set of applications and constraints. Getting the choice right at design stage saves significant remedial cost later.

What Each System Is

Beam and block (B&B): prestressed concrete beams of inverted-T profile span between the foundation walls, with infill blocks (typically 100 mm dense concrete, lightweight aggregate or aircrete) laid between them.¹ The joints are grouted, and a levelling screed usually goes on before the insulation and floor finish. The whole system is suspended above ground level, so it is tolerant of variable ground conditions, tree roots and poor fill.¹

Ground-bearing concrete slab (solid slab): a reinforced or unreinforced slab cast directly on a compacted hardcore bed (well-graded crushed stone or MOT Type 1), DPM (damp-proof membrane), and insulation. The slab bears on the ground beneath it, so it requires ground that is competent, not subject to tree root heave, and free of made ground or significant fill. A standard domestic slab is typically 100–150 mm of concrete, often with A142 mesh reinforcement.

Suspended timber: traditionally used in pre-1919 housing, suspended timber involves floor joists (47 × 100 mm to 47 × 175 mm) spanning between sleeper walls built off the strip foundation. Still used in heritage-sensitive projects and where site levels make a solid floor impractical.

Beam and Block: How It Works

Standard B&B spaces the beams 440 mm apart, the length of an infill block, which spans between them. For higher spans or loads the blocks are turned sideways and the beams close up to a 215 mm gap.¹ Infill blocks are available in:

• Dense aggregate (7.3 N/mm²): standard option, suitable for most loads¹
• Lightweight aggregate (Lytag-type): reduces weight and eases handling
• Insulated infill block (EPS core): reduces cold bridging through the floor system

Typically a 4:1 sand and cement grout is brushed into all the joints, and it can double as a levelling screed.¹ A separate levelling screed is generally recommended, and the insulation and floor finish go on top. In many new builds, underfloor heating (UFH) pipes are clipped into the screed layer.

Beam and block is delivered to site as precast beams, made in standard lengths, and loose infill blocks.¹ The crew lifts beams by hand or with a telehandler (for long spans), drops in blocks, then screeds. Weather protection is not critical: the system is inherently robust to rain during installation.¹

Ground-Bearing Slab: How It Works

A ground-bearing slab is simpler in concept but demands good ground preparation:

1. Strip topsoil to undisturbed ground or engineered fill
2. Lay 150–300 mm well-compacted hardcore (Type 1 MOT or clean crushed stone)
3. Lay DPM (polythene at least 300 µm, 1200 gauge, with sealed joints, as Approved Document C describes)²
4. Lay rigid insulation (EPS 150, XPS, or PIR), see below
5. Pour and level the concrete slab (Approved Document C describes at least 100 mm of concrete, thicker where the structural design needs it; 100–150 mm is typical)²
6. Cure and then lay floor finish as required

For tree-affected sites, a ground-bearing slab over engineered fill must account for heave, the slab may need to be thickened and reinforced, or replaced with a suspended system entirely.

Cost per m²

Prices are 2026 UK averages, ex-VAT. London and South East rates typically sit at the upper end; Scotland and Wales at the lower end.

Insulation Requirements

Under Building Regulations Part L 2021 (England), the limiting U-value for a floor is 0.18 W/m²K, both for a new dwelling and for a new floor in an existing one.³ 0.25 is the swimming pool basin figure, not the domestic floor one.³ In practice most specifications land between 0.13 and 0.18, because the notional dwelling that a new home’s targets are set against uses 0.13 W/m²K for floors, so a floor built only to the 0.18 limit leaves the rest of the design to make up the difference on the emission rate, primary energy and fabric efficiency targets.¹ New homes then move to the Future Homes Standard, whose regulations come into force on 24 March 2027, so a new-build floor designed now should be checked against the edition that will apply to it.⁴

Thickness depends on the insulant and on the floor’s perimeter-to-area (P/A) ratio. As a rough guide, the Modern Masonry guide puts 0.18 W/m²K at about 90 mm of PUR/PIR or 125 mm of EPS for a P/A of 0.4, and the notional 0.13 W/m²K at about 150 mm of PUR/PIR or 200 mm of EPS for a worst-case P/A of 0.7.¹

Insulation continuity at floor perimeter is critical. Cold bridging at the edge of a ground-bearing slab (where the slab meets the cavity wall foundation) is a common weak point. Perimeter upstand insulation (EPS or PIR) should be installed to prevent edge losses; the LABC beam and block junction detail reproduced in the Modern Masonry guide asks for an R-value of at least 0.8 m²K/W.¹

Where Each System Suits

Radon and Damp

Every ground floor has to keep ground moisture out to satisfy Building Regulations Part C (site preparation and resistance to contaminants and moisture), but Approved Document C does not ask the same of each system.² A ground-bearing slab gets a damp-proof membrane continuous with the damp-proof courses in the walls.² A suspended concrete floor such as beam and block instead relies on a ventilated void at least 150 mm deep, with a membrane only where the ground beneath has been dug out below the outside ground level and will not drain.²

In high-radon areas (parts of Cornwall, Devon, Northamptonshire, Derbyshire and others), Approved Document C points to BRE Report BR 211 for the protective measures, which typically include:²

• A radon-resistant membrane (a purpose-made radon barrier, or heavy-gauge polythene with sealed joints)⁵
• Radon sump provision (sealed sump with upstand for future fan connection)⁵
• Suspended floors such as beam and block have an advantage here: the void beneath can be ventilated, naturally or with a fan⁵

Check the UKHSA radon map (ukradon.org) at design stage for any new-build site.⁶ For a building site, the level of protection the Building Regulations call for (none, basic or full) is confirmed through the British Geological Survey’s GeoReports service.⁷

Building Regulations

Ground floor systems are governed primarily by:

• Part A (Structure): beam sizing, slab reinforcement and bearing design
• Part C (Site preparation and resistance to contaminants and moisture): DPM, radon barriers, subsoil drainage²
• Part L (Conservation of fuel and power): U-value targets and SAP compliance

A beam and block floor is normally designed from the beam manufacturer’s span tables, and the precast components are made to BS EN 15037 (precast concrete products: beam-and-block floor systems).¹ Ground-bearing slabs are designed to BS 8500-1 (specifying concrete) and Eurocode 2 (BS EN 1992-1-1) for reinforced slabs. On a new home with an NHBC warranty, the floor also has to meet the NHBC Standards, whose Chapters 4 and 5 cover foundations and ground floors.⁸

Always confirm the ground floor specification with your structural engineer before tendering, the difference in total floor thickness between systems has knock-on effects on finished floor levels, door thresholds, external steps and drainage connection heights.