Block and Beam Floor: A Practical Guide for Extensions

How the system works

Inverted-T prestressed concrete beams span from foundation wall to foundation wall at set centres. Standard concrete blocks sit on the beam flanges to fill the gaps, forming a deck. Grout is brushed into the joints to lock everything together, and the deck then takes insulation and a screed or a floating floor finish.

Because the floor spans rather than resting on the ground, it is unaffected by what the soil beneath it does. That is its main structural advantage.

When it beats a ground-bearing slab

A cast concrete slab on compacted hardcore is the familiar alternative. Block and beam tends to win in a few situations:

• Sloping sites, where a slab would need a deep build-up of fill
• Clay soils prone to shrinking and swelling, especially near trees
• Where deep foundations are already needed, so the floor can span between them
• Tight programmes, as there is no curing time before following trades
• Where a drainage run or a void needs bridging

Ventilation and the void beneath

A suspended floor leaves a void between the ground and the underside of the beams. That void needs cross-ventilation through airbricks, and where a ventilated void meets the existing house, any existing ventilation to the original floor must be kept open — sleeved through the new extension if necessary. Blocking an old house’s subfloor ventilation is one of the more common causes of damp problems after an extension.

Ground gas protection, where the area requires it, is designed into the void and membrane details.

Insulating a block and beam floor

Insulation usually goes on top of the deck: rigid insulation boards, a separating membrane and then a screed — often with underfloor heating pipes set into it — or a floating chipboard floor. The thickness is set by building regulations for heat loss and by the floor level that needs matching. Some systems use insulated polystyrene infill blocks instead of concrete ones to improve performance and lighten the deck.

Matching the existing floor level

In an extension, the finished floor almost always needs to meet the existing house at the same level, with no step. That level is worked backwards: finish, screed, insulation, deck, beam depth and bearing. Getting it right at the foundation stage is far easier than adjusting at the end, so it is one of the first details fixed on drawings.

How it is laid on site

Once foundations and the substructure walls are up to level, a damp-proof course goes on, the beams are delivered to a layout drawing and placed — by hand for short spans, with lifting gear for longer ones — and the infill blocks follow. A crew can deck a typical extension floor in a day. The deck is grouted, then left ready for the walls above and the floor build-up later in the programme.

What affects the cost

Span and loading set the beam type and centres. Site access matters, because beams are long and heavy. The infill choice, insulation thickness, screed or floating finish, and any underfloor heating all change the floor build-up. Against that, block and beam can reduce excavation, fill and waiting time compared with a slab on a difficult site.