How the Beam and Block Calculator Works

Enter floor length and width along with span direction. The calculator divides span width by beam centres to find the number of beams, then works out infill blocks from remaining floor area.

The standard system uses 150mm or 225mm deep prestressed concrete T beams laid at 515mm centres, with 440 x 215 x 100mm infill blocks dropped between the beam flanges. At those centres blocks work out at roughly 5.5 per square metre of floor, so a 50 m² floor needs around 275 blocks or just over 300 once 10 percent wastage is added.

Beam Depths and Spans

Beam depth is set by clear span, loading and for standard domestic floors indicative maximums are:

• 150mm beams: spans up to about 4.5m
• 175mm beams: up to about 5.5m
• 200mm beams: up to about 6.5m
• 225mm beams: up to about 7.5m

Most UK house footprints span comfortably on 175mm or 200mm beams. Longer spans need either deeper beams, doubled up beams or a central sleeper wall built off its own foundation with honeycombed blockwork to keep air moving through void. Every beam layout should come from manufacturer’s design service or a structural engineer working to actual loads, so treat calculator quantities as ordering estimates around a proper design.

How the Floor Goes Together

The build sequence is straightforward, which is a large part of why system took over from ground bearing slabs. Beams land on a mortar bed on inner leaf with 100 to 150mm of bearing, camber side up, since prestressing gives every beam a slight upward bow. Infill blocks then drop in dry between flanges with no mortar. Once the floor is fully blocked out, whole surface is grouted by brushing a wet cement and sand slurry across joints and after 24 hours of curing floor takes foot traffic and becomes a working platform.

Two details protect floor for its lifetime, a minimum 150mm ventilated void beneath beams with airbricks giving cross ventilation and no load bearing walls built off the floor itself, since those need their own foundations.

What It Costs

For supply only, prestressed T beams currently run around £16.50 to £20.50 per linear metre depending on depth and span. A typical 50 m² domestic floor comes to roughly £1,400 to £1,900 in beams plus the infill blocks, with delivery adding £150 to £350 because beams arrive on a flatbed and usually need a hiab or crane to offload. Insulated systems replacing concrete infill with polystyrene panels cost more per square metre but hit U values new build regulations demand, down to 0.08 W/m²K with best undercloaking systems.

Worked Example

A 7m x 6m extension floor spanning the 6m direction on 200mm beams:

• Beams at 515mm centres across 7m: 14 beams, each 6.3m long including bearings
• Beam metres: 14 x 6.3 = 88.2 linear metres
• Infill blocks at 5.5 per m²: 42 m² x 5.5 = 231, plus 10 percent = 254 blocks
• Grout: cement and sand slurry for 42 m² of brushed joints
• Airbricks to give cross ventilation on opposing walls

At current beam prices that is roughly £1,450 to £1,800 of beams before blocks and delivery.