Concrete Floor Beams

Concrete Floor Beams are strong in compression, but weak in tension. For this reason it is best practice to have a beam supported at each end carrying a load.

A beam and block floor is a suspended concrete floor made from prestressed concrete beams spanning between walls, with concrete blocks dropped into the gaps between them. The beams are shaped like an inverted T so the blocks sit on the flanges, and the whole surface is then brushed with a sand and cement grout to lock it together before insulation and a screed or boarding go on top. It is the standard ground floor for new houses, extensions and garages on clay or soft ground, because the floor is carried on the walls rather than resting on the soil, and it is often used for first floors too. Our prestressed concrete floor beams are 135mm wide and 150mm deep. 

The main downsides are weight, planning and ventilation. The beams are heavy, so they need two people or a lifting machine to place and cannot be adjusted on site the way timber joists can, which means the floor has to be designed and the lengths ordered before the walls are finished. The void underneath must be kept ventilated with air bricks and a minimum clearance to the ground, which adds height to the build. The finished floor is not level enough to use as it is; it needs insulation and a screed or boards on top, and without insulation it is a cold floor. 

Not under the beams, but you do need one on top. A beam and block ground floor sits over a ventilated void, so unlike a ground-bearing slab there is nothing under it to need a damp proof membrane. The membrane goes on top of the grouted beams and blocks, under the insulation and screed, and is lapped and sealed into the damp proof course in the walls so moisture from the void cannot rise through the floor. In areas with high radon levels, a radon barrier does the same job and should be continuous across the floor and walls. Your building control officer will confirm which barrier you need for your site.

A 3m length of our 150mm deep floor beam weighs around 110 to 130kg, which works out at roughly 35 to 45kg per metre, and the 4.8m beam is nearer 200kg. That is why beams are placed by two people with lifting hooks or by a telehandler or crane, not carried like timber. Order with access in mind: the delivery vehicle will offload at the kerb or onto hard standing, and you need a clear route and firm ground to move the beams to the walls. Ask us for the exact weight per metre when you order, as it varies slightly between manufacturers’ profiles. 

Standard 100mm thick building blocks go between the beams, laid flat so they rest on the beam flanges on either side. Dense 7N solid concrete blocks are the usual choice because they are strong and cheap; lightweight aircrete blocks such as Celcon or Plasmor can be used instead to reduce weight and improve insulation, as long as they are rated for floor use. Both are on our concrete blocks page. Hollow 215mm blocks are not suitable. The beams are set at centres to suit a 440mm block with a bearing each side, and any gaps at the edges are filled with cut blocks or concrete before grouting. 

Our floor beams come in four lengths: 3.0m, 3.6m, 4.2m and 4.8m. All are the same 135mm x 150mm prestressed section, so you choose the length to suit the span between your supporting walls plus a bearing of at least 90mm on each wall. Prices run from around £39 to £60 per beam including VAT depending on length. Spans beyond 4.8m, or floors carrying heavier loads, need a deeper beam or beams placed in pairs, and that is a job for a floor design rather than a product page, so talk to our concrete team before you order. 

Only by a small amount at the ends, and ideally not at all. Prestressed beams get their strength from steel strands tensioned inside the concrete, so cutting into the beam releases that tension. Trimming 50mm or so off an end with a diamond disc to fit a bearing is accepted practice, but a beam should never be cut mid-span, notched for pipes or shortened by a large amount. The right approach is to measure the opening, allow for the bearings and order the nearest length from the four we stock. Services run through the void beneath the floor or between the beams, not through them.

In practice, yes. Concrete joists is simply another name for prestressed concrete floor beams, and people searching for either are looking for the same product. Both describe a reinforced concrete beam that spans between walls to carry a floor, in the same way a timber joist does. The difference from a timber joist is what goes on top: concrete beams carry blocks between them to make a solid deck, rather than floorboards or chipboard laid across. They are also rot proof and fireproof, and they carry more load over the same span, which is why they have replaced timber for most new ground floors.