Solid Concrete vs Beam and Block

Ground Floor:-

Solid Concrete vs Beam and Block

Many older homes have a ‘suspended’ ground floor, with timber joists spanning from wall to wall, supported by sleeper walls. The problems are manifest and are exacerbated by the modern requirement for insulation within the floor zone which, although possible, is hard to achieve, and is made even more difficult by the need to ventilate beneath the floor. The Building Regulations also require that the ground beneath the floor is concreted and, once this has all been done, most self-builders decide that they might as well build a solid concrete floor.

The Building Regulations require ground floors to be insulated to attain a U-value of at least 0.22. This can be achieved using 70mm rigid foam flooring slabs but it is often better to upgrade to 100mm to account for future requirements. The slabs can be laid beneath the concrete oversite, above it or in a combination of the two. With beam and block flooring the insulation is usually laid on top of the beams, beneath the screed and, as such, is laid at a later date. There are, however, systems that employ rigid foam infill blocks that form the floor. These provide excellent insulation but require that the finishing screed is laid at this early build stage.

The Costs

NB: These costs assume a 100mm solid concrete oversite with an average fill of 300mm in each of the bays.

Preparation

2 labourers 1 day @ £100 each

£200.00

Subtotal

£200.00

All vegetable soil or topsoil will have been cleared from the oversite in the earlier stages of the building work, but it is important that any vegetable or organic matter, and tarmac or seams of coal, are removed. The ground within each section, or ‘bay’, of the building must be levelled out.

Consolidation and filling
Ganger plus semi-skilled labourer plus another labourer @ £41 per hour, 3 days	£984.00
38 tonnes hardcore @ £13 per tonne	£494.00
VAT @ 20%	£98.80
Vibrating plate compactor @ £27 a day, 3 days	£81.00
VAT @ 20% (not recoverable)	£16.20
13 tonnes sand blinding @ £16 per tonne	£208.00
VAT @ 20%	£41.60
2 labourers, 1 day @ £100 each	£200.00
Subtotal	£2,123.60

Each bay must be filled with a minimum of 150mm and a maximum of 600mm hardcore, compacted and then blinded with clean sand so as not to puncture any damp-proof membrane. The consolidation process should be in 100mm layers with each layer compacted prior to the next being laid. There can be a temptation to use brick rubble or hardcore, but this must be broken up into 50mm pieces and the labour involved in that often means that it is better to buy in clean stone or Type 1 hardcore.

Damp-proof membrane, Radon gas and installation
2 rolls polythene membrane @ £59 per roll	£118.00
VAT @ 20%	£23.60
3 rolls sealing tape @ £6.77 each	£20.31
VAT @ 20%	£4.06
Ganger plus labourer @ £27.63 per hour, 1 day	£221.04
45 sheets 100mm flooring insulation @ £32 per sheet	£1,440.00
VAT @ 20%	£288.00
6 rolls foil tape @ £6.77 each	£40.62
VAT @ 20%	£8.12
Ganger plus 2 labourers @ £39.63 per hour, 1 day	£317.04
25mm perimeter insulation, 2 sheets @ £11.50 each	£23.00
VAT @ 20%	£4.60
1 labourer, 1 day @	£100.00
Subtotal	£2,608.39

The damp proof membrane must be continuous over the whole floor zone, dressed up the walls to link in with the damp proof course. It must be sealed at all joints, laps and junctions, including service entry points, and, to protect against Radon gas, sealed to a cavity tray in the inner leaf of the cavity walls, supported by the cavity fill and brought through the external wall leaf with weep holes to the outer skin. The membrane must be 300mu (1,200 gauge). The insulation slabs, if they are to be below the concrete, must be laid on the damp proof membrane and then sealed together using foil tape. Around the perimeter of the building an upstand of 25mm insulation must be provided to prevent cold bridging. Some builders prefer to lay a further layer of damp proof membrane above the insulation prior to pouring the concrete oversite.

Concrete oversite
18 sheets A142 mesh @ £14.70 per sheet	£264.60
VAT @ 20%	£52.92
Ganger plus labourer @ £27.63 per hour, 1 day	£221.04
12 cubic metres concrete, (mix type ST2 or GEN 1) @ £75.68/m³	£908.16
VAT @ 20%	£181.63
Ganger plus 2 labourers @ £39.63 per hour, 1 day	£317.04
Concrete pump hire	£560.00
VAT @ 20% (not recoverable)	£112.00
Subtotal	£2,617.39

Preparation is everything with concrete. If you’re ready for it and all the levels have been set out correctly then the pour will go well. Some will prefer to barrow the concrete in. This is labour intensive, prone to waste and liable to cause harm to the structure due to the inevitable accidents. If the oversite is close to where the ready-mix concrete lorries can get and they can pour straight in, then the chances of accidents are minimised but, even then the force of the concrete delivery can disturb the membrane, and the concrete also has to be shifted from bay to bay. A concrete pump, though expensive, saves on all of this and saves time, making it possible to place huge amounts of concrete in hours rather than days. Most local authorities now ask for reinforcement within the slab.

ABOVE: 1. Concrete block/ inner wall; 2. Outside wall; 3. Hardcore infill; 4. Damp-proof membrane; 5. Sand blinding; 6. Minimum 100mm concrete

Ground Floor Total	£7,549.38
(of which recoverable VAT)	£703.33

Alternate Costs for Beam and block

The need for reinforcement within the solid concrete oversite is one reason why the use of beam and block flooring becomes a more attractive option. If the slab has to be thickened or is required to sit on the foundation walls due to the fill exceeding 600mm or the fear of differential settlement, then the reinforcement may have to be laid in two layers and may have to be of a stronger type.

In a Radon gas area, which is gradually being extended to the whole country, the beam and block floor will require a membrane and the same precautions at the junction with the walls as with the solid slab oversite.

Although a concrete pump won’t be needed, on many sites it will be necessary to hire a crane to move the beams around, and the labour element of placing the beams, building in the necessary cranked ventilators and infilling the beams with blocks will increase.

The flooring insulation wouldn’t go in at this early stage but, for the sake of comparison, is left in these costings.

ABOVE: 1. Concrete block; 2. Outside wall; 3. Concrete block/ inner wall; 4. Sleeper wall; 5. Concrete beam

Levelling out oversite
2 labourers, 1 day @ £100 each	£200.00

Laying floor beams and infilling
Floor beams – package including beams, split coursing bricks, cranked ventilators and airbricks	£1,762.00
VAT @ 20%	£352.40
125m² infill blocks @ £5.95/m²	£743.75
VAT @ 20%	£148.75
Crane hire, 1 day	£350.00
VAT @ 20% (not recoverable)	£70.00
Ganger plus semi-skilled labourer plus another labourer @ £41 per hour, 4 days	£1,312.00

Membrane/ Radon protection
2 rolls polythene membrane @ £59 per roll	£118.00
VAT @ 20%	£23.60
3 rolls sealing tape @ £6.77 each	£20.31
VAT @ 20%	£4.06
Ganger plus labourer @ £27.63 per hour, 1 day	£221.04

Insulation (costed for comparison; in practice left until later)
45 sheets 100mm insulation @ £32 per sheet	£1,440.00
VAT @ 20%	£288.00
6 rolls foil tape @ £6.77 each	£40.62
VAT @ 20%	£8.12
Ganger and 2 labourers @ £39.63/hour, 1 day	£317.04

Beam and Block Ground Floor Total	£7,419.69
(of which recoverable VAT)	£824.93

Which floor type?

On paper the costs of the two systems are remarkably similar but builders favour beam and block because, with fewer variable factors, there is a greater certainty of costs. The drawbacks with the beam and block floor are the need for absolute accuracy and the fact that the floor cannot be ordered – unless you take a chance – until the foundation walls have been built and measured, after which there may be a waiting time for delivery.

How to reduce these costs

This is a fairly labour-intensive part of the house but self-builders may decide that they can take on much if not all of the work involved at this stage, perhaps with a little help.

The concrete pump is an expensive item but if the pour was spread over more than one day and the labour requirement increased, its cost would pale into insignificance.

Mixing your own concrete may seem a good option but the mix won’t be consistent and the structure could suffer.

Our Benchmark House

The floorplan for the house costed within this series measures up at 227m² — much larger than the average family home. Four bedrooms on the smaller first floor share three bathrooms. Downstairs, the living spaces flow easily onto one another, with an open plan family kitchen being the hub of home life.