What Will it Cost? - Constructing Floors

There are three common designs for floors in new housing and all may well be suitable for your needs:

• a ground-bearing concrete slab (obviously only an option on ground floors)
• a precast concrete floor
• a timber joisted floor

The latter two floors are examples of sus­pended floors, which means that their weight is borne by supporting walls, whereas the slab sits on the ground. When comparing floor construction costs, it’s normal to ignore the choice (and cost) of the final floor cover­ing, but this sometimes has an important bearing on the choice of the floor it sits on. Specifically, it is usually simpler and more convenient to lay a timber-finished floor on a timber sub-floor, be it joists or a timber board covering. And conversely, it is easier to lay ceramic tiles or stone on a cement screed. With sheet floor covers and carpet, it really makes no difference.

Ground-bearing slabs

This is invariably a concrete floor, poured and finished on site, but there are, in fact, a num­ber of elements that go to make up such a floor.

Working from the base:

• 150mm of hardcore
• 50mm of sand
• polythene damp proof membrane
• 75-100mm of expanded polystyrene insulation
• 100mm of poured concrete, with or without anti-crack mesh.

In addition to this, many people will choose to top all this off with a cement screed of 65 or 75mm depth, often incorporating under­floor heating pipes, before fitting the finished floor cover. Overall, the construction measures upwards of 450mm depth. Every element of this floor has a cost attached to it. Including insulation and a screed topping (but not the actual floor finish), in situ slab/screed floors work out at around £50/m² for both labour and materials.

Precast Concrete

Precast means simply that the concrete for the floor has been made in a factory and delivered to site. The commonest type of precast flooring is the concrete T-joist, used in conjunction with regular building blocks to fill the gaps between, and known as a beam and block floor. This type of floor became widely used in the 1980s as a way of over­coming problems with difficult site conditions where it was felt a ground-bearing slab might not be suitable. But the speed of installation is such that precast flooring is now used widely on sites where there are no specific problems to overcome. And, being suspended, it’s also widely used on intermediate floors.

The simplest precast floor, the beam and block floor, tends to cost much the same as an in situ slab, provided there is a decent size of floor to be laid: typically this will be something in excess of 100m². On smaller jobs, the cost of transportation makes the precast floor too expensive. Other common types of precast floor are the hollowcore slabs which don’t require infilling and a few designs that use polystyrene infill blocks in place of the more normal concrete blocks. These require a ready-mix concrete to be poured on top, so they are a hybrid between the in situ slab and the precast floor. The main advantages of solid upper floors are that they provide a solid platform to build off straight away, they provide good sound insulation between floors, and they allow all upper storey partition walls to be built from masonry.

Timber Joists

The simplest and cheapest way of building intermediate floors is to use timber joists. You can still build a floor for around £30/m² (though of course it will still require a ceiling under it and a floor cover above). In compar­ison, precast concrete intermediate floors are between £10/m² (beam and block) and £25/m² (the higher specification hollowcore floor) more expensive.

On ground floors, the difference in price is not nearly so marked because you have to pay attention to various sub-floor details that don’t affect intermediate floors: these being specifi­cally that insulation, ventilation and a concrete sub-floor are required. The current trend in timber floors is to switch away from traditional joists towards engineered timber I-beams. These lightweight beams can span longer distances than timber joists and are very quick to install. Plumbers and electricians also like them because they are much simpler to cut through. They also boast that they eliminate squeaking floors, something conventional timber joisted floors are notorious for. In cost terms, I-beams are more expensive to buy than joists, but they do claw back most of this added cost by reducing the labour at first fix stage.

Sound Reduction

Good sound insulation is one of the key fac­tors behind self-builders choosing precast concrete intermediate floors. Many people have experienced modern houses with poor soundproofing and the intermediate floor built from chipboard with plasterboard ceilings under­­­neath is a prime culprit. By and large, concrete intermediate floors are excellent at absorbing sound but, don’t dismiss timber floors out of hand — because they can be ‘improved’ so that they can perform just as well as concrete floors. Besides adding sound insulation in the void between the joists (now mandatory in new houses), you can double up the plasterboard on the ceilings or use a special sound-deadening board. You can also add resilient layers under the floor covering. And avoid the use of downlighters in the ceiling below — this makes a pathway for sound to travel through.

Of course, there are extra costs involved. Constructing a really good timber intermediate floor, capable of forming a party floor between flats, will double the original cost of £30/m², and make it about the same price as a precast hollowcore floor. But in performance terms, they will be equal. As the old saying goes, you get what you pay for.

Underfloor Heating

Underfloor heating can be either incorporated into a cement screed or under a timber floor. However, do be careful to specify what type of floor you are laying and your choice of floor covering, because the heating char­ac­teristics are quite different and the designed-for tem­pera­tures have to be adjusted. On solid floors, the pipes have to be laid prior to the screed being laid but with timber intermediate floors the pipes can often be fixed from below, once the floor cover is fixed.

About These Articles

This series is based on a typically constructed, 4/5 bedroom house, with an internal floor area of 200m² (2,150 sq ft) plus an integral garage. Its raw build cost (that is only labour and materials) is £135,000. Professional fees, contractors’ overheads and profit, insurances and warranties would all be in addition to this. The raw build cost presumes a straightforward job finished to a fairly basic standard; the sort of finish you would expect from a professional housebuilder. In this series we will be looking at how the costs break down for all the various component parts of a house, and looking at the cost implications of choosing alternatives. This house is also featured in Mark Brinkley’s latest book, The Housebuilder's Bible (6th edition).

OTHER COST GUIDES:

• Getting out of the ground
• Insulation
• External Joinery
• Installing Services
• Roofing
• Internal Finishes