The timber industry has been losing out to concrete in the domestic floors market, but is fighting back with prefabricated structural kits for faster and cheaper installation.
Timber’s on the way Back. AS Patios get the heave-ho and decking becomes de rigueur, householders are rediscovering their parquet floors from the 1950s and turning to the latest Scandinavian imports from Ikea. But it is not just in respect of surfaces that wood is undergoing a renaissance – it is also making a comeback in the structural market.

Timber-frame manufacturers have introduced several ideas to make wood the favoured choice of housebuilders after 10 years of being largely shunned in favour of concrete.

Having already started using standardised components and extensive pre-assembly for homes, timber-frame manufacturers are extending the approach to components such as structural floors. By offering prefabricated panel systems and engineered joist floor kits, manufacturers are helping to re-establish timber floors as a real alternative to concrete.

There are a number of reasons for the resurgence of interest in wood. The public is now more environmentally aware, and timber has gradually shed its low-tech image. Industry initiatives such as the Egan report, which extolled prefabrication and greater site efficiency, have also played their part, and the time and cost benefits inherent in timber-frame construction, particularly for hotel rooms and large housing developments, have won over other doubters.

The resurgence has come in the nick of time. In the decade to 1997, timber use in domestic ground floors fell from 17% of the total to 10%. In the same 10-year period, the use of suspended precast concrete rose from 23% to 60% of the total market, with a further 5% accounted for by insitu concrete.

But this paints rather too gloomy a picture of the timber industry. Despite its ground-floor battering, it retained its dominance at higher levels: 95% of intermediate floors in domestic construction still use timber. And timber has a lot going for it, even if concrete beam and block, which retains 5% of the domestic upper-floor market, may be about to increase its share –particularly at the executive end of the housing sector where the perceived acoustic superiority of concrete is winning over buyers. Not least, timber is the only fully sustainable and renewable construction material. It is also flexible, retains heat, and because it is lightweight, can reduce foundation requirements.

However, the way to persuade more builders to use timber is by introducing them to the benefits of prefabrication. One system that Trada Technology has been promoting is factory-assembled timber floor “cassettes”. These slot into place, providing a faster alternative to traditional stick-built floor joists and boards. Craned into position as construction proceeds, the cassettes are secured by nails and straps to provide a fast, safe working platform. The long, clear spans mean that the internal layout flexibility usually associated with concrete floors can now be just as easily achieved with timber. Floor cassettes comprise a system of timber joists or I-beams, typically 200-300 mm deep, supporting a floor platform that can be chipboard, plywood, tongue and groove or oriented-strand board. Costs are about £20/m2, supply only, for the floor cassette and deck. This cost will vary, particularly if bought as part of an overall timber-frame house kit.

Cassettes can be delivered with moisture-resistant plasterboard attached to the underside or left open for on-site fixing. If panels are to form a suspended ground floor, they can be pre-fitted with insulation and an underside liner. The wearing surface is faced with a removable waterproof membrane for weather protection.

Cassette size is determined by individual project requirements, transport and crane capacity.

They are typically 3 m wide and up to 12 m in length and can be delivered with waste and water pipes pre-installed. Three men and a crane operator are required to install each cassette. Depending on panel size, non-insulated cassettes weighing 50 kg/m3 can be lifted manually. Less labour is therefore needed onsite and the range of skills required is no more than that associated with traditional timber joists and boarding.

None of the major housebuilders has yet adopted the system, but Mark Wilson of manufacturer Panel Agency, says installing floors, walls and roofs of similar panel construction means that 60% less timber is used than in a standard timber-frame house. The clear spans usually associated with concrete are now available in timber frame, allowing owners and tenants to say where they want partitions. Wilson adds: “If you can work off pad foundations and have no internal loadbearing walls, the use of cassettes and panels will bring substantial savings in the ground.”

It may entail some considerable effort to overcome conventional thinking, but the next logical development of cassettes and structural panels is to make them fully self-contained, with factory-fitted services, plasterboard and insulation. And when that happens, timber frame will have really taken off.

The total wood solution

Fillcrete’s Tradis range of prefabricated structural timber panels has been used to form floor “cassettes”, wall panels and roof plates at CDS Housing’s Harlow Park development in Liverpool. The panels were specified by architect Parry Boardman & Morris because they can be installed much more rapidly, and because they meet performance and environmental criteria. Three men and a crane driver were able to erect 10 complete house shells in only four days, as opposed to the eight or nine weeks that would have been required using traditional construction methods. Each floor deck comprises three cassettes, and storey-height walls for ground or first floors are formed by a single panel. A pitched roof is formed by four cassettes, craned into position at the correct incline by adjusting the length of the four lifting chains on each panel. Many of the panels incorporate Tradis’ “super insulation” and breathing technology, achieving a U-value of 0.19W/m2°C and allowing the passage of household water vapour through the structure. Made from recycled or sustainable materials, each cassette, panel and roof plate can be anything up to 12 m long, 3.1 m wide and 400 mm deep. The internal structure is provided by British Board of Agrément-approved Masonite beams, with flanges of Swedish whitewood and webs of 8 mm Masonite K40 structural board made from forest thinnings and wood waste. These beams are lighter than the equivalent solid softwood joist and are designed not to warp or twist. Wall panels can accommodate wiring and pipe runs on the internal side before the plasterboard is fixed. Floor cassettes for ground floors are delivered factory-filled with Warmcell 500 insulation made from recycled newsprint; flooring-grade chipboard provides the walked-on platform, and the underside of the cassette is clad with Panelvent sheathing. Integral service zones can be provided to allow for services installation without disrupting the integrity of the floor. Complete structural floors can be installed in as little as 30 minutes.

Concrete battling to take a step up

In recent years, the site-built timber joist floor has lost out to precast concrete delivered as customised components. Concrete has several advantages over traditional wooden flooring. It requires cheaper, less-skilled labour for installation, does not need special site storage conditions and takes less time to install. More than 60% of the domestic floor market now uses a precast concrete floor system – up from 23% in 1986. Beam and block is the most widely used method of forming suspended concrete floors, followed by hollowcore planks. However, both these methods are far less common at first-floor level, which is why the concrete industry is focusing on upper-floor domestic construction as a potential growth area – and why the timber manufacturing industry is fighting tooth and nail to retain this section of the market with recent product innovations, such as floor cassettes. Typical supply and install costs for beam and block (excluding top screed) are about £12-15/m2. Those for hollowcore planks are higher, at £22-26/m2.