Summary
• The market for metal-web joists has increased to nearly 15%.
• MiTek’s sales doubled last year.
• Engineered I-joists eliminate movement and squeaks.
• Timber floors require greater acoustic insulation.

Not so many years ago, when it came to constructing the sub-floor, timber frame house builders had little option but to use conventional timber floor joists.

Fast-forward to today, and the choice has grown significantly. Unsurprisingly, competition for specification among sub-floor manufacturers is hotting up, as suppliers of metal-web joists go head-to-head against I-joists in the battle to see who will command the lion’s share of the ‘alternative joists’ market.

According to suppliers, metal-web joists represent the fastest growing alternative to timber floor joists. A couple of years ago, the market for metal-web joists was probably less than 5% but that has now increased to nearly 15%. “That growth is against all alternative joists and is increasing substantially year-on-year as builders and specifiers realise the benefits of the metal-web floor system,” said Alan Ritchie, sales and marketing director at Wolf Systems, which markets the easi-joist. Metal-web joists like easi-joist and MiTek’s Posi-Joist are favoured by builders and clients, not only for their open areas for running services such as electrics, plumbing and ducting (even close to the end of the joist near the bearing which was typically a problem with other joist methods) but also because they are light and easy to handle.

They have a wider surface that speeds up the laying out of joists and provides easier fixing for decking materials. All metal-web joists are engineered and manufactured to suit the specific job, which ensures there is no wastage. A variety of different support conditions are available which enables the joist to be used in all types of construction. The joists are also dimensionally stable, eliminating squeaks and call-backs for builders.

“We continuously hear of builders who, once they have used easi-joist, will not revert back to their previous joist methods,” said Mr Ritchie. “easi-joist complements the design, manufacture and supply elements of a roof truss supplier’s business and these manufacturers have used their knowledge and experience to develop the metal-web floor market.”

Market opportunities

MiTek’s sales and marketing director Roy Troman is equally bullish about the market opportunities for metal-web. “Our sales have doubled in the last year, although I don’t expect this to happen every year,” he said. The jump was due in part to Posi-Joist becoming the first metal-web manufacturer to be granted Robust Standard Detail (E-FT-4) for a metal-web joist, following months of exhausting tests at four separate developments.

The Posi-Joist, which first appeared in the UK in 1981, is a combination of a high strength steel web and strength-graded TR26 structural softwood chords. The system allows a high degree of speed and flexibility in the installation of both floor and through services, and can be top-hung, which means no joist hangers. Posi-Joist combines the lightness of structural timber with the strength of the Posi-Strut steel web, producing a fully engineered quality joist capable of achieving greater spans than conventional joisting.

Mr Troman estimates that around 12% of timber floors will be made using open metal-web joists, of which MiTek claims a 50% share.

“Most houses have en suite bathrooms, some have two and houses with three floors have a bathroom on every level,” he said. “In the past, notches and holes would have been drilled to make room for pipes. Nowadays, architects have a choice – if they don’t want the en suite to back onto the bathroom, they don’t have to. If they want to link to a soil pipe 10ft away they can: there’s enough space to put a 2in pipe through the floor and link up that way. It is a massive advantage.”

He said Posi-Joist had also become popular with timber frame manufacturers because of its inherent stability and the fact that it could be easily handled in the factory, which meant they could make cassette floors.

Despite their success, metal-web systems have still got some way to go to catch up other ‘alternative’ joists. Over the past 10 years engineered I-joists have been replacing the traditional timber joist for intermediate floors in new house builds, with most, if not all, of the top 10 housebuilders now using them.

Engineered I-joists are designed to eliminate the challenges faced when using traditional timber joists. “Traditional timber joists tend to dry and as a result can twist and warp, creating an uneven floor – resulting in the number one complaint in new house builds: squeaking floors,” said a Finnforest spokesperson. “The engineered I-joist eliminates this movement and results in a much more stable floor construction.”

Service provision

Service provision is also simpler, with services being passed through the centre of the joist, the web, rather than having pipe holes cut in the top of a traditional joist. Service work is now generally carried out from under the floor, through the ceiling of the room downstairs, whereas, before, trades interrupted the construction process as they lifted, re-laid, lifted and re-laid the floor covering to install electrics and pipe work. So the I-joist can unlock savings in terms of construction and the management of various trades on site.

But, while the I-joist has been adopted by the larger well-known housebuilders, many regional firms are yet to adopt the product and reap the rewards.

“The I-joist is a wonderful working example of the difference between the word ‘value’ and the word ‘cost’,” said Simon Jones, business development director Boise, UK and Ireland. “Yes, they typically ‘cost’ more per metre than the solid timber they replace but they are so much more effective in the ‘built’ system.”

I-joists are strong and stable and reduce potential shrinkage in the floor, keeping the floor even and minimising squeaks. “The I-joist is light, easy to handle and cut and services can be run through the web for plumbing, ventilation and electrics,” said Mr Jones. “Full software support provides accurate, designed plans for the builder to use and expert processing by dealers provides accurate, precision-cut product packages to site on a just-in-time basis, minimising construction costs and effort for the builder,” he added.