Flawless flooring

Joist to the world

Precut board proveS big - and we mean big - hit
When Avada Country Homes put the first floor in for a three-storey home near Cromer in Norfolk, everyone on site gathered round to watch. The Parsonage, a 2,600 sq ft detached house, was Avada’s very first new-build project and the company had decided to eschew traditional building materials. The barn conversion specialist wants to cut construction time so it can save money on labour and get a faster return on its investment by finishing and selling houses more quickly. Time-saving innovations at the Parsonage included thin-joint blockwork and a beam and block ground floor. Avada designer Ray Gurney was one of those who stood and stared at the two men building the first floor with I-joists from Boise Cascade. “It amazed me,” he says. “The joists were huge - about 40ft long - but they were so light the two blokes could carry them around easily.” Gurney reckons it would have taken two days to cut to length and hoist into place the solid timber beams for a carcassing floor; it took the Avada pair only half a day to put the first floor in. An I-joist is a length of 9.5mm-thick wafer board - like chipboard but with bigger flakes of wood - with wooden flanges running along the edges, giving it its ‘I’ profile. Its big advantage over solid wood is just that: it’s big. Knots and other natural imperfections make it a struggle to get timber beams of any quality that are over 6m long. I-joists, however, come in lengths up to 14.7m. So while timber joists can span rooms, I-joists can span between external walls, which reduces the number of joists to install, typically by about a third. The wooden flanges are made of sections of wood that overlap every 2.4m. These short lengths give the joists greater strength and stability than solid wood. As a result, I-joists are laid 600mm apart compared with 400mm centres for carcassing. Solid wood is vulnerable to twisting, shrinking and bowing as it dries out. When this happens, the wood pulls away from the decking nailed to it and squeaks as nails rub up and down. I-joist floors, however, are less likely to squeak because all I-joists share the same properties and are more stable than wood. Gurney says there were no squeaks at the Parsonage even when workmen were walking over the floor getting on with their own jobs. I-joists cost about 25% more than solid joists to buy but construction companies win back the higher material cost in lower installation costs. Boise Cascade draws up a floor layout plan and supplies all I-joists cut to length, which makes building a floor like putting up a flatpacked wardrobe but with a better set of instructions. “It’s so easy to fit you could do it without skilled labour,” says Avada’s construction manager, Rob Cooper. “There are big savings in labour because everything comes pre-cut, and the preformed knockouts for service runs save a huge amount of time at first-fix.”
Enquiry number 201

Sold on Celcon

JAMERA Aircrete BEAMS give builder faster finish
For Matthew Haynes, anything that lets him build houses quicker has got to be good. “Cutting the build time makes a huge difference now,” says the partner with developer and builder Abell Haynes. “The housing market is so buoyant we can’t build houses fast enough.” On a five-bungalow site at Savernake Road in Leicester, Haynes has successfully used Jamera aircrete beams from Celcon to cut the time it takes to install flooring. He says it took less than a day to put in a 115m2 floor, compared with the three days he estimated it would have taken using beam and block. The steel reinforcement in Jamera beams allows them to span up to 6m, and Haynes just cranes them into position and slots them together. “It’s like fitting very large pieces of tongue and groove floorboard together,” he says. For builders looking for speed, the advantages of Jamera outweigh the extra expense of cranage and finding a crane operator familiar with the flooring system and layout. The Jamera beams cost much the same as beam and block. And because aircrete has good thermal performance, Jamera typically provides enough insulation to satisfy Part L by itself, unlike aggregate block floors, which require a dedicated layer of insulation before screeding. The steel reinforcement in the aircrete beams stops 75mm from each edge, so holes for services can be drilled easily. Once the beams are in place, grouting the joins and laying a 10-15mm levelling screed completes the floor installation. Celcon’s Jamera floor beams come in a range of lengths up to 6m, widths up to 600mm and depths up to 300mm. The company also produces steel-reinforced aircrete beams for all the other elements of the external envelope: walls, lintels and roof. Jamera elements can be used in isolation, as with Haynes’ floors, or together. Available in Finland for the last 20 years, Jamera - the word is Finnish for ‘hard’ - first appeared in the UK just over two years ago.
Enquiry number 202

Emission: impossible

DUPONT gas membrane is safe, simple... and cheap
As far as Gordon Dunne is concerned, gas barrier membranes are about as different from each other as Tweedledum and Tweedledee. “To be honest, it’s all about price,” says the boss of Dunne Building. On a site at Hopeden Street in Edinburgh, Dunne is using DuPont’s Total gas barrier, which excludes radon, carbon dioxide and methane. “It does the job,” he shrugs, “just like other membranes do.” His man on the job, site manager Conor Byrne, isn’t quite so dismissive. Byrne is putting up a four-storey development of offices and flats on a brownfield site. At the edge of the block now going up there used to be a petrol station, so the ground was contaminated. All over the UK, gases leak out of the ground into the atmosphere. Natural emissions of radon and carbon dioxide are common on greenfield, while methane is released by landfills. It’s not usually a problem - until you start building. Instead of dispersing, the gases get trapped and can present a health risk as they build up. On a previous job, building on top of a landfill, Byrne used one of the many other barrier membranes on the market. His gripe? The sheets had to be heat-welded together on site, which meant bringing in subbies to do it. Lengths of DuPont’s 3m-wide Total membrane, on the other hand, can be taped together, which makes for a simpler installation. “You control your own destiny,” says Byrne. Laying the barrier was straightforward and, with the double-side tape, very quick. The sand-topped hardcore had vent pipes running into it every 2.5m so gas wouldn’t build up. Byrne then laid bitumen-impregnated protection boards to prevent punctures from the concrete floor’s steel mesh, which was poured subsequently. Easy to install, it was also the cheapest per square metre . And as even Byrne has to admit: “It’s always down to square metreage.”
Enquiry number 203

Foundations in the past

PETER KERNAN brings you a brief history of concrete
With the world pouring around five billion tonnes of concrete a year - nearly one tonne per person per year - concrete is probably the most common material in modern construction. The name comes from the Latin concretus, which means ‘grown together’, and concrete consists of hard, inert materials (aggregate) such as gravel and pebbles embedded in cement (a mixture of limestone/chalk and clay that turns hard when water is added). While cement mortar was well known in the ancient world as a way of binding other building materials together - the Egyptians used it in the pyramids, and the Chinese in the Great Wall - the Romans were the first to make widespread use of cement-encased aggregate as a building material in itself. In 13BC, Roman architect Vitruvius wrote a guide on how to make cement, and his rules were adopted throughout the Empire. Roman concrete consisted of broken bricks and small stones embedded in wet lime and volcanic ash. The material was used in roads, bridges and public buildings, including the still-surviving Pantheon in Rome, where the 42m-diameter dome (the largest in the world up to the 20th century) is made of poured concrete. Medieval Europe made far less use of concrete, typically as foundations or infill material for castles, churches and cathedrals. Modern concrete dates from English stonemason Joseph Aspdin’s invention of Portland cement in 1824, a standardised, repeatable process that worked for huge quantities. By the middle of the century, mixing Portland cement with aggregates for use as concrete in housing and factories had taken off. In 1854 Newcastle builder William Wilkinson invented reinforced concrete, embedding a network of flat iron bars or wire rope in concrete. Successful reinforcement, which allows concrete to resist forces of tension as well as compression, had eluded the Romans, whose use of bronze rods led to cracking as the metal expanded faster in heat than the concrete. Reinforced concrete bridges and buildings started to appear from the 1890s. The last major advance was prestressed concrete in the 1920s, achieved by tensioning the steel. But future directions include:

• aesthetic concrete, where the material is treated as an artistic medium and coloured concrete becomes the norm
  
• eco-concrete, where concrete’s thermal mass is exploited to cool and heat buildings over their entire life
  
• clever concrete, with properties such as self-compaction