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It’s 11am on a quiet residential street in north east London, and all is calm. Birds twitter, the occasional car drones by and children chatter as they assemble in a nearby playground before class. It’s hard to believe, but just metres away there’s a live construction site for a new school.

Our ears do not deceive us. Lauriston Primary School in Hackney is being built using cross-laminated timber panels for the walls and frame, which slot together with minimal machine fixing and neighbour nuisance.

In fact, the strategy of minimising disturbance to pupils at the existing school has been almost too successful. ‘There’s a joke going round that there’s more noise on the school side of the hoarding at play time than on site. The contractors are getting more disturbed than the kids here!’ says Ann Griffin, director of project architect Meadowcroft Griffin.

Ikea fanatics will warm to this flat-pack form of construction, which is based on precision engineered panels from Austrian company KLH. At Lauriston, a team of just six carpenters is required to assemble the kit of parts, which includes a remarkable 50,000 screws and 3,500 angle brackets needed to support the walls.

Already hailed by some architects as ‘the concrete of the 21st century’, cross-laminated timber panels are proving a popular choice for schools, where reduced disruption, fast erection and sustainability credentials top clients’ wish lists. But as well as enjoying these advantages, main contractor Neilcotts had to get to grips with new techniques and challenges.

Dubbed ‘the treehouse’, the £8m project involves replacing the single-storey 1970s school with a new three-storey building twice its size, while creating double the amount of external space on the same tight urban site. Comprising 16 classrooms for 480 children, it’s being built in four phases, the third and largest of which is due for completion in June this year. The existing school has remained open throughout.

With the site team working to tolerances of +/- 3-4mm across the entire length of the building there was great pressure on Neilcotts to create a perfectly level concrete slab for the wall panels to rest on. ‘The panels more or less sit directly on the slab so it had to be pancake flat. We spent a long time on the finish,’ says project manager Ian Berger.

Unlike a traditional concrete foundation with starter bars rising from the slab, this is basically a flat podium with steel angle brackets fitted at 250mm centres to provide bracing for wall panels. The panels are delivered from Austria on a just-in-time basis, and lifted individually from the truck onto the building by a mobile crane.

Then the KLH UK team jumps into action. One operative loads the panel from the truck into the crane’s sling, one unhooks the panel once it is lowered into place, one gets it level, and one screws in the steel bracing. Adjacent wall panels interlock via a half-lap connection joint held together with 8mm-diameter timber screws, with about seven used to fix a typical panel. At corner connections, panels meet and are screwed together.

Once all vertical panels for each floor are in position, glulam ceiling beams are inserted in rooms that require extra stiffening, such as the classrooms which have a glazed facade on one side, or over the large-span sports hall. Next, the thicker cross-laminated timber flooring panels are dropped into position. It took about two days to complete the walls on the ground and two upper floors, but just half a day to complete each floor. (KLH has also recently completed a nine-storey building, London’s Murray Grove, which is the world’s tallest timber residential block.)

Externally, the building will be clad in various materials, including brickwork slips, timber and coloured panels. These will be attached as a rain screen using fixing batons screwed to the cross-laminated timber over a layer of insulation.

Compared with alternative framing solutions, cross-laminated timber provides a solid wall that doesn’t require infilling with insulation, plywood or plasterboard. As Berger explains, this speeds up the overall programme, even though the panels went up at a similar speed to a steel frame. ‘In most classrooms two walls are left fair faced, and the rest just need a layer of plastering,’ he says. ‘Joints between panels have chamfered edges, making the connection details very neat and tidy.’

For following trades, there is the added benefit of having a flat timber surface to fix cables and other services to. ‘We went one step further by predicting service routes and designing recesses into some walls and the underside of floors at the factory,’ adds Ann Griffin of architect Meadowcroft Griffin.

One of the trickiest jobs was erecting the first of three 13m-tall panels that form the stairwell and run the full height of the building. This required extra temporary support in the form of a tubular steel prop bolted at its base to the concrete podium and fixed to the timber wall at an angle to ensure stability before adjacent panels went in. Once all three panels were in position, extra nail plates were positioned across the lap joint connections higher up the panels to increase lateral stability. ‘When we start new jobs like this one people always watch in excited anticipation to see what will happen, especially with the 13m panels,’ says Sebastian Popp of KLH.

But Griffin warns that on more restricted sites getting similar-sized panels could be problematical. ‘You might have to start breaking panels down into smaller sections, making it less efficient,’ she says.

Denting and damage during transport can be an issue with prefabricated timber, so was it a problem here? Berger hastily positions himself in front of a suspicious-looking scuff mark before answering: ‘With huge panels there’s inevitably going to be some damage, but it hasn’t been too much of a problem and we will be able to in-fill any blemishes.’

With panels interlocking along their full lengths, it was vital to get details accurate during the design and manufacturing stage. However, Griffin was surprised at how much time this took. ‘We spent 50% more time than we set aside to work on refining drawings with KLH and the design team. Prefabrication doesn’t make the whole project shorter, it only reduces time on site,’ she says.

KLH’s Popp says connection details between the cross-laminated timber and other structural glulam beams and steel beams proved particularly tricky to detail as each had different tolerances. ‘We haven’t had many problems on site and most panels fit perfectly well, although we had to sand off the ends of some glulam beams to fit into steel hangers. But luckily with timber you can be a bit flexible with the chainsaw,’ he jokes. cm