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2 August, 2008
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Making light work
Published:  18 June, 2007

Timber provides the wow factor at 28 Percy Street

An airy, open glulam-framed extension has transformed a listed London Georgian terrace – and its prefabricated core was built by tea-time. Mike Jeffree reports


Take two truckloads of glulam framing, I-joist based floor and wall cassettes, one 35m tower crane, close off the street and in a day you have the core structure of a three-storey, mixed-use apartment building. That, in a nutshell, was the story of the erection of Satellite Architects’ extension to 28 Percy Street, just off Tottenham Court Road, in central London.

In fact, a day is an exaggeration.

“They started lifting the first pre-made panels at 9am and I was walking inside the frame at 4pm,” said Satellite co-founder director Stewart Dodd.

“The new building is in a locked-in courtyard behind No 28 so the contractors used a self-erecting crane – a Liebherr Mk 80 - with a 42m boom to swing components up over neighbouring buildings,” said Satellite’s other co-founder Neil Wilson. “It was pretty impressive. I even took my son along to watch.”

Preparing the site for the building’s crowd-pleasing rapid lift-off naturally took somewhat longer. The conversion of No 28, part of a listed Georgian terrace, into a ground floor and basement office space, four studio flats and two two-bed apartments, one of them the duplex on top of the old building, involved substantial refurbishment and alteration to the latter. The contractors, Lakehouse Design & Build, also had to knock down an old 1960s extension where its successor would stand.

The next step was to erect the steel chassis supporting the new structure. “This was quite complex in both design and construction,” said Wilson. “We wanted as much natural light into the below-street level office area, so, besides cutting in the old concrete ground floor plate at the end, we designed the chassis using transfer beams rather than columns.”

 


 

Timber provides an impressive backdrop - especially at night
The demolition and steel work took around 12 weeks. In the meantime the main components of the new building were being pre-fabricated by Global Timber Frame of Basildon. Wilson and Dodd stress that they select materials and construction methods on suitability for the task in hand. At the same time, they clearly have timber sympathies. Both took post graduate diplomas and Dodd now teaches at UCL’s Bartlett School of Architecture where the curriculum focuses on understanding materials generally and wood in particular. The partners have also used wood extensively, structurally and decoratively, in previous projects, the latest being a range of new-build and refurbishment work at the headquarters of River Cottage, Hugh Fearnley-Whittingstall’s food and cooking school operation in Devon. Wood, said Dodd, is being selected on aesthetic grounds and “more and more for reasons of sustainability”. 

“At the same time it’s being increasingly seen as a cost-effective, time-efficient method of construction.”

“That’s been proved with Percy Street,” said Wilson. “Cost before fit out was £100sq/ft, which is good, and total build time was 40 weeks. We estimate it would have taken 10% longer if we’d used other methods for the extension.”

The Percy Street client Halcyon property also bought enthusiastically into a wood-based structure, even though it was its first. The only slight problem was with the contractors. “Lakehouse wasn’t one of those contractors who try to convince the client they’d be better off with steel portal frame!” said Dodd. “But, while it had used timber frame before, it had been on buildings where it was eventually covered up. Here the frame is left exposed and untreated and we had to remind people not to pencil messages and phone numbers on the wood.”

The PEFC-certified larch glulam for the Percy Street frame was made by Ante-Holz in Germany using a bespoke 250x75mm beam size, and delivered by its UK agent Lamisell. To ensure everything fitted and minimise remedial work on site, Global Timber Frame fixed the frame together in its factory and bolted in the pre-made panels, with the latter all based on Masonite TGI I-joists, comprising solid timber flanges on K40 wet-process fibreboard web, 100mm deep for the walls, 240 mm for the floors.

 

 

The conversion of the listed terrace took less than a day
“They didn’t build up the whole structure, but put together various elements to check jointing and so on,” said Wilson. Dismantled for delivery, the components were loaded onto two flat-bed trucks and, minus the glazing, were transferred to site in just 21 lifts. The new building is fixed to the existing structure using steel straps running diagonally through the floor plates and connected to a wind post slotted into the brickwork. Apart from this, the structural strength is all down to the glulam frame and the floor and wall panels, where the I-joists run in two planes for added stability. “The engineer asked if we would consider more diagonal racking elements, but we said no,” said Wilson. “We wanted a clean and honest structure. I was OK with the steel straps because I couldn’t see them!”  

Underlining the robustness of the structure, there is no discernible difference underfoot when crossing the threshold from the old Georgian building, where five of the six flats have bedrooms and bathrooms. “You don’t detect any more bounce in the floors,” said Wilson. “The key difference is the new building is so much lighter thanks to floor to ceiling windows.”

For the latter, architects and clients opted for Crittalls steel frames because they liked the thin profiles and materials contrast.

Each apartment also has a western red cedar balustraded balcony, and these too are supported by glulam beams cantilevered into the frame.

As a conversion, the building was not bound to meet Building Regulations Part L, but to satisfy building control, had to comply with (NHBC) SAP ratings. It did this easily thanks to the highly insulating windows and the fact that the multi-layer wall and floor build up includes 100mm of Rockwool.

The apartments have been fitted with hard wood flooring, a veneered Kahrs oak strip, and the terrace of the duplex, which covers the roof of the building, is finished in tanalised softwood decking. However, the building still came up to scratch on sound transfer thanks to the 400mm floor structure, in addition to the I-joists and Rockwool, comprising two layers of plasterboard, resilient bars top and bottom and 18mm and 15mm ply subboards.

The finished wood structure, insists Satellite, should effectively be maintenance free. “In previous projects where we’ve left timber exposed, purchasers have asked whether they should varnish it,” said Wilson. “But we always say no. Externally the larch will simply silver and weather. You’ll get some raising of grain in the wood, but that’s not a problem.”

“Internally, you can get oily stains on the wood where people touch it,” said Dodd. “But that’s only an issue in busy public environments, not prime residential flats.”

Certainly the expressed timber structure hasn’t deterred prospective residents of No 28. If anything, quite the opposite. The office space was occupied halfway through the construction programme and the duplex was snapped up almost as rapidly as the frame was erected.

“The duplex does have real wow factor, with terraces front and back, and sold within a week of completion,” said Dodd. And it went, he added, for £850,00, just £30,000 less than the whole project value.



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