Residential

The developers were John Pringle and Penny Richards of architect Pringle Richards Sharratt, who planned a two-storey block of four one-bedroom loft-styel flats with two of them earmarked for their own student-aged children. But the tiny site had defied previous development plans, as it was 20 m deep and just 7 m wide, lay close to a noisy railway viaduct leading into Waterloo Station and stood on ground consisting of fill, with negligible bearing capacity. And if all this were not challenge enough, Pringle and Richards aspired “to use sustainable materials from renewable sources and well insulated and energy-efficient construction”.

The architects’ aspirations, along with the poor ground conditions, pointed to lightweight timber construction. The architects took this approach further by using industrialised timber components fabricated in Germany by timber engineering specialist Merck, which they had introduced to Britain several years earlier.

The solution eventually adopted by Pringle and Richards was a novel combination of high-tech and low-tech construction methods. They decided to use large prefabricated timber components made by Merck (since taken over by Finnforest), but at the same time to retain the old concrete ground-slab as well as the old brick walls that bounded three sides of the site. They reckoned that the existing masonry walls should be recycled as ready-made external walls with good weather-resistance, fire-resistance and sound insulation from overhead trains.

Conventional foundations, which could have been extensive on the poor-quality ground, were obviated by laying a 200 mm reinforced concrete slab over the existing ground slab. Solid load-bearing walls of laminated timber were then erected on top of the slab – three of them as inner leafs behind existing brick walls. These walls were made using Merck’s Lenoplan panels, basically large, solid boards of chunky strips of spruce 105 mm thick which criss-cross over each other and are glue-laminated together. They are strong enough to be self-supporting without needing studs. Glass mineral-wool thermal insulation was injected to achieve a high U-value of 0.15 W/m2K.

For the low-pitched roof, prefabrication was taken another step higher. Lenoplan boards 80 mm thick and supported on laminated timber beams were prefabricated to the exact shape of the double-pitched roof. Eight of these cassettes were dropped into position over the timber walls and connected by a combination of rebated joints and horizontal screws. Then cellulose insulation was packed between the beams and 27 mm thick boards of laminated veneer lumber were laid as a top layer of sarking. The LVL used was Kerto, which was also made by Merck, this time out of 3 mm thick veneers of spruce a bit like a robust version of plywood. The roof was finished with Terne-coated stainless-steel sheeting over a geotextile mat.

The only timber-framed wall to have its own external cladding is the one facing the narrow courtyard to the four lofts. This was clad in what looks like plain tongue-and-grooved boarding but turns out to be another innovative Finnforest Merck product. Called ThermoWood, it consists of spruce boarding that has been heat-treated to 212°C to make it rot-resistant and durable without further treatment.

Inside the lofts, the laminated-timber roof boards have been left self-finished and the walls finished in plasterboard. Solid laminated-timber panels were also used for the first floor, this time 169 mm thick and lined below by two layers of plasterboard and above by a floating floor of cardboard honeycomb infilled with a chalk-based Ballast, a resilient layer of wood-fibre, gypsum fibre-boards and finally oak floorboards. In combination the composite floor achieves an impact sound reduction of 56 dB.

Project team

Client and architect John Pringle and Penny Richards of Pringle Richards Sharratt
Main contractor Eurban
Structural engineer, joinery subcontractor and timber supplier Finnforest Merck