An extra CLT timber floor adds art gallery chic to Lower James Street

CLT

A beautiful old Soho office building needed a refurb. Mass timber was the perfect material to provide an extra floor at the same time.


“At the time, looking to extend a building with CLT wasn’t all that common,” recalls Matt Hale, Director at Hale Brown Architects. “I think Lower James Street was possibly the first time anyone had looked to do a single-floor extension to a London office building in CLT.”

The extension and refurbishment complemented the building’s classic 1920s look. Image courtesy of Hale Brown architects.

The client was Knight Frank Investment Management. The brief was to update a 1920s office building in Soho and extend it upwards, giving it an extra floor. They were very keen for the new top floor to have an art gallery feel. “We wanted an exposed, textural look, rather than the standard white box that you immediately associate with a gallery space,” remembers Matt. “We wanted the material to be visibly identifiable.”

Heyne Tillett Steel, the project’s structural engineers, have plenty of experience in mass timber. They suggested cross laminated timber for the project. While Hale Brown hadn’t worked with CLT before, they liked the idea. And whitewashing the CLT could give it that art gallery look.

The client was open to it, albeit with some reservations. But timber is a good solution for a tight site like Lower James Street. The timber panels for the new storey could be made offsite and put together quickly once delivered.

“There was a big old lead mansard roof on top of building when we started, with ‘plant’ on top,” Matt recalls. “As part of the planning conditions, we couldn’t increase the height of the building. So the walls and foundation didn’t require significant further strengthening.”

The challenge of old, wonky buildings

Being a 1920s building, the plan wasn’t entirely square. Matt found it highly instructive to work with timber frame consultants Eurban on such a nonregular plan.

There were many walls with 85 or 96 degree corners, he remembers. This underlined the need for a very accurate site survey. Some of the old roof had to be stripped off for the survey purposes, in order to accurately assess the walls that the CLT would come down and sit on. Throughout the design process, Eurban were fastidious about the need for precision. That said, timber does have a certain amount of malleability.

The new floor and roof. Image © Heyne Tillett Steel.

“When it came to the installation, there were a few places that were very slightly off, where a few sledgehammer blows were necessary to shift it by a millimetre or so. That’s one advantage of timber – there is a relative degree of flexibility. If a window panel, say, is slightly off it can be plane sawed. It’s a lot harder to do that with blockwork or concrete.”

A cranked roof

“For planning reasons we had to have a crank in the roof, almost like a mansard shape,” Matt tells us. “We thought that would also add some interest. And from very early on we’d decided the extra floor would be duplex – the top two floors would be linked so there’d be volume and space to it.”

“The cranked rooflights were a very nice feature – these came pre-cut in the panels. The joins where the panels fit together are centred on the middle of the rooflights – it was very neat.”

That whitewashed gallery look

The client was not keen on the look of exposed CLT for the new floor’s interior – they thought it was too sauna-like. So the CLT would have to be whitewashed and its surface fire-rated. At the time, ticking both those boxes proved difficult. Fortunately, as they were developing the scheme, Envirograf were developing a new product: a matt whitewash with the requisite fire rating.

The new gallery-style extension, with cranked rooflights and whitewashed timber walls. Image courtesy of Hale Brown.

Putting everything together

Originally the floor slab was going to be CLT too, but in the end the CLT was limited to the roof and walls. This was the first time that the contractors had used CLT, but the construction went smoothly and quickly. The timber arrived in the narrow street on trucks on a Friday evening. CLT fabricators Eurban then craned the 2.4m wide panels into place over a few days and the work was completed in late 2016.

The panels slot together via a ‘toothed’ or recessed joint. All the CLT had been cut to shape, with fixings in plan and section that had required a lot of design input from Heyne Tillett Steel and Eurban. Kelly Harrison, an engineer from HTS, designed a lot of the fixing details that held the timber frame to the existing brickwork walls. Irregularities, such as triangular sections of old brickwork gable, had been accounted for.

Achieving a neat, minimalist look

Slots had been factory precut into the interior ceiling’s panels, which later had the lighting tracks set into them – so no cables or conduits
are visible in the ceiling. The air conditioning comes up though the concrete deck. Wall-mounted air handling units are supplied with air from below.

The original steel columns were retained. They had been encased in concrete, which was removed at Heyne Tillett Steel’s suggestion, to give a more modern, exposed look.

Maintaining the timber vibe throughout

“It’s a lovely old period building, and we tried to complement the top floor’s use of CLT in the rest of the building’s refurbishment,” Matt says. “The panelling behind the desk in the reception area is CLT, with the same whitewashed finish as the roof. The doors aren’t CLT – they’re spruce – but they have the same finish.”

Reclaimed timber flooring was installed on the lift levels, where it had previously been terrazzo. It is a darker, warmer colour, providing a pleasing contrast to the CLT and spruce.

Making a timber project work

Lower James Street completed in February 2017, a good few years before initiatives such as the RIBA 2030 Challenge or LETI’s carbon targets were drawn up. Back when the project was being planned and designed, the decision to go with timber was mainly about design: the material’s sustainability was a nice bonus, though it was not part of a drive to meet any particular embodied carbon or operational energy ambitions.

That said, engineers Heyne Tillett Steel have calculated that the use of CLT in the walls and roof sequestered some 32.8 tonnes of CO2, with the project’s total carbon emissions being some 23.5 tonnes of CO2e.

“It only takes one person to be uncertain about timber and it probably won’t happen,” Matt ponders.

“But everything fell into place neatly on this project. It showed that a mass timber office extension was eminently practical and functional.”

CLT

Installation of the cranked roof.

*This article is taken from issue 5 of Designing Timber magazine