Jettying was a common technique used in medieval timber frame building. It entails the upper storeys, supported by a cantilever, jutting out above the ground level. It maximised floor space in houses that could only have a minimal footprint due to streets being narrow and crowded with people, carts, hawkers, assorted livestock, and sundry other medieval clutter. It might look precarious to the uninitiated, but experienced medieval craftspeople clearly had the knack, as many of their buildings survive centuries later.
Now, dRMM Architects, renowned as an engineered timber construction pioneer, has put a modern twist on the approach with WorkStack, a five-storey hybrid wood and steel workshop development.
The building, located within the Charlton Riverside business zone in London and commissioned by the Greenwich Enterprise Board, is billed as “challenging the generic, land-hungry tin-shed default of light industrial construction”. The dramatic cantilevered design, with the top floor façade projecting 9.3m from the ground floor, has resulted in a building providing a total of 1,335m2 of floor space on a footprint of just 229m2.
Besides dRMM, the team that worked on WorkStack included engineers Arup, the builders F Parkinson, and timber specialist sub-contractors B&K Structures.
Initially the ground level was to be in concrete, but, after value engineering, this was switched to a ‘podium’ floor on pile foundations and a concrete slab, comprising a steel-braced frame infilled with Lignacite masonry blocks. The core structure of the upper floors is in CLT, and, while the stairs themselves are steel, that includes the stair core and lift shaft.
The floors comprise 19.2m-long CLT panels from Hasslacher Norica Timber, while the walls are 12m-long CLT panels from Stora Enso. All the engineered wood was delivered in a mere 15 truck loads and, said dRMM, building in wood resulted in fewer overall materials deliveries to site and, consequently, a less disruptive construction process than if it had been a standard concrete or steel build.
“A concrete or steel building requires many more construction elements but also, as it’s a much heavier structure than a CLT one, needs more material to support itself,” said dRMM associate director Steve Wallis. “The use of CLT as the primary structure also meant that the timber panels could be left exposed and negated the need for plaster boarding / partition walls and fire boarding. So, in short, fewer materials and fewer components.”
The “rigorous” fire safety analysis of the building was undertaken by Swiss specialist Ignis and the fire safety methodology included making each of the 14 work space units a separate compartment, achieved through the “integrity of the CLT, detailing of connections and interfaces of the other elements with the CLT”.
“WorkStack was initially conceived before the Grenfell Tower fire incident,” said Mr Wallis. “Whilst our building is not a residential one, the shift in regulation, fire safety best practice, along with changes to the insurance market that followed, meant that the fire strategy needed to be reviewed after the initial design. The CLT, which is left exposed virtually everywhere, has been coated with a surface spread of flame retardant. But the building fire strategy does not rely on sprinklers, which was a noteworthy cost saving for a project of this scale.”
No timber preservative treatments were required, he added. “The entire building above ground floor is timber, which reduces the need for preservatives and also brings it above any possible flood height from the Thames,” he said.
There were some hold ups in the build process. “Construction started during 2020, in the midst of the Covid-19 pandemic, and progress was affected by supply chain issues,” said Mr Wallis. “Most challenging were CLT factory closures in Europe, as well as material cost inflation. Progress onsite was stalled due to these factors and the site was closed for a period while the CLT fabrication was moved to another factory.”
Once the erection started, however, it progressed rapidly. The CLT panels were craned into place and fixed with around 30,000 screws. A team ranging from five to eight people was all that was needed to build the timber structure and the frame was completed in nine weeks.
Each floor extends 1.5m beyond the one below, giving a total cantilever of 6.2m, while the building also steps three times, resulting in the 9.3m distance between the façades of the ground floor and the top storey. “The progressive overhangs work both as solar shading and covered delivery area,” said Mr Wallis.
The exterior is clad in a mix of corrugated steel and translucent polycarbonate panels.
In terms of sustainability, WorkStack is described as “punching above its weight” and dRMM says it’s “on track to be BREEAM Excellent”. The CLT stacked structure has sequestered 343 tonnes of carbon during timber growth. The building also has around 21% less upfront embodied carbon than the London Energy Transformation Initiative’s (LETI) 2030 Design Target of 350kg CO2/m2, and 44% less than RIBA’s 2030 Built Target for whole life carbon off 750kg CO2/m2.
“The panelised construction also reduces time on site, creates a safer and cleaner working environment and minimises material waste,” said Mr Wallis. “As the load bearing timber walls and ceilings are exposed throughout, this beautiful and economical solution eliminates the need for additional internal finishes and reduces associated material resources and environmental impacts, while offering biophilic benefits to users.”
WorkStack’s 14 units range from 55 – 110m3 “offering tenants choice and growth potential within the building”. They provide work space for about 60 people, equating to an employee density of 428 employees per hectare, compared to the London industrial average of 69 employees per hectare. Occupants include furniture makers, knitwear producers, work wear manufacturers and a bicycle/motorcycle workshop.
“It wasn’t fully occupied from the outset but is now, a little over a year after the first tenants moved in,” said Mr Wallis. “We’ve interviewed a few of them and feedback has been good. They like the timber – of course – and the people on the floors above ground appreciate the security and privacy they get from not being as exposed as in a singlestorey, shed-like industrial premises. It’s also very light. Their clients have been interested in visiting too, more so than in other places they’ve been. Also, the acoustic properties of the timber means the sound travels less between studios than in their previous places.”
It’s acknowledged that insurance for timber buildings can be complex. “However, there are several insurers that maintain reasonable terms for well-designed and considered timber buildings,” said Mr Wallis. “While we do know it was not as straightforward as for other buildings in their portfolio, our client for WorkStack has been able to find insurance for the building.”
Overall, concludes dRMM, WorkStack “demonstrates the viability of timber building in an urban environment”.
“It’s backed by the Greater London Authority as an exemplar for industrial intensification, and a recipient of Good Growth Funding,” said Mr Wallis. “WorkStack is designed to create a new class of highdensity industrial buildings which can be integrated within mixed-use development across cities, and we are now working on different treatments for the concept.
