This central London rooftop extension project, recently completed by TDUK members ConstruktCLT, demonstrates the architectural, structural, and environmental potential of using cross laminated timber (CLT) to sensitively modernise existing buildings.
Royalty Studios, in Ladbroke Grove, is undergoing a full refurbishment and redevelopment, involving the addition of two new CLT storeys on top of the existing four-storey Post Modernist-style office building. The lightweight timber frame element – 300m3 of timber, which increases the building’s floor area by 40% – was designed and installed by specialist CLT contractor ConstruktCLT. Due to open in Q4 of 2025, the scheme will offer up to 23,000 square feet of flexible commercial space.
CGI of Royalty Studios; courtesy of ConstruktCLT
Founded eight years ago, ConstruktCLT initially focused on small, unique and challenging CLT projects. Now, director Jim Johnstone tells us, they work regularly with a network of sustainability-minded architects and are increasingly moving into commercial-scale developments – with Royalty Studios being their largest and most ambitious to date.
“At ConstruktCLT, we’ve always looked for projects that push the boundaries of what’s possible with CLT, and this project is a great example. It brings together principles of Optoppen – adding storeys to an existing building with timber – with innovative solutions, such as our use of Rothoblaas’ Spider connections, and improved waterproofing, to create a beautiful, resilient building with large, open spaces.”
Turning the design timber
The aim of the client, UCG, was to update and transform the existing building into a landmark contemporary workspace – and they were keen to do so using low-carbon timber construction.
Although the original architectural design by Mike Mahon Studio did not specify CLT, Jim explains that ConstruktCLT were brought onto the project in the early design stages to help develop the design in timber, and model it for costing. “We worked collaboratively back-and-forth with the Engineer, Architect and client to rework the design to suit timber construction. It was good to be involved at a very early stage as it meant we didn’t have to change too much.”
Installation of the timber extension; © ConstruktCLT
“We later conducted a high-level survey of the existing structure after internal partitions had been removed – but with the external walls and roof in place – which allowed us to check compatibility with the CLT design.”
As it was the client’s first time working with mass timber, the predictability of cost and construction timelines were reassuring factors; with ConstruktCLT able to provide a fixed price based on detailed modelling of known material quantities.
Balancing old and new
Externally, the building’s postmodernist character has been preserved, with most heritage considerations focused on the façade, including the brickwork and round window details. The interior however allowed for more design freedom, with the new floors characterised by exposed CLT providing a warm and contemporary feel to the workspaces.
“The client wanted to future-proof the building, to make it into something notable, and they were keen to see timber.”
Tolerances for interfacing the CLT structure with existing brickwork and structural elements were managed carefully. “For some details, we built in tolerance, and in some places, small adjustments were made during installation – but this is part of the beauty of CLT construction, that it can be easily manipulated.”
Structural upgrades with a Spider
Internally, the structure features large open-plan spaces, made possible by the innovative use of Rothoblaas’ Spider connectors. These enabled the design to remove internal columns and structural partitions, resulting in a clean and flexible layout, with just two internal glulam columns (each measuring 240x240mm) on each floor.
The Rothoblaas spider connector
The load from the timber extension is transferred directly through the two glulam columns down to the existing building’s load-bearing points. Two Spider connectors are used at the top floor, where they distribute loads and prevent the columns from punching through the CLT panels. This allows the columns to go straight to the ceiling, which works as a monolithic slab, without the need for internal beams or down stand beams. Rothoblaas’ engineers provided hands-on support throughout the engineering process collaborating with ConstruktCLT and their engineering partner Studio Allen.
From forest to structure
The client had been inspired by high profile commercial spruce CLT offices like the Black & White Building, but they ultimately decided to opt for pine instead, as a visual preference. The CLT was manufactured by Xilonor using Radiata Pine grown in Galicia, Spain.
The timber was transported by boat to Birkenhead and then by lorry to London, resulting in a significantly lower environmental footprint than traditional transport routes for CLT. The installation of the CLT took a crew of five people and a 60T Telescopic Crawler crane just seven weeks and involved seven articulated lorry deliveries.
Access and logistics were a challenge due to the site’s location. “Building something like this in the middle of Ladbroke Grove had its difficulties for delivery access and getting the crawler crane on site. We were building through winter with visual grade timber, so it was crucial to maintain the integrity of the finish on the internal face and be very careful with water management.”
During the project, tenants were fully decanted, allowing the ConstruktCLT team to work efficiently and keep better control of environmental conditions.
Moisture and fire safety considerations
A proactive and methodical approach to moisture resilience was taken throughout the build, with advice from industry experts. ConstruktCLT collaborated closely with Rothoblaas on waterproofing solutions, which resulted in the project using Rothoblaas’s Defence 200 temporary waterproofing layer, which was factory-applied to the CLT floor and roof slabs. This product helped maintain the timber’s moisture content between 12–14%, a level confirmed during on-site checks just before the roof was closed.
During construction, panels were stored on the existing concrete level to avoid any prolonged exposure to wet conditions. Early warning moisture sensors from Tector will be installed to monitor the building throughout its lifecycle.
To ensure fire-safety, the CLT elements were all designed with sacrificial layers to achieve the targeted fire resistance. All visual timber was treated with a spread of flame product. The treatment had to be applied directly to bare timber, requiring careful specification and experienced contractors.
“This project really showcases what CLT can do. It’s been a great example of industry collaboration, technical innovation, and attention to detail, all coming together to make a building that will hopefully set a benchmark for future projects.”
