‘ROOFLAG’ is the showroom of Daito Trust Construction, a Japanese construction and real estate business, with a focus on rental housing. 

The building consists of a column-free atrium, a triangular roof using cross-laminated timber (CLT) beams of the largest cross-sectional size in Japan, and a 4-storey reinforced concrete structure which houses a theatre room, lecture hall, exhibition facilities, offices and business meeting rooms. 

The shape of the building is simple but technically challenging to realise. Arup was responsible for the structural design and site supervision. Working closely with MOUNT FUJI ARCHITECTS STUDIO, we developed innovative solutions to realise this stunning structure. 

Arup developed innovative solutions to realise the eye-catching triangular roof made of cross-laminated timber (CLT) and the column-free atrium for this showroom. 

Supporting the roof 

The huge wooden triangular roof is supported by the roof of the 4-storey reinforced concrete section, the thin mullioned columns of the glass façade along the front road and the steel columns in the side walls.  

All of the horizontal seismic forces acting on the triangular roof are transmitted to the ground via the rigid reinforced concrete section, thus allowing the creation of a transparent glass façade without the need to place any seismic elements on the street side. 

Timber roof design for rooflag showroom

Arup developed innovative solutions to realise the eye-catching triangular roof made of cross-laminated timber (CLT) and the column-free atrium for this showroom. 

Largest roof of its kind in Japan 

The northward sloping wooden triangular roof over the vast atrium is made of 128 pieces of CLT panels. The maximum size of the CLT panels used is 270mm x 2,300mm x 12m, almost the largest size that can be produced in Japan. 

The maximum span of the structure is approximately 30m in the short side direction, much longer than the CLT panels. Therefore, we customised two types of connectors: shear connectors to transmit shear forces and lag screw bolts to transmit axial forces and bending. The joint system is as rigid as the beam members while keeping the aesthetics of the roof. 

Arup developed innovative solutions to realise the eye-catching triangular roof made of cross-laminated timber (CLT)

The northward sloping wooden triangular roof over the vast atrium is made of 128 pieces of CLT panels. 

Exterior view of Rooflag showroom

External view of the building

The column-free atrium for the Rooflag showroom.

The shape of the building was simple but technically challenging to realise. 

Facilitating assembly 

As this is a dense structure with huge 2,300mm CLT beams intersecting each other in three directions at each node, it was key to determine the appropriate winning and losing positions of the members at each node. We took into account the stresses on the beam members after completion, and also the erection sequence and onsite joining methods. 

To facilitate smooth and correct assembly, the designers and builders shared a 3D model and made a full scale mock-up to ensure the quality, safety and workability. As a result, the assembly of the large CLT structure was completed as planned in just 54 days. 

Rooflag exhibition drawing

Awards

  • 6th JAPAN WOOD DESIGN AWARD 2020 : Life style design 

Partners & collaborators

Project owner: Dato Trust Construction Co., Ltd. / Key collaborator: Tokyu Construction Co., Ltd