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Working Detail: Sevenoaks School, by Tim Ronalds Architects

[WORKING DETAIL 24.04.10] Auditorium roof construction

Sevenoaks School − The concert hall’s timber roof, a major feature of the building, was designed to provide optimal acoustics. The roof is supported on a slender concrete frame structure. The trusses span 18m with a 50° pitch, and compression members are solid Douglas fir in lengths up to 9m. The tensions members are steel rods with Daversteels couplings. Trusses were fabricated by Construction Timber. Douglas fir common purlins span 3.6m between the trusses, and support Douglas fir boarding. The form of the roof provides the acoustic volume needed, and its constructional detail diffuses and modulates sound reflections. Above the visible construction, 11 further layers of construction create a lightweight but highly soundproofed roof.

The upper part of the roof is a giant linear acoustic silencer for the natural ventilation, as well as a space for acoustic banners and performance lighting machinery. A steel frame structure works with the boarding on the slopes below to laterally brace the entire roof structure. The angle seam zinc roofing, with zinc sheets 13m long, was a tour de force by specialist roofing subcontractor Albany Brent.

Construction of the roof was not easy, as the Douglas fir timber and the insulation layers had to be kept dry throughout. Special measures were taken to ensure the trusses did not exert any outward thrust on the supporting columns. Only when the full dead load of the roof was in place was the tension in the truss ties adjusted, the temporary tie rods removed, and temporary supports removed - before the truss base plates were finally welded to their bearings.

Tim Ronalds, director, Tim Ronalds Architects

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