The curved roof of the new first-floor extension runs beyond the walls of the single-storey office to create more space and rises to overhang the parapet wall of the two-storey office, incorporating a row of clerestory rooflights. It is clad with pre-patinated zinc with standing seams.The west gable wall is clad with prepatinated zinc in diamondshaped 'fish-scale'panels.The south elevation is glazed and set back from the parapet to create a terrace, protected by a glazed balustrade.
The curved roof/wall structure consists of a series of paired 254 x 45mm glulam beams at 3m centres; the curve is created by curved 254 x 103mm UB members flitched to the glulam beams.
The glulams taper to 100mm towards the clerestory, where they are flitched to plates welded to 88.9mm-diameter CHS props supported on a new concrete upstand.The zinc roof extends beyond the windows, its soffit clad with zinc with coulisseau (flattened seam) joints.
At their bases the glulams are flitched to a 200 x 200mm RSA beam which is bolted back to a new 450 x 350mm concrete beam set in place of the original parapet wall.
The curved enclosure is formed of proprietary lightweight steel C-shaped studs and purlins, chosen to avoid overloading the original foundations.The lower vertical part of the wall - a series of C-section studs at 600mm centres and horizontal purlins at 600mm centres - is suspended from a 200 x 100mm RSA beam at the back of the UB members and is bolted to them with steel brackets.The purlins are infilled with mineral fibre insulation quilt and lined on the external side with 35mm rigid insulation.