[WORKING DETAIL 13.05.10] Terraced seating and entrance staircase
West Buckland School − The terraced seating offers a sheltered meeting place for students, and the courtyard that separates the existing Victorian school and the new arts building acts as a social hub.
The paved surface of the courtyard continues up into the new building. Using the timber frame as the supporting structure for a precast concrete staircase seemed counterintuitive, but once the engineers had demonstrated that the loadings were acceptable, it was a matter of ensuring that the timber frame was protected from water ingress.
The changes in level across the site made it necessary to install a reinforced concrete retaining wall along the western edge of the courtyard. The KLH structural frame abuts the retaining wall, separated by a drained and ventilated cavity to prevent any moisture transfer into the timber. The inside face of the retaining wall is covered with a mechanically fixed studded membrane that drains to a cavity tray at the base of the wall.
The KLH staircase is wrapped in an EPDM membrane. Precast tread profiles were set on a continuous mortar bed along their entire length, with each level of treads fixed to the previous one with stainless-steel pins. Once the double-height terraces were completed the intermediate steps were installed, bedded on mortar and located with stainless steel pins.
The soffit above the stair is clad with 90mm-wide Siberian larch boards, fixed to softwood joists above a continuous layer of black insect mesh. The larch cladding is fixed with stainless steel, self-countersinking screws in a board-on-board arrangement, which provides ventilation behind the timber. A 50mm-deep PPC aluminium coping caps the roof parapet, providing a crisp edge to the elevation.
Daniel Burt, project architect, Rundell Associates