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A STONE RAINSCREEN WALL

Working details

The architect describes the form of the new museum as 'a lower perimeter building wrapped round the higher rectilinear core gallery'. The perimeter, with walls of Scottish sandstone, is reminiscent of the curtain wall of a castle, and the core gallery rises above it like the castle keep.

Clashach sandstone was used to relate the new museum to the adjacent historic stone buildings, but its entirely modern application lays it as a rainscreen with open joints, with courses of random height and length 'to reflect the non-modular spaces inside'. The honey-coloured stone, from a small quarry on the coast of the Moray Firth, has unique circular rhythmic banding, caused by the successive movement and deposition of iron by groundwater. The quarry invested in advanced extraction equipment to accommodate the 2m-long slabs.

The building is of cast in-situ concrete; a 250mm concrete wall lies behind the stone rainscreen.

The concrete wall is waterproofed with epdm membrane and the cavity is part-filled with Celotex insulation batts with taped joints. Each stone slab is individually supported at its base by purpose-made ss angles, and restrained by anchors at the top or sides. Each fixing has a pad which compresses a silicone compound against the epdm. The 6mm joints are left open and the edges of the slabs exposed. The window frames are recessed behind the stone and fixed behind internal linings.

The ground-floor gallery is lit by a row of 4m-high glazed slots with column-like mullions between. The inner and outer parts of the mullions conceal a stainless-steel post to which the window frames are fixed. The window sub-frame at the head is fixed to the concrete with allowance for movement. It lies behind a stone soffit slab fixed to an extended angle. A flashing above it directs water away from the window head.

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