Timber Cladding in Scotland
ST MAGNUS CENTRE, KIRKWALL, ORKNEY
GORDON FLEMING IN ASSOCIATION WITH ARP LORIMER & ASSOCIATES
The St.Magnus Cathedral in Kirkwall dates back to the 12th Century and forms the centrepiece of a group of new and existing buildings which are now collectively known as the 'St Magnus Centre'. A new upper floor has recently been inserted into the volume of the Victorian church hall, with the resulting function hall below able to operate independently. The newest building sits perpendicular to this, creating a new south-facing entrance court as well as a series of flexible function rooms inside.
In selecting materials for the building, reference was made to the relationship between Orkney and Norway and the significance of timber structures in Norwegian traditional architecture. Additionally, the use of timber cladding emphasises the non-loadbearing function of the upper clerestorey by separating its steep monopitch roof from the roughcast masonry walls which enclose the accommodation below.
The principle criteria in species selection for the timber cladding was that it should have a long service life, and for it to weather to a silver-grey colour without any surface coating. European oak, imported western red cedar, and iroko are all classed as durable timbers and so each fulfilled these requirements. However, because the architects had uncertainties about the staining effects of western red cedar and European oak, it was decided to use iroko even though this is a tropical hardwood.
The horizontal timber cladding was fixed to vertical, preservative treated, softwood battens on a breather membrane on sheathing ply to a non-loadbearing timber stud background, with insulation, vapour barrier and birch-faced ply to the interior. With each board drawn onto large-scale elevations, the architects' established rules on spacing and the arrangement of fixings to ensure a regular pattern of screw-heads. Stainless steel screws and washers were countersunk into the face of the board by the depth of the washer.