Timber Cladding in Scotland
IMPORTED EUROPEAN OAK
SCOTTISH POETRY LIBRARY, EDINBURGH
MALCOLM FRASER ARCHITECTS
European oak was selected for the Scottish Poetry Library because of its associations with landscape, its material character and, not insignificantly, its limited maintenance requirement. The timber cladding is combined with glass as infill to a simple steel frame, with large timber-framed screens sliding to transform the building into a timber box at the end of each day.
The River and Rowing Museum at Henley by David Chipperfield had just been completed at the time the Library's detailing was being developed. Unlike the green oak used by Chipperfield's office, however, the cladding at the Scottish Poetry Library was dried to a moisture content below 19% to avoid the large shrinkage associated with green oak as it dries.
European oak from France was selected for the Poetry Library. The practice has since made efforts to specify Scottish oak for large external sliding doors and windows, but sawmills, industry experts and ironmongery suppliers have all recommended against its use for these specific applications (While it is suitable for many cladding and other external joinery purposes, homegrown oak is not generally suited to designs in which movement is not easily accommodated. Also, homegrown oak is usually too expensive to be used as cladding except for green oak designs).
The cladding was specified as 2300mm long vertical tongue and grooved boards with 19 x 175mm profiles machined from 22 x 200mm sections. Lateral movement and board fixing were the practice's principal detailing concerns and - working to the rule of thumb 1% movement for every 3% change in moisture content - the tongue and groove profile was determined. Fixed with stainless steel self-tapping screws with stainless steel and rubber washers, each board's weight is carried by the screw head/washer and not by the screw shank, which is fitted through an over-sized pre-drilled hole. In principle, each board can move unconstrained by its screw fixings.
The specification precluded the use of sapwood and tolerances for knots, cracks and splitting were indicated, as was the need for temporary protection of the fixed boards. Intriguingly, the practice was advised to use diluted Toilet Duck(!) to clean off the black stains caused by acetic acid corrosion of mild steel shavings left in the drilled holes (a 5-10% solution of oxalic acid would be the more normal recommendation).