Timber Cladding in Scotland
HOMEGROWN EUROPEAN LARCH
GLENCOE VISITOR FACILITIES
The new National Trust for Scotland visitor facilities at Glencoe comprise a series of buildings containing cafe, kitchen, shop, toilets, interpretation centre and education room. Boardwalks connect these with administration buildings, storage and on-site staff accommodation. A steel framework raises the whole complex off the ground to allow ventilation beneath the floors and to ensure minimum disturbance to the ground and it's waterways.
Timber-framed and using 'breathing' wall construction, the entire complex is fabricated from home-grown woods. None of the timber is preservative-treated, and various measures have been taken to ensure acceptable levels of durability and maintenance. All boards and supporting battens are of European larch (heartwood), with 144 x 25mm vertical cladding boards set 6-8mm apart on 44 x 25mm inner battens. The supporting horizontal battens are fixed at 600mm centres on vertical battens over a high performance breather membrane. The boards are face-sawn with planed edges (whether to plane or saw the cladding boards was a moot issue - by reducing the amount of surface presented to the atmosphere, planing might reduce moisture ingress. Conversely, sawn surfaces may permit moisture to evaporate much more readily, and could reduce the time boards spend saturated). No coatings of any sort have been used.
Screwing the boards proved too expensive and so each is nailed, with torque carefully gauged to prevent nail heads penetrating too far into the timber's surface (ideally, they should be flush with the surface to avoid creating mini-collection points with exposed end grain). Screws would have facilitated ease of replacement of boards, but nailed connections were accepted by the design team once it was agreed that boards would only be removed when they were unlikely to be useful for other applications. Nails are applied centrally, allowing for any amount of movement to either side.
Differential weathering between boards is a disadvantage of vertical cladding - the lowest 150-300mm can suffer 'splashback' whilst higher areas protected by eaves stay in good condition. To avoid replacing whole boards when only their lowest parts are in poor condition, a detail was developed using a 150mm horizontal board to take the brunt of the weather whilst the vertical boards above remain in good condition.
Untreated European larch boards have also been used to clad the roof. While this is contrary to recommended practice in Scotland and Norway, the wider environmental remit of the building precluded the use of preservative treated roof boards. The designer and client accept that the roof timbers will require regular maintenance and have detailed it accordingly.