External cladding subject to intermittent wetting but away from ground contact and well ventilated requires "Moderately durable" timber. Homegrown species which meet this classification are: oak, sweet chestnut (both durable and western red cedar (moderately durable). In all cases only the heartwood is suitable. Other softwood species like larch, spruce and douglas fir are suitable if treated either with surface finishes or impregnation. (note home-grown larch and Douglas are less durable than slow-grown imported stuff).

With appropriate detailing unseasoned (green) timber can be used and this will then dry in situ. Shrinkage is predictable and must be allowed for. Wherever possible single fixings should be used to allow for shrinkage or if not possible, oversize or elongated holes used to fix cladding boards to battens. Narrow boards (75 or 100mm wide) are more reliable than wider boards and generally cheaper. Thin boards (25mm or less) dry more evenly and are less prone to surface checking than thicker boards.

All three species contain soluble tannins which leach out in the early stages of weathering. This will stain concrete surfaces and is unsightly for a while. A pebble run or similar at the base of the wall is preferable. In time all timber turns a silver grey colour and the species are then indistinguishable from a distance. Tannins react with iron to produce a blue-black stain. Use non-ferrous fixings or stainless steel to avoid this.

There is a wide range of choices of profiles including horizontal, vertical and diagonal boards. Square or feather-edged cross sections are most commonly used. It is important to ensure a free movement of air around all faces of the cladding, particularly if heavy sections are preferred. Tongue & groove arrangements should be avoided as moisture is retained in the groove. Probably the most adaptable profile is a feather-edged board with a rebate to fit over the board below or alongside. Simpler arrangements like batten on board or board on board can be used in vertical cladding. Batten on board is preferred as it requires only one fixing, leaving room for movement. Sawn shingles can be produced from all of these species and are suitable for vertical surfaces. Oak and chestnut can also be used for roof shingles; chestnut is preferred because it is more stable. Open rain screens with horizontal boards should be chamfered at 10o to shed moisture.

Coed Cymru has experimented with a number of other profiles for special applications.

Coed Cymru, The Old Sawmill,
Tregynon, Newtown,
Powys SY16 3PL
Tel 01686 650 777

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