Plywood sheets can be used as exterior cladding. However; the risks are greater than in the case of solid wood boards. If anything other than the very highest quality of sheet is applied, and if anything but the greatest possible attention is devoted to the finishing of the edges and to the mounting, there is a substantial risk of 'delamination': the layers of the sheet will become detached from each other Only plywood sheets with a facing of gaboon (Aucoumea klaineana) or sapele (Entandrophragma cylindricum) can be finished with a layer of transparent varnish. With all other types, a full-cover coating is required to provide adequate protection against ultraviolet light. Because the edges must also be protected, the sheets are cut to size and treated in the factory.The order dimensions must be extremely accurate since the sheets cannot be sawn again. (Suppliers impose these requirements in connection with the guarantee.)
The vertical joints between sheets must be at least 10 mm, preferably 20 mm. A drainage profile is often fitted to the horizontal joints in order to protect the upper edge of the sheet (in the same manner as in figure 8.20).
The sheets can be mounted using staples, nails or screws. The heads of any nails or screws can be countersunk, or screwheads can be fitted with a plastic cap.This solution may be somewhat less attractive, not least due to the risk of streaking.
The sheets can also be affixed to the supporting structure with adhesives. As with all glued joints, this must be undertaken during dry weather; which poses a restriction.The adhesive itself must be thick enough and remain elastic enough to accommodate swelling by the sheets. A good mounting method is to affix the sheets to a wooden frame in a factory and then fit it to the supporting structure. The frame remains adjustable and can be removed if required.
In addition to plywood, there are also sheets of heat-hardened resins, reinforced with wood fibres. These are produced under high temperature and high pressure. The surface can be coloured or have a woodgrain photoprint applied. In effect, these are plastic sheets with the appearance of real wood.The sheets can be sawn into lengths and can be curved. The edges require no special treatment but they will remain dark in colour; which renders them less authentic in appearance than solid wood when subject to close inspection.This effect is exacerbated by the fact that the sheets are only 6 mm to I 0 mm thick.The mounting method is the same as that for plywood. Although water permeation at the mounting points does not present any risk, it should be remembered that these sheets will expand and contract more than the plywood equivalents, and a greater tolerance in the mounting holes must therefore be allowed.
Shakes and shingles
Both are small boards with a size In the region of 50 x 300 mm. The difference between both Is that shingles are sawn from a block and shakes have been split from a block. The surface of the first is smoother; the latter has the advantage that the vessels are not cut open, as in splitting the block the cut follows the structure of the wood. This prevents water from penetrating the vessels.The boards are nailed directly to a structure of battens and counterbattens with a
ventilated cavity (Figure 8.25) or to a closed timber wall of butt-jointed, rough-sawn boards (Figure 8.26). The boards are applied with a double overlap in such a way that the nails fixing the lower board are covered by the higher board.The vertical joints between the boards alternate. In wet climates shingles and shakes are made of western red cedar, larch, chestnut or oak. In dry climates less durable types of wood have traditionally been used. An outstanding example of the use of shingles is Peter Zumthors' St Benedict's Chapel in Sumvitg, Switzerland (Figures 8.27, 8.28).
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