Wood claddings

Wood and wood products in various forms can be used as an exterior cladding material in all climate zones. The type of wood to be used and the detailing of the construction will depend largely on the climate in the region concerned. In countries which have long, dry winters (with temperatures below freezing point) and dry summers, moisture within the structure has more time to evaporate than in a maritime climate. Cladding which could last many decades in Sweden, Austria, Switzerland or Germany would fare considerably less well in the Netherlands, United Kingdom or Norway. In the dry parts of Canada and the United States, Red Cedar is regularly affixed directly to the supporting structure without any cavity, a practice which is inadvisable in the European maritime climate zones.

post and beam can be made on 8.15a Metal connectors in timber curtain wall (Raico, Germany) site with two screws.

The cladding may comprise boards applied horizontally or vertically, or may be panels or shingles. A number of cladding options are shown in Figure 8.17. The method of attachment will depend on the dimensions of the wood and the degree to which these change with the weather. Panels which have undergone thermal modification will present fewer problems in this regard.

Table 8.3 shows the suitability of certain types of wood for use in exterior cladding and whether treatment is required. In principle, all woods in Durability Grades 3 to 5 should be treated before use.

Application

When used without a cavity (which is only possible in extremely dry climates and when using woods of Durability Grade 1 or 2), a breather membrame must be applied behind the cladding if the inner sheeting is not waterproof, because external timber cladding can only act as a rainscreen. The structure to which the

8.17b Cladding types with horizontal boards. a. battens with open joint; b. boards with open joint; c. weather boards; d. weather boards with profiled edge; e. feather edge or bevel siding; f.rebated feather edge/bevel siding; g. shiplap; h. channel siding.

cladding is affixed must be waterproof on the outside and vapour-proof on the inside. Moisture must not be able to accumulate behind the cladding structure.The use of a cavity (of 20-40 mm) is recommended in all cases and is indeed essential in most.This cavity must allow vertical ventilation. An advantage of horizontally laid cladding boards is that ventilation becomes easier to achieve. When the boards are horizontal, the supporting battens are upright, allowing a clear vertical ventilation flow. If the boards themselves are vertical, the battens behind need to be placed horizontally, which will impede the vertical airflow. It will be necessary to provide vertical counterbattens to ensure ventilation up the cavity (Figure 8.18). Horizontal boards also reduce the risk of water permeation. One advantage of vertical boards is that, used unfinished, they will bleach at a more even rate. Moreover, if the length of the cladding boards does not exceed the height of one storey and if a horizontal joint is made on the level of the floor, it is not necessary to include butt joints (since the length of the boards will corres pond with the height of each storey) (Figure 8.19). Figures 8.20 and 8.21 show other solutions for a continuous horizontal joint construction.

8.18 Two solutions for vertical air flow behind vertical boarding.

8.19 Typical butt joint in verbal boarding.This 8.20 Horizontal joint between vertical joint can be appplied when joints between boards with aluminium profile, used when

8.21 As figure 8.20, but with a wooden profile. In this case the joint is not ventilated, which gives a higher risk of rot.

8.19 Typical butt joint in verbal boarding.This 8.20 Horizontal joint between vertical joint can be appplied when joints between boards with aluminium profile, used when boards differ in position.

the length of the boards corresponds with the height of each storey

8.21 As figure 8.20, but with a wooden profile. In this case the joint is not ventilated, which gives a higher risk of rot.

Sizes and relationship with the supporting structure

In most cases, the thickness of the cladding material will be between I 8 mm (hardwood and WRC) and I 9 mm (European spruce). Boards are commercially available In widths of 75 mm to 225 mm (In 25 mm Increments) but boards wider than I 50 mm are not recommended due to the likelihood of excessive swelling or shrinking. After planing and the cutting of any edging profile, the boards will be 145 mm.With an overlapping joint, the face width will be some 20-25 mm less again. With open joints, 7 mm must be allowed for the joint itself and 3 mm fortolerance.The face width will then be I 55 mm.The centre-to-centre distance of the battens should not exceed 600 mm. Fixing materials such as screws and nails must be used at least 40 mm from the end of the board in order to prevent splitting. Accordingly, if butt joints are used, the battens must be more than I 30 mm wide: an extremely uneconomical option. In practice, therefore, the fixing points are generally placed much closer to the edge of the wood, the risk of splitting being reduced by pre-drilling or the use of a single fixing point. A further option is to align all the butt joints and to use two supporting battens. All elements will then be of equal length and there will be a very visible pattern (Figure 8.22). Using this approach, it is also possible to prefabricate larger exterior wall elements which may already have the cladding fitted, or to mount the entire cladding on a frame which can then be affixed to the underlying structure.This is a good option where the cladding consists of battens instead of boards, since accurate placement is difficult on site. Any aberration will be immediately visible and it is therefore preferable to mount the entire assembly in the factory. Usually the boards are affixed to battens of treated softwood.

8.22 Cladding of green oak on the Rowing Museum, David Chipperfield, Henley-on-Thames.

Joints

The joint between the boards can be an open joint, lap joint or mated joint. Butt joints are not recommended because no matter how well the boards are lined up against each other; any unevenness, distortion or mounting space will cause a small chink in which water can be absorbed (by capillary action). The water will then be dispersed by ventilation only very slowly, if at all. The risk of rot is therefore greatest at the joints. Due to the natural swelling and shrinkage of the wood, there will always be a gap at certain times of year; or conversely the boards will press so tightly against each other that they will buckle.

According, open-joint constructions are now increasingly used. A waterproof but vapour-permeable membrane is then applied behind the battens at the back of the cavity, to prevent water from penetrating into the wall construction. The joints are designed in such a way as to minimize the penetration of rainwater and prevent absorption by capillary action. It is advisable to make the joint large enough to ensure that no capillary water remains, even when the wood reaches its maximum dimensions in damp weather In practice, this means a joint of I 0 to 13 mm, when the boards are applied during the dry season. Open jointed boards should be chamfered so that they overlap and shed water to the outside. A tapered joint, becoming wider on the interior side, will promote drainage and reduce air pressure and thus reduce the amount of water which can pass into the cavity.This will also protect the breather membrane from ultraviolet light. Where boards are applied vertically with open joints, the supporting structure must be designed to ensure proper drainage, and the waterproof membrane must be resistant to ultraviolet light. David Chipperfield's design for the Rowing Museum in Henley-on-Thames (UK) provides a good example of the use of open joints.

The lap joint is seen in the non-profiled weather board, in bevel-siding for horizontal planking and in overlapping planking for vertical boards (Figures 8.1 7a, 8.1 7b).Where feather-edged siding (or bevel-siding) is applied, less wood is required and the total package is slightly narrower If a better seal is required, elements with mated joints can be used (with a simple or 'Swedish' rebate). All edges of the boards must then be prepared accordingly.'Channel siding' or'shiplap joint' is a type of cladding joint that is particularly popular in North America. Tongue-and-groove joints are not recommended since they create a closed space in which water can accumulate. Moreover; particularly thick boards are required in order to provide a sufficiently robust tongue.

Mounting

Boards can be fixed with nails, screws, bolts and clips. Where boards wider then 150 mm are applied, especially in the case of open joints, two fasteners at each fixing point are needed.When nails are used, it is not necessary to take the swelling and shrinkage of the boards into account The nails are flexible enough to follow the swelling and shrinkage of the boards due to seasonal variations in the equilibrium moisture content. At the ends of the boards, it is best to use only one nail, or predrilled holes, to prevent the boards from splitting.The fixing length of the nail should be twice the thickness of the board when annular ringshank nails are used. With ordinary nails this length should be 2.5 times the thickness of the board.

Where screws or bolts are used, and where the boards have a starting moisture content in the region of I 6%, holes for screws or bolts should be approximately 2 mm larger then the diameter of the fasteners in order to prevent splitting of the boards. If boards are used with a higher moisture content, as with 'green wood', that diameter should be 4-6 mm.The fasteners should be positioned one quarter of the board's width from the edge. If the holes are much larger than the shaft of the bolt or screw, it is possible that inadequate fixing strength will be achieved, because the screwhead is not big enough unless washers are also used.The use of bolts or screws with an integrated rubber-covered washer will ensure that the screwhead is always centred and that the fixing will remain secure even when the wood shrinks. The screwheads and washers can also be countersunk if desired, but it then becomes important to ensure that enough space is allowed between washer and wood to allow for movement.

Where smaller overlapping boards are used only one fastener is needed at each fixing point.The fasteners should be positioned in the overlapping edge of the board, but allowing the overlapped board to swell and shrink (seeTable 8.5).

The number of screws used, their size and their position are not so much determined by the weight of the boards as by the stresses that may be caused by the drying of the wood, and the resultant distortion.

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