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TAKING TIMBER ON BOARD

FOCUS ON: CLADDING

Timber cladding is an attractive option and one that is both affordable and durable. For these reasons, and with a rising green agenda, it is finding increasing popularity. However, as with all materials, it needs to be treated with respect and knowledge.

TYPES OF CLADDING Although long boarding is the image that first springs to mind, this is not the only type of timber cladding that is available. In countries such as Austria and Switzerland, shingles are the traditional approach, and these are winning wider popularity, often in western red cedar. Plywood cladding is also increasingly popular. Each type of cladding needs to be considered differently in technical terms.

If timber cladding is to be durable, it is important that it does not become saturated.

Although it can cope with wetting, it must be designed in such a way that it can dry out.

A separate barrier, or membrane, as well as a cavity, should be formed behind the cladding. This should both allow water passing through the cladding to escape, and ensure that the internal and external surfaces of the wood have a similar moisture content, reducing the potential for movement and distortion.

The cavity should be at least 19mm wide, but this may vary depending on the thickness of fixing battens used.

Horizontal boarding can be butt jointed, rebated or profiled.Often 'green oak' is used as horizontal boarding with consistent open gaps between the boards. This allows it to act as a rainscreen, providing an exit for water behind it, and allows for the inevitable shrinkage of a material that is not yet fully seasoned. Because of this shrinkage, designing appropriate fixing and jointing that can accommodate movement is essential. Putting fixings closer together than when using seasoned timber should avoid splitting.

Horizontal boards should be fixed to vertical battens.

Support battens should have a maximum spacing of 600mm.

Boards should not be more than 150mm wide and overlaps should be at least 25mm.

Shiplap-type boards can have a minimum overlap of 15mm, but allow 2mm gaps between the upstand in order to achieve a nominal movement.

Tongued and grooved boards should have a maximum face width of 100mm, with a 2mm clearance above the tongue, again for expansion.

Installation should be tongue upwards.

Open-joint boards should have a 12mm gap at the water face. Chamfered edges allow the boards to overlap slightly, reducing cavity exposure.

Diagonal boarding can, in general, be regarded like horizontal cladding, but the maximum spacing for battens should be reduced to 400mm.

Shiplap profiles work best, as their design channels water away from the boards.

Vertical boarding often comprises tongued and grooved boards, rather than overlapped or profiled sections. Tongues must be long enough to allow for slight movement, so that open gaps do not develop as the timber moves naturally.

Other methods of vertical cladding include staggered overlaps with boards on top of boards. Again, fixing and detailing is vital to ensure that longevity and low maintenance are achieved.

Vertical boards should be fixed to horizontal battens, with vertical counter battens to facilitate drainage and ventilation. The face width should not exceed 100mm.

The most versatile fixing method is board on board.

The overlap should be a minimum of 25mm.

FIXINGS For softwood, annular ring shank nails are usually the most appropriate, but round-head nails work best with western red cedar. The nail length should be 2.5 times the thickness of the board being fi xed, and be slightly below the surface of the wood.

Boards over 100mm wide should have double fixings.

Butt joints should always meet on sufficient batten support width.

Stainless-steel nails should be used on species of wood that have a high tannin content, in order to avoid permanent staining with the reaction on ferrous metal.

For hardwoods, the preferred method of fixing is screwing to treated battens.

Slight overdrilling of the screw holes will allow for enough movement in the wood without causing splits. Countersinking is also recommended.

Where green wood is chosen it may be necessary to fit washers to the screws in order to maintain the fixing security of the board.

TIMBER SELECTION

If the timber is to be used in unpainted form, visual appearance is key. In some instances architects may deliberately welcome a rough appearance, so a certain number of knots will be acceptable. It is, however, vital to eliminate all dead knots.

Other natural defects such as splits and shakes must also be addressed. BS 1186 Part 3 deals with these in detail. Technically, the most important consideration is durability.

Moderately durable timbers can be used without being preservative-treated but should have their sapwood removed.

Generally it is easier to use an appropriate timber preservative and finishing to enhance the service life of the cladding and keep maintenance to a low, practical level.

The selection of preservation type should be considered with respect to the final surface finish. For example, using a high-pressure treatment process will give the timber a green appearance.

Softwoods are most commonly used for cladding.

They include western red cedar, European redwood and whitewood and Douglas fir.

In addition, coming to the market this year is chemically modified (not preservative treated) certified radiata pine.

A native of North America, it now grows in plantations in Australia, New Zealand, Chile and Spain.

Temperate hardwoods are increasingly popular, in many cases replacing tropical hardwoods. European oak and sweet chestnut are used widely and there is a growing market for English oak. It is not usually available in such long lengths, but careful design can accommodate this. If the timber is untreated, sapwood (from the edge of the tree) should be excluded. Like all timbers, if left untreated the surface of temperate hardwoods will turn a silvery grey. This does not affect strength or durability, and in many instances is a desired aesthetic effect.

It is susceptible to a visual patterning known as 'surface checking' as its moisture content varies, but again this doesn't affect its durability.

Tannin is a natural constituent of all timbers; oak and sweet chestnut in particular will exude this as they dry.

The result is a black deposit, sometimes in copious amounts.

Although it will wash away, it is corrosive, so consideration should be given to the selection of appropriate fixings and adjoining materials.

Tropical hardwoods are the most difficult to source, but independent certification bodies such as FSC and PEFC can provide guidance, and reputable supply companies will meet certification requirements.

Tropical hardwoods are generally stronger and more durable than softwoods. They are denser in structure and contain more heartwood than sapwood. While the need for preservative treatment is greatly reduced, the timbers are also much more resistant to preservative treatment. It is still advisable, however, to specify species where the sapwood has been eliminated so that its resistance to degradation is maintained.

MOISTURE A moisture content of less than 22 per cent will prevent fungal decay. All cladding used in the UK is normally below this level.

Timber is hygroscopic (i. e. it absorbs and releases moisture), but it does tend to maintain its average moisture content in UK climatic conditions.

A controlling factor to help facilitate this is the provision of suitable ventilation, in the form of a cavity between the cladding and the main construction surface.

The end grain of the wood is absorbent and should be protected. In addition, where vertical cladding is used, sufficient distance should be left between the ground and the first board - typically 150mm - to ensure that the timber is not in constant contact with moisture.

SURFACE PROTECTION If the natural colour changes that occur in timber over time are not considered appropriate, a chemical preservative and/or surface finishes will be needed.

If a surface coat is used, the finish coat must be applied to all faces prior to fixing, so that any shrinkage will not produce an exposed surface.

This is particularly relevant for tongued and grooved sections.

Surface coats should be exible and vapour permeable.

Normal oils, paints and varnishes should be avoided as they are prone to crack, allowing the possibility of moisture ingress. Water may become trapped beneath the coating, leading to degradation of the timber. In contrast, microporous finishes will prevent moisture build up, and are more suitable.

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