Weighing up the benefits
The clamour for environmentally friendly building materials has seen demand for timber cladding increase.However, in order to get the right product, it is important to consider the specific needs of each project
The growth of interest in 'green' construction since the late 1980s has had a knock-on effect on the use of timber in construction in the UK. Carefully sourced, specified and detailed timber - softwood in particular - can provide environmentally friendly, economical and durable buildings.
Softwood board cladding can be recommended on many levels as an external cladding. Its popularity may in part be attributed to the fact that its environmental credentials are good, especially in terms of embodied energy and toxic emissions that emanate from it in the manufacture, construction and life of the building. The amount of energy used in the felling of trees and the conversion of them into boards is estimated at 10 per cent of that for facing bricks and 50 per cent of that for concrete block*. However, in deciding whether it is right for a particular project, the following points should also be considered:
Does the building have an appropriate form? Good protection from the inhospitable British climate is important for the durability of softwood cladding.
ls programme an issue? Softwood cladding is relatively quick to erect and versatile in that it can be readily altered on site if necessary. It lends itself well to projects with tight programmes - cedar cladding was used untreated on the new classrooms built for the King Alfred School in Golders Green, north London, which were constructed in the summer break.
Is the weight of the building an issue?
Softwood cladding can have advantages over other cladding materials such as brick and metal.
These positive qualities should not be compromised by the use of inappropriate or damaging preservatives and finishes, or with detailing that commits the end user to high levels of maintenance.
Picking your material
The most commonly used softwoods for cladding in the UK are western red cedar, European redwood, larch, Douglas fir and European whitewood. These species have varying degrees of natural durability and this will be the main consideration in the choice of species. EN350 parts 1 and 2 define this natural durability. Depending on the durability rating of the species, some can be left untreated against fungal and insect infestation.
Several different types of treatment are available to protect softwood cladding from insect and fungal attack, should it be needed.
Again, in deciding on an appropriate preservative treatment the choice will depend on some fundamental questions:
ls it part of the aesthetic that the timber is used unfinished, or will it have a decorative/protective coating?
ls the timber likely to get wet or is it protected from the weather?
Preservative treatments are, by their nature, unfriendly to some forms of natural life. This must be borne in mind when specifying. The pros and cons should be weighed up carefully depending on individual circumstances.
Pressure impregnation and double vacuum treatments are more effective than immersion. CCA is bound into the timber once it is dry and, as a result, tends not to leach if the timber gets wet. This means that care must be taken with disposal at the end of the life of the cladding, which should be done through a competent and authorised disposal company. This treatment will give the timber a slightly greenish tinge, though this normally fades with time.Waterborne, mixed-emulsion treatments are more susceptible to leaching if the timber gets wet.An alternative treatment is boron, which has relatively good environmental credentials but has a shorter life than those treatments already mentioned. Boron treatment can be applied in a variety of ways, including the insertion of pellets or rods that release the preservative over time, and again must also have a protective coating due to the leaching effect.
One treatment, based on copper and biocides, is similar to CCA but omits the chrome and arsenic, though this is not in widespread use.
Doing the detail
To get the best out of softwood cladding it is important to get the building form right first.
The old maxim of 'good hat and boots' is especially important to the durability of the cladding. My practice's timber-clad buildings, such as the Sutton Hoo visitor centre in Suffolk, and the Gateway to the White Cliffs visitor centre in Dover, benefit from generous roof overhangs and proper plinths to lift the cladding away from the ground.
Softwood cladding should only be used as a rainscreen, with secondary waterproof protection provided behind the cladding.
This allows air to circulate freely around the timber and should ensure the boards have a sensible lifespan. The rainscreen will give the secondary membrane some protection from wind, rain and sunlight, helping to reduce the degradation caused by these conditions.
Other important criteria when detailing the cladding are board profile, thickness, width and length, and fixing method. All of these issues are, to a degree, interrelated.
All in the thickness
Considerations that should be borne in mind are, first, that the board profile relates to the thickness of the boards. The more that is removed from the thickness of the board in terms of rebates or tongues and grooves, the thicker the boards that must be used in the first place. Second, in terms of board width, the wider the board the greater the potential for 'cupping'. The width of board also depends on the quality of the timber.
Boards wider than 150mm are commonly used in Scandinavian countries, where the quality of the timber is higher, but are not recommended in Britain. Third, when detailing boards for horizontal, vertical or diagonal use, thought should be given to the most appropriate detail to prevent penetration or entrapment of water, as well as appearance. Fourth, and finally, the decision on how to express vertical joints is an aesthetic one depending on personal preference, but in all cases care must be taken to allow air to circulate around the end grain, ensuring it remains dry. This also applies to, among other things, abutments at window frames and corner posts.
Timber is a natural material and is subject to changes in size due to fluctuations in its moisture content. Boards with the correct moisture content for a particular situation should be specified. Even so, the boards will still change in size as the moisture content changes. Limiting board widths can help to counter this movement, but it is also important to allow for movement when detailing.
Joints between boards must allow for movement, but also provide a weatherproof covering. Overlapping or open-jointed cladding will allow for movement relatively simply, but tongued-and-grooved or rebated boards should be detailed to allow for movement within the joint itself. Narrower boards can have single fixings, avoiding problems of movement, but wider boards may need two fixings across the width, and this can set up stresses within the board. Fixings can be expressed in a number of ways: nails or screws can be left visible or the holes filled or pelleted. Alternatively, the fixing locations can be hidden behind cover strips.
You can guard against insect and vermin infestation by installing small-gauge mesh at the top and bottom of the cavity behind the boards, and also as an extra layer behind open-jointed boards.
Maintenance and durability
The issue of natural durability of different species has been referred to above. The addition of a decorative coating will improve the durability of the cladding, but it also has maintenance implications for the building user.
Leaving timber unfinished will not guarantee an even appearance; this can only be achieved with a decorative coating that is also maintained. Any surface coating should be flexible and allow vapour to pass through.
Finishes such as oil paints or varnish are not suitable - they will crack when the boards expand and contract and will not provide the protection required. Lead-based paints, though banned for all work other than on listed buildings, still provide good performance, however. More suitable for most projects are micro-porous stains and paints that will move with the timber and not trap moisture behind them.
Chris Wilderspin is a director of van Heyningen and Haward
References * Baird, G and Chan Seong Aun (1983), Energy costs of Houses and Light Construction Buildings, Report No 76, New Zealand Energy Research and Development Committee, Auckland, NZ.
Baird, G and Chan Seong Aun (1983), Energy costs of Houses and Light Construction Buildings, Report No 76, New Zealand Energy Research and Development Committee, Auckland, NZ.
Hislop, P J, External Timber Cladding, TRADA Technology, High Wycombe, UK.
McCartney, K, The Culture of Timber, University of Portsmouth, Portsmouth, UK.