Timber Cladding in Scotland
WOOD PRESERVATIVES AND COATINGS
Providing it has sufficient natural durability, the heartwood of a timber species can be used without preservative treatment even where a biological hazard exists. Sometimes, however, the natural durability of a particular timber may be insufficient for the Hazard Class in which it will be used. If so, the designer may have to switch to a more durable timber, or increase the durability of the timber through the use of a suitable preservative to avoid the risk of subsequent decay or insect attack.
DEFINING THE RISKS
Sustainability is currently a significant market driver for timber cladding in Scotland and so any discussion of wood preservation involves the recognition of two - sometimes conflicting - risks, the balancing of which is not always straight-forward:
- Environmental risks
Like all other pesticides, timber preservatives are biocides that carry environmental risks throughout their life cycle from manufacture through to eventual disposal. Consequently, when wood preservatives are used in a cladding assembly one could argue that sustainability may be reduced and this might limit its market acceptance.
- Risk of product failure
Within the confines of construction contract management, product selection will be partly determined by quantifying the risk of each product's failure and subsequent reduction of the risks - either through redesign or, more commonly, through transferring any associated liabilities to the supplier or contractor. Thus whilst sustainability-related issues may be important to the designer or the client, implementation will be limited to those situations where the designer, main contractor or supplier is content to underwrite any contingent liability. This tends to engender a conservative approach, and whilst designing for durability can minimise the risk of premature failure, many in the construction industry will still require to reduce the perceived risk of failure by using the 'insurance' of wood preservation.
USING WOOD PRESERVATION APPROPRIATELY
Depending on service location, many timber products are susceptible to biological degradation from fungi and insects. It is therefore essential that - when used in a particular location - timber products have a durability appropriate to the performance required of the component. European standards provide a decision-making sequence for selecting the level of natural durability or wood preservation required in a particular situation:
1 The desired performance and the Hazard Class of the component are decided first, taking into account the cost of failure (e.g. health & safety; replacement costs etc). This assessment is described in BS EN 335 parts 1 and 2:1992;
2 The timber species is then selected using the timber properties guidance in BS EN 350-2:1992;
3 The durability of the chosen species is then assessed against the performance and Hazard Class using BS EN 460:1994. The options are either: that the timber has sufficient natural durability for the envisaged use; or that the durability of the component needs to be increased either by using a more naturally durable species or through preservative treatment;
Where required, the performance of wood preservatives are then specified by taking into account the biological agencies against which protection is required. With some timber species it may be impossible to achieve the appropriate preservative specification, in which case a different timber species should be selected or the design changed to reduce the decay hazard. Guidance on the specification of wood preservatives is given in BS EN 351-1:1996 and BS EN 599-1:1997. The draft standard DD 239:1998 provides additional information and, along with BS EN 460:1994, gives a general introduction to the specification of naturally-durable or preservative-treated timber.
Preservatives can be applied in several ways. Some methods (e.g. brushing, spraying and dipping) will only result in a surface deposition of preservative and are therefore relatively ineffective. Deeper penetration and distribution of the preservative chemicals in the wood generally require industrial vacuum or pressure-impregnation methods. Some wood preservatives are prone to leach from the timber and where this is a risk, either the timber component is given a water-repellent coating (as is common with window joinery) or the preservative chosen should be resistant to leaching.
It is vital to ensure that the quality of wood preservation practice is consistent with the requirements of the European Standards. For example, inappropriate preservative or poor site practice may lead to situations where the level of durability conferred by timber preservation may be much less than that required by the Hazard Class. Consequently, where preservative treatment is specified, care should be taken to ensure that the preservative chosen, its depth of penetration and retention, as well as the subsequent handling and installation of the component, are appropriate to achieve the level of protection required. This is a complex process and there may be benefit in adopting formal quality assurance procedures to ensure compliance with the relevant standards.
On this exposed site on Shetland the housing designers chose to pressure-treat the softwood cladding with a leach-proof preservative before applying a decorative surface coating.
This batten has had to be cut to length on-site which has exposed untreated timber. In order to preserve the ˜envelope of protection', the joiner has followed best practice by liberally coating the endgrain with a recommended preservative.
The main function of a surface coating is decoration and should never be seen as a substitute for good detailing, natural durability or timber preservation.
Oak external joinery with the remnants of a transparent coating which has failed after a couple of years. It would have been better not to have used a coating at all.
If a designer wishes to avoid using preservatives then both the design and the timber species need to be chosen with care using the guidance given in the standards. The designer also needs to be confident that the subsequent maintenance programme is suitable and that, should a component fail, it would not constitute a danger to persons or property; that it could be easily replaced; and that the owner of the building is prepared for the possibility of replacement.
It is always worth being cautious with timber preservation. Adherence to environmental criteria for selection or avoidance should be rigorous, and emotive arguments and rationale avoided. The discussion of wood preservation is littered with ill-informed good intentions and so performance requirements, fitness for purpose, and risk of failure should always be the guiding principles. Designers should always ensure that any supplier's claims in relation to preservative performance can be backed-up, especially under service conditions which reflect the design's intended end-use. Boron-based preservatives are, for example, frequently advocated as having less environmental impact than other systems. Yet while suitable for some applications, the fact that boron preparations are water-soluble and can therefore leach out of the timber generally limits their use to Hazard Class 2 applications, unless the timber is subsequently painted with a water-repellent coating which is then regularly maintained.
HEAT TREATMENT OF TIMBER
Because of growing environmental concerns about some wood preservatives there is increasing interest across Europe in the potential for the use of thermally modified wood for cladding. To date there are four commercially available technologies: the Finnish THERMO WOOD; the Dutch PLATO WOOD; the French RETIFICATION and the German OIL HEAT TREATMENT. Timber cladding using these products is beginning to be available in Scotland.
According to a recent review of such technology, all four modification treatments are based on controlled heating of solid wood to temperatures close to or above 200C for several hours in an atmosphere with low oxygen content in order to modify the wood's inherent properties. Whilst such treatments reduce some mechanical properties they also increase the dimensional stability and biological durability of the wood without adding outside chemicals/biocides to the wood (4).
However, it should be noted that the use of heat-treated timber is not recommended in Hazard Class 4 or 5 situations (e.g. in ground contact) and it is not yet supported by a body of long term in-service performance evidence. Because of these and other uncertainties careful selection of timber species and process are critical if adequate in-service performance is to be achieved and hard evidence should always be sought from suppliers as to the appropriateness of their treatment for the timber species concerned and its intended end use.
Manufacturers of surface coatings sometimes use confusing terminology such as 'preservative coatings', but the preservative ingredients contained in their coatings are mainly intended to increase coating life and in most cases do not in themselves confer significant additional durability to the timber substrate (5). There are, however technical benefits from coatings which, in some circumstances, can be important:
In Hazard Class 3 conditions a superficial coating which is water-repellent may slightly reduce the speed of water uptake by the timber and will consequently tend to:
- reduce the susceptibility to fungal decay;
- reduce the amount of moisture-induced movement in the timber;
- protect susceptible wood preservatives from leaching out of the timber.
All of these effects depend on an intact coating being maintained.
Timber exposed to sunlight and not protected by some kind of coating will quickly bleach to grey due to photo-degradation of its surface by ultraviolet light. Surface checking and splitting can also occur. In some circumstances these effects may be desirable but, where bleaching of the timber is not acceptable, it is necessary to provide some kind of surface protection. Beyond the bleaching effect, timber damage from UV light is not in itself a significant degradation issue under Scottish conditions.
SELECTING A COATING
There are two key points to remember when selecting a coating for use on external joinery:
- Relatively opaque coatings are more durable than non opaque coatings -
While UV degradation of timber is not in itself a major problem, UV damage to the timber surface is a problem if it occurs under a surface coating. This can cause the adhesion between the coating and the timber substrate to fail, resulting in the coating peeling off. Only coatings that offer long term resistance to UV penetration are suitable for use on cladding and a high degree of opaque pigmentation is generally the most effective long-term UV protection for the coating. Some less opaque coatings use UV absorbers, but these tend to be used up quite quickly and may only offer limited protection. For this reason, exterior varnishes and low-build woodstains should generally not be used on timber cladding. There are no completely clear coatings that will maintain the original colour of the wood over the service life of the cladding.
- Exterior coatings need to be moisture vapour permeable -
The use of a moisture-vapour-permeable coating that allows any moisture accumulating in the wood to evaporate away without causing damage is generally considered beneficial. The more impermeable conventional gloss paints should be avoided on exteriors and a specialised timber coating used instead.
The generally accepted minimum service life of the best modern coatings is about 6 years in most conditions, with less opaque coatings having a shorter service life. Claims of service lives of 10 or even 15 years for particular coating products should be treated with extreme caution.
ENSURING THE MAXIMUM SERVICE LIFE OF AN EXTERIOR COATING
Timber coatings are often resisted because it is argued they create an ongoing maintenance cost. This can be minimised through the use of the following measures:
- Opaque moisture-vapour-permeable coatings can more than double the life of the coating when compared to using a semi-transparent stain.
- Application of a primer and a first top-coat to timber cladding before it is fitted onto the wall avoids any uncoated areas becoming exposed should the boards shrink even slightly after installation;
- The use of design details that protect endgrain from wetting is very important, otherwise moisture will be absorbed, resulting in premature coating failure;
- Coatings tend to weather better when applied to timber that has been pressure treated with a preservative;
- Rough-sawn boards hold coatings longer than smooth boards. Although rough boards initially absorb more of the coating, their use can extend the life of most coatings by several years;
- Avoid sharp edges or arrises on profiled cladding, as the coating is always thinner and less durable on a sharp edge. TRADA recommend a radius of 3mm on profiled sections (5);
- Avoid timber that needs to be coated on very exposed elevations. Protect eaves with metal sheeting or use the heartwood of a durable or very durable species that needs no coating;
- Channel water off the wall by using flashings at regular intervals;
- Avoid exposed horizontal or near horizontal timber surfaces, as these are vulnerable to moisture;
- Coating timber under properly controlled conditions always gives a more cost-effective and durable finish than site application. Factory coating is well established for window joinery and would also be of benefit for cladding;
- External cladding boards that will be coated should be separated from other materials by a gap of 10mm. This makes the coating much easier to apply as less care is needed at the junctions.
The alternative to using a coating is to allow the timber to weather naturally to a silver-grey. This approach is only suitable for timbers of moderate durability or better, and will not be visually acceptable in all circumstances. The use of uncoated timber offers particular qualities of surface and is increasing in Scotland.
1 Hislop, P., 2000,, External timber cladding, TRADA Technology.
2 Byggforsk, 1998, Byggforsk kunnskapssystemer ver 1.0, Norwegian Building Research Institute
3 Riddout, B., 2000, Timber Decay in Buildings, E & FN Spon.
4 Rapp, A.O., 2001, Review of Heat Treatments of Wood. Proceedings of Special Seminar held in Antibes France, February 2001, COST ACTION E22, Environmental optimisation of wood protection, European Commission Research Directorate
5 TRADA, 1993, WIF 1-20, External Timber Cladding
BS 1186-3: 1997. Timber for and workmanship in joinery. Specification for wood trim and its fixing.
BS EN 335-1: 1992. Hazard classes of wood and wood based products against biological attack. Classification of hazard classes
BS EN 335-2: 1992. Hazard classes of wood and wood based products against biological attack. Guide to the application of hazard classes to solid wood.
BS EN 350-2: 1994. Durability of wood and wood based products - natural durability of solid wood - Guide to the natural durability and treatability of selected wood species of importance in Europe.
BS EN 460: 1994. Durability of wood based products. Natural durability of solid wood. Guide to the durability requirements for wood to be used in hazard classes.
BS EN 351-1: 1996. Durability of wood and wood based products. Preservative treated wood. Classification of preservative penetration and retention.
BS EN 599-1: 1997. Durability of wood and wood based products. Performance of preventive wood preservatives by biological tests. Specification according to hazard class.
DD 239: 1998. Recommendations for the preservation of timber.