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Landscape and exteriors Cladding

At present, no European test standards are available to examine the durability of modified wood; tests designed for solid wood are employed, including laboratory and field tests. Some modified species have also been tested for resistance to wood-boring insects and marine borers. Most modification processes enhance the durability of wood against decay fungi. Modification offers an alternative to preservative treatment for some applications. Some modified woods can provide service lives equal to or longer than preservativetreated wood or some naturally durable timber species.

Dimensional stability Improved stability has been demonstrated using all types of modification processes (see Figure 1). Since the cell structure of modified wood products absorbs less water, the equilibrium moisture content of these products is also reduced. Improved dimensional stability reduces movement in service and the risk of cupping and other associated distortion often found in cladding boards. Splitting or checking around fixings over time is also reduced.

Independent tests to evaluate movement characteristics of timbers have shown some modified woods outperform even timbers considered to be very stable, such as western red cedar and teak.

Reaction to fire properties As for unmodified wood, the only practical way to improve performance of thermally and chemically modified wood exposed to fire is to apply a fire-retardant treatment. Effectiveness of fire-retardant treatments applied using conventional techniques has been evaluated. For further information, contact the manufacturers of the modified wood products. Impregnation modification technologies can improve the reaction-to-fire properties significantly. The application of flame retardants is often required for applications, such as cladding, depending on its proximity to a boundary or another building. Further information on the need for fire protection can be found in the Building Regulations.

Modification processes The modification type tends to consist of either a thermal, chemical or impregnation process. Examples of some of these products, suitable for exterior cladding, are presented in Table 1 below.

Modification type

Commercial name of modified product


ThermoWood® S and D Plato®WOOD Lunawood® Brimstone® Durawood®

Figure 1: The percentage of total swelling and shrinkage for five modified woods tested against reference species when exposed to changes in ambient conditions (taken from WIS 2/3-63 Modified woods)

The enhanced stability of the modified products also improves the quality of finish, particularly if coated. For example, when a high build film-forming surface coating is applied, it will be less likely to be subjected to higher amounts of shrinkage and swelling frequently occurring with natural timber species. Therefore, the elasticity of the coating is less likely to be compromised, which will help improve its service life, and subsequently, increase maintenance intervals.

Natural Cladding® Chemical

Accoya® Medite® Tricoya® Extreme Kebony®


Lignia™ XD®

Table 1: Taken from WIS 2/3-63 Modified woods

Timber 2019 Industry Yearbook

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Profile for BM TRADA

Timber 2019 Industry Yearbook  

The annual publication of the Timber Research and Development Association (TRADA) includes topical and technical features on all aspects of...

Timber 2019 Industry Yearbook  

The annual publication of the Timber Research and Development Association (TRADA) includes topical and technical features on all aspects of...

Profile for trada