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Building Products April 2017

TIMBER AND TREATMENTS to protect them from the worst of the elements. However, it was in the 1990s when the technology started to develop in its current form. Today, the treatment involves heating timber to over 200°C, with steam used to prevent cracking and burning. Once cooled, the wood is remoisturised to around 5%. The principle is simple: heating timber removes most of the moisture, making it less likely to warp or swell and removing the issue of corrosion caused by acetic acid. As the treatment also removes resin, sugars which fungi would survive on are eradicated. Installers report that thermally modified timber is easier to cut with reduced wear on tools, and as the resin has been removed it is lighter too, making installation easier. Unlike chemical methods, the treatment penetrates all the way through the wood so thermally modified wood will not be subject to rot at the core. However, because the cell structure of the timber is permanently altered, it becomes more brittle and loses much of its tensile strength – that is why thermally modified timber is unsuitable for structural use. When fitting thermally modified 50 BUILDING PRODUCTS | APRIL 2017 timber cladding, installers must take extra care and follow fitting instructions carefully and it is best to use pre-drilled boards and hand-nail boards into place to avoid splitting. Thermally modified timber should be installed using stainless steel fittings only. As a key component of cladding is weatherproofing, thermally modified timber’s increased weather resistance makes it a sound choice. This increased resistance to moisture means that the cladding is suitable for outdoor and indoor use, such as in a bathroom, wetroom or steam room. Thermally modified timber has aesthetic advantages as well, which can make it preferable even to exotic hardwood cladding or some of the more durable softwoods. For example, many architects specify Western Red Cedar despite its expense because of its lengthy life-cycle, attractive knot-free straight grain and famously rich colour, but in fact results can vary with some timber showing light pinkish tones rather than the desired reddish brown. Kiln-dried Western Red Cedar is rare, and air-drying results in variance in moisture in the timber so it weathers unevenly, resulting in a patchy or even grubby-looking appearance. Both thermally treated Redwood and Clear Pine products offer the same rich colour as Western Red Cedar, but the treatment means that the timber will weather evenly, giving a better aesthetic effect with significant cost savings. As both timbers are sourced from Europe and are available with PEFC or FSC certification, there are clear sustainability benefits. The thermal treatment alters wood in such a way that timbers such as frake, which otherwise has little commercial value other than in plywood, are suitable for cladding. The enhanced durability of thermally treated frake and its availability in wide boards and long lengths, as well as visual factors like its dark vein, has made frake one of the more popular and high-end cladding options in the past year. With a host of benefits, easy installation and unbeatable environmental credentials, thermally modified timber will be part of the cladding landscape for a long time to come. Phil Barman is a timber specialist with Howarth Timber <<< Continued from page 49


Building Products April 2017
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