Timber iQ April - May 2019 // Issue: 43 | Page 35

FEATURES come from the vast Northern forests which span the globe across North America, Western Europe and Russia. The natural regrowth of these forests exceeds the annual harvesting rate and as such the timber being produced is highly sustainable. DURABILITY OF TIMBER CLADDING • Durability class 1: Very durable • Durability class 2: Durable • Durability class 3: Moderately durable • Durability class 4: Slightly durable • Durability class 5: Not durable Timbers rated as class 3 and better can be used untreated, as long as only heartwood is used. Sapwood is always considered as class 5 and should only be used if a preservative treatment is applied. The table below gives an indication of the expected service life for various timbers according to their durability class. RECOMMENDED DURABILITY CLASSES FOR OUT-OF-GROUND CONTACT USE: CONDITION DESIRED SERVICE LIFE 15 YEARS 30 YEARS 60 YEARS Coated 4 3 2 Uncoated 3 2 1 Source: TRADA Regardless of these guidelines, there are many examples of timber clad structures that have lasted centuries in harsh conditions without issues, including magnificent 13th century churches in Scandinavia and Russia, as well as traditional alpine dwellings. TYPES AND SPECIES OF TIMBER Rare Woods SA stocks a wide variety of timber suitable for cladding applications, and samples of all of these can be viewed in their showrooms in Cape Town and Knysna. In general, these fall into one of several categories: www.timberiq.co.za One of the key areas of focus when selecting or specifying timber cladding is its durability, which refers to the extent to the expected life-in-service that the timber provides before needing replacement. This can be influenced by numerous factors including the natural durability of the species, the use of preservative treatment or surface coating, or thermal or chemical modification of the timber. Commonly-used cladding species usually naturally durable, which means the timber can be left entirely maintenance-free, weathering to a mellow silver-grey patina that is widely considered as desirable. Timber species are rated in classes, according to their natural durability, as follows: Western Red Cedar was selected for the treehouse project by Malan Vorster Architecture Interior Design. SOFTWOODS Coniferous softwoods have always been the most popular choice for timber cladding. This is because they are often relatively inexpensive, widely available and have the benefit of lighter weight and fantastic sustainability. The undisputed king of softwoods for cladding is Western red cedar (Thuja plicata), which is world renowned for its fine warm grain, high strength-to-weight ratio, stability in service and Class 2 durability. Western red cedar has natural oils and resins that discourage fungal and insect activity and ensure its high durability. Sourced from Canada and the Pacific North West of the US, it is high priced relative to other softwoods, but is considered the superior option for the most discerning of clients. Other more affordable softwood options include Siberian larch (Larix siberica), Oregon pine / Douglas fir (Pseudotsuga menziesii) and Japanese cedar (Cryptomeria japonica), which are generally less than half the price of Western red cedar and each have significant strengths of their own. For softwoods, an important design consideration is the extent to which knots can be tolerated. For Western red cedar, the typical grading in South Africa is largely free of knots. Oregon pine is available in a range of grades. Other options like Siberian larch and Japanese cedar are usually ungraded and will typically be knottier. Staff at Rare Woods // APRIL / MAY 2019 33