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