Radiata pine (Pinus radiata), a softwood, has been used in New
Zealand homes since the 1950s. The main disadvantage of radiata pine is that it
is a non-durable timber if it is not treated and will deteriorate very
rapidly when wet. Fungi grow rapidly and attack the timber, destroying its
structural integrity. This will occur in all external use, and where
there is a leak in the cladding it will occur internally too. Fungal
growth can also spread rapidly, transporting moisture to other areas causing
widespread failure.
From untreated to treated radiata pine As native timbers became in short supply in the 1950s, the first plantation forests of radiata pine became available. Initially untreated radiata pine was used, but it was found to be susceptible to borer attack, so it began to be treated with boron. This treated radiata gradually took over from rimu for house building in New Zealand.
Copper based treatments were developed for protecting radiata from moisture for use outside and for piles and poles. The most common of these treatments was Copper Chrome Arsenic (CCA). This treatment was not considered necessary for interior or general framing use, instead boron treated radiata continued to be used for this.
In the 1980s it was found that kiln drying the radiata at a higher temperature than usually used to dry it, gave some resistance to insects. Based on this, it was decided that high temperature kiln-dried (untreated) radiata could be used in place of boric treated timber in certain specified conditions where the timber could be kept dry and clear of the ground. This decision was entrenched in the New Zealand Standard 3602 Code of Practice for Specifying wood and wood based products for use in building in 1995.
Unsuitable timber and unsuitable cladding - a recipe for disaster
The introduction of untreated radiata pine framing also coincided with
the introduction of new construction materials and new fashions
in buildings such as the
fitting of insulation into timber frame cavities. This prevented ventilation of
the frames, and the effectiveness of waterproof coatings meant water couldn’t
escape, ensuring if the timber frame got wet it remained wet.
In April 2004, Acceptable Solution, B2/AS1, adopted the new amended
version of NZS 3602 dealing with the use of treated and untreated timber.
These amendments required the use of treated timber where there was any
risk of water getting into the timber frame. New housing has been required
to incorporate treated timber from this date. This has dramatically reduced
the effects of water that might get through the cladding.
All of the above means: houses constructed between 1995 and 2004/2005 may be
entirely built of untreated framing. (CB)