Radiata pine has been used in New Zealand homes since the 1950s. We look at when untreated radiata might have been used in homes and the effects of moisture on treated and untreated timber.

Radiata pine

Radiata pine is a softwood timber that is a native of California, but is extensively grown in plantation forests in New Zealand. It is fast growing and a sustainable source of timber. Its advantages include that it can be:

• Silvi-cultured.
• Grown with consistent strength and physical characteristics.
• Used to make processed components.
• Chemically treated.

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.

Radiata pine use in New Zealand

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.

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.

Houses constructed between 1995 and 2004 may be entirely built of untreated framing.

Effects of water on timber

Almost all building materials deteriorate when they are exposed to moisture over time. Moisture causes fungal decay and mould in timber. Also, chemicals from corroded metal fixings can cause damage to timber.

Rots
There are three broad classifications of fungal decay or rot:

• brown rots
• white rots
• soft rots

All rot is caused by fungi, spread by airborne spores. Rots require food, moisture, oxygen and warmth. Spore germination needs the presence of free water.

Brown rots are the most common in wet framing. They tend to darken the appearance of the timber as they consume the cellulose[define] only, leaving behind the darker lignin[define]. In its early stages the rotted timber may not change much in appearance but will be easily penetrated with a knife. Brown rots are considered to be more ‘malignant’ than other rots because they decay timber more rapidly. One particularly malignant brown rot is dry rot. It has the ability to transport moisture and is very destructive once established.

White rots appear to bleach the timber as they consume both the lignin and cellulose. They are less prevalent in softwoods such as radiata pine.

Soft rots leave very little indication of their presence. The decay takes place within the cell wall and will generally occur when the timber is in contact with the ground.

A damp wall cavity can form an ideal environment for fungal growth. Typical fungi are Penicillin, Aspergillus and Stachybotrys.

Moulds
Moulds are a non-rotting fungi that usually consume sugars in wood without causing decay. The moulds we are most familiar with grow on the interior surfaces of walls and ceilings. They are associated with poor ventilation and heating or excessive interior moisture levels. These kinds of moulds are easily treated with diluted bleach.
  
Metal corrosion
Water can also cause metal corrosion. Moisture will rust steel fixings and the chemical by-products of corrosion can attack timber, weakening the area around the fixing. This is known as nail-sickness. Where copper treatments are used, galvanised nails will lose their zinc protection due to the interaction of the copper, zinc and moisture.