A history of timber treatment

Timber treatment became a major building issue in the late 1990s due to the “Leaky Homes” issues.

Originally timber used in house framing or components was used untreated. This was not a huge problem for native timbers such as rimu or matai as these timbers have some natural protection against deterioration by borer or fungus, etc. As long as the timber was kept at a low moisture level, ie, below 18%. Borer requires high humidity and some dampness to establish itself in untreated timber.

Pine, when not treated, is readily attacked by mould and fungus if damp and quickly deteriorates and loses its strength. Treatment of pine with boron salts was introduced in 1952, and this gave pine some useful protection. Pine is not affected by borer due to the pine resins.

Councils could allow the use of non-treated pine as an Alternative Solution from September 1995, as long as it was kiln dried and had a moisture content under 18%. This was fully incorporated into the Building Code in February 1998.

These of untreated timber was okay as long as the pine did not get wet. Then it could cause major structural issues, and this was a major contributor to the “Leaky Homes” rectification costs.

It took a few years for the problems of untreated timber to be noticed and for changes to be made to the timber requirements. Treated timber was made compulsory once again in all building work from April 2004. The buildings under construction needed to be completed by April 2005.

The current situation of timber treatment was required from April 2011.

(H stands for the level of required timber treatment for a particular Hazard Class.)

H 1.2 treated. All enclosed wall, roof, and sub-floor framing.

H 3.2 treated. Any weather-exposed decking, weatherboards, or exterior plywood.

H 5 treated. House piles, retaining wall poles.

Some instances of untreated Douglas fir are allowed on low-risk buildings where weather protected.

These dates are important as it is possible that untreated timber was used in house construction in Nelson/Tasman during the years 1995 to 2005, although the time frame is more likely to be from 1998 to 2004. Unfortunately, it is not possible to check this in the crucial places like bottom plates and studs as these are well covered up by plasterboard or exterior wall cladding. Looking at the building specification can help, but there is no guarantee that what was specified was actually installed.

Keeping water out of the framing is important to a house’s integrity in the long term.

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