Douglas-fir vs radiata pine for framing lumber

Friday 17 Jul 2009

 
Douglas-fir has been used as an untreated framing timber for over 70 years. That changed following the Leaky Building Crisis and the resulting revision of the New Zealand Building Code. The amended Building Code significantly restricts the use of untreated radiata pine and Douglas-fir in domestic buildings, making no allowance for differences between the species. Scion has recently completed a study that could prompt a review of this ruling.

Suppliers of Douglas-fir argue that Douglas-fir is more resistant to moisture uptake, more durable, and is stronger and stiffer than radiata pine. This latter distinction implies that even if Douglas-fir framing became decayed it would still retain greater stiffness than un-decayed radiata pine. This is an important point as observable decay is the main criterion for replacing framing when a "leaky building" is repaired.

Scion was contracted to design and run a three-year experiment to directly compare the loss of stiffness between radiata pine and Douglas-fir in leaky buildings. Indications are that some decay in untreated Douglas-fir does not cause the same amount of stiffness loss as it does in untreated radiata pine. Therefore when leaks are rectified and the framing timber dried, the structural integrity of Douglas-fir would be retained.

The purpose of these comparative tests was to provide information to allow a possible review of the status of Douglas-fir framing in the New Zealand Building Code, as regards situations in buildings where it can be used untreated or preservative (boron) treated.


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