FRAMING MOVEMENT

Douglas Fir wins new stiffness tests

For more than 70 years leading up to the leaky buildings crisis, Douglas fir was used as an untreated framing timber. But the subsequently amended New Zealand Building Code significantly restricts the use of untreated radiata pine and Douglas fir in domestic buildings – making no allowance for differences between the species.

Douglas fir suppliers argue it is more resistant to moisture uptake, more durable, stronger and stiffer than radiata pine. The latter distinction implies that even if Douglas fir framing decayed it would retain greater stiffness than radiata pine unaffected by decay. This is an important point as observable decay is the main criterion for replacing framing when a ‘leaky building’ is rehabilitated.

The New Zealand Douglas Fir Cooperative contracted Scion to run a three-year experiment to directly compare the loss of stiffness between radiata pine and Douglas fir under conditions promoting decay.

Results indicate some decay in untreated Douglas fi r does not cause the same amount of stiffness loss as it does in untreated radiata pine (see graph). Therefore, when leaks are rectifi ed and the framing timber dried the structural integrity of Douglas fir is retained.

Summary of test conditions:

• timber was allplaner gauged 100 x 90 x 45 mm
• samples had partially decayed feeder block ‘infected’ with brown rot attached at the centre of one 45 mm edge
• samples were immersed in water for two hours before installation
• every four to eight weeks samples were removed from tanks (in a partly shaded area), weighed, assessed for decay/mould and tested for deflection
• because moisture content of all samples exceeded fibre saturation (26% MC) after 12 weeks, no corrections for moisture differences were required.

The purpose of the comparative tests, conducted by Mick Hedley, Dave Page and Jackie van der Waals, was to provide information to allow a possible review of the status of Douglas fir framing in the New Zealand Building Code – particularly buildings where it can be used untreated or preservative (boron) treated.

A Scion spokesperson told in-wood that Douglas fir (particularly from South Island sawmillers) could gain a signifi cant commercial benefit if it could be used untreated in more diverse framing situations – as had once been the case.

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