Comparative moisture uptake of Douglas fir and radiata pine structural lumber when exposed to rain wetting as an indicator of relative decay resistance
M E Hedley, G Durbin, L Wichmann-Hansen, L Knowles
Trials were undertaken to determine the relative resistance of radiata pine and Douglas-fir to wetting when exposed to the weather.
Douglas-fir samples were obtained from one Central North Island and three South Island sources and had a heartwood/sapwood mix typical for each resource. Radiata pine sapwood and heartwood samples were obtained from a Central North Island source. Material was exposed to the weather as horizontal studs in the first trial, and as horizontal and vertical studs in the second trial. The first trial ran over later winter from 29 July to 22 September 2003; the second, and more comprehensive investigation, from 22 October to 17 December 2003.
In the first trial, after seven days exposure, radiata pine reached a moisture content which would sustain decay (~27% mc), and remained well above that moisture content for the remaining 48 days of the trial. However, the maximum moisture content attained by Douglas-fir throughout the trial was only 21.8 % mc.
In the second trial, radiata pine sapwood rapidly attained a moisture content conducive to decay, and Douglas-fir did not. Because of the warmer and sunnier weather, fluctuations in moisture content were more pronounced than in the winter trial. Samples exposed horizontally attained higher moisture contents than those exposed vertically, irrespective of wood species or relative heartwood/sapwood content.
It is concluded that Douglas-fir timber shows significant positive differences from radiata pine in terms of susceptibility to moisture uptake. This trial confirmed the ‘refractory’ reputation of Douglas-fir, and the ‘absorbent’ reputation of radiata pine. At a practical level, Douglas-fir heartwood and sapwood can be regarded as equally impermeable, and independent of where in New Zealand it was grown.
Keywords: Wetting, decay resistance, Douglas fir, radiata pine