Responding to moisture: How do Douglas fir and radiata compare?
New Zealand Tree
Grower August 2007
Three research projects undertaken for the Douglas fir Cooperative have
set out to quantify the differences between Douglas fir and radiata
pine in terms of the responses to moisture by the two species.
Untreated Douglas fir and untreated radiata pine are assumed to be at
similar risk of decay when used in the same structural situations.
Neither is approved for use in the untreated state as framing for
exterior walls except in buildings at a low risk of inadvertent
moisture ingress. The first study was set up to determine the relative
resistance to wetting of radiata pine and Douglas fir structural timber
when both were exposed to the weather.
Difficult to impregnate with water
It is well known that Douglas fir is a refractory species and its
timber is difficult to impregnate with water, even under pressure.
Radiata pine sapwood on the other hand, is much more permeable to
water. Radiata pine heartwood has more variable permeability, some
being as permeable as sapwood, some being almost as refractory as
Douglas fir.
Previous research has shown that a minimum wood moisture content of 27%
is necessary for decay to be initiated in radiata pine sapwood when it
is in contact with decaying wood. The conservative assumption was made
that the minimum moisture for decay initiation is the same for Douglas
fir sapwood and heartwood and for radiata pine heartwood. However, once
initiated, the rate of decay would be less in most examples of radiata
pine heartwood and even less in Douglas fir heartwood because of their
comparative and greater natural durability than sapwood.
Research trials
Co-operative Report No 36 describes two trials using Douglas fir
samples from one central North Island and three South Island sources,
with a heartwood and sapwood mix typical for each resource. Radiata
pine sapwood and heartwood samples were obtained from a central North
Island source. This 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
The first trial ran over later winter 2003, the second more
comprehensive investigation was from October to December 2003. Although
both timbers would differ little in susceptibility to decay if they
attained the same moisture content, there would be significant
differences in resistance to moisture uptake if both were exposed to
the same wetting regimes, such as that represented by
rainfall. Being the more permeable, radiata pine would be expected to
attain a moisture content suitable for decay much more readily than
Douglas fir.
In the first trial, after seven days exposure, radiata pine reached a
moisture content of 27% which would sustain decay, 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%. 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 during
this trial, 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.
Conclusions from the trial were that Douglas fir is more difficult to
wet than radiata pine, and under the conditions of the test, failed to
reach a moisture content where there would be a risk of decay if it was
in contact with decaying wood. Conversely, radiata pine reached this
moisture content after just six days exposure to rainfall and never
went below that moisture content for the remainder of the trial.
More detailed investigation
Following this preliminary trial, the Douglas fir Research Co-
operative commissioned a more detailed investigation to assess the
moisture absorption characteristics of Douglas fir from South Island
sources and to compare this with that of Douglas fir from the initial
trial. This trial also aimed to establish whether there were any
significant differences in moisture absorption between Douglas fir
sapwood and heartwood relative to that of radiata pine sapwood and
heartwood.
It was 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. Independent of where in New Zealand it was grown, all New
Zealand grown Douglas fir timber can be similarly classified with
respect to moisture uptake, regardless of its origin.
Wood under water
A subsequent study was set up to determine whether there was any
fundamental difference in the moisture uptake between Douglas fir and
radiata pine framing timber when they were submerged in water.
This project was set up following concerns about the moisture uptake of
house framing, and the introduction of the requirement to use treated
framing for external walls. Douglas fir growers and processors were not
convinced that this requirement for treatment of external walls for
Douglas fir framing is necessary. The Building Industry Authority has
stated that information showing the fundamental differences in relative
moisture uptake of Douglas fir and radiata pine is needed before timber
of the two species can be differentiated in the
New Zealand building code.
Water uptake of Douglas fir and radiata pine samples was compared.
Short blocks of 100 mm by 50 mm by 25 mm, and long samples 1.2 metres
in length, were selected to demonstrate the differences between grain
orientations of both sapwood and heartwood samples. This was a tougher
test compared with the earlier work, where under intermittent rain
wetting conditions, the
Douglas fir did not exceed the 27% threshold.
The blocks were submersed in water and the weight change recorded at
specified intervals over four days. At the end of the 96-hour
monitoring period, all samples of both species attained a moisture
content above the 27% threshold moisture content for decay. However the
water uptake rate was slower for Douglas fir samples. For
the long samples both samples of radiata pine reached 27% moisture
content after 15 hours, Douglas fir sapwood took 48 hours and the
Douglas fir heartwood 96 hours. Uptake was greatest in the longitudinal
direction, followed by tangential then radial.
At the completion of the study, radiata pine long samples averaged 50%
moisture compared to 30% for the Douglas fir.
Stability study
A third study was to compare the stability between New Zealand grown
Douglas fir and radiata pine structural timber when subjected to
moisture cycling. There is a perception in the building trade that
Douglas fir framing remains more stable during construction both while
drying to equilibrium moisture content, if erected green, or when
erected dry, than kiln dried radiata pine.
Stud-length samples of radiata pine and Douglas fir were subjected to a
number of wetting and drying cycles. Samples of Douglas fir, both green
and dry, and kiln-dried radiata pine, were obtained from one central
North Island and two South Island sources representing a typical mix of
frame grade for each resource.
Each stud sample was weighed and then scanned for any initial
distortion using the Ensis Warpmaster. Samples were then positioned
individually in a rack so they were free to move while equilibrating,
and in the case of the green studs while air drying. When the green
Douglas fir samples reached approximately 15% moisture content, all
samples were re-measured for weight and distortion. Samples were then
given a one hour soak in water to simulate rain wetting and re-measured
for weight and distortion. This sequence was repeated a second time
after re-drying the samples to approximately 15%.
Refractory reputation confirmed
The trial confirmed the refractory reputation of Douglas fir, and the
absorbent reputation of radiata pine. During the wetting and drying
cycles, the radiata pine absorbed three to four times more water than
the Douglas fir. The soaking period confirmed previous findings that at
a practical level, Douglas fir heartwood and sapwood can be regarded as
equally impermeable, independent of where it is grown in New Zealand.
Increases in timber warp during the course of the trial were greater in
green Douglas fir than in either dried Douglas
fir or kiln dried radiata pine. The increased warp for green Douglas
fir occurred during the initial two month period of drying rather than
in the subsequent wetting and drying cycles.
As a result of repeated wetting and drying cycles, the greater
permeability and water uptake of radiata pine led to greater timber
movement and warp when compared with Douglas fir. However, the
difference in level of movement between the two species induced by
these cycles was small, being generally less than one millimetre per
stud length.
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