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THE INTERNATIONAL RESEARCH GROUP ON WOOD PRESERVATION
Section 2 Test methodology and Assessment
A comparison of rates of decay and loss in stiffness of radiata pine and
Douglas fir framing lumber
Mick Hedley, Dave Page and Jackie van der Waals
Scion Wood Processing, Private Bag 3020, Rotorua, New Zealand
SE-114 86 Stockholm
A comparison of rates of decay and loss in stiffness of radiata pine and
Douglas fir framing lumber
Mick Hedley ([email protected]
), Dave Page ([email protected]
and Jackie van der Waals ([email protected]
Scion Wood Processing, Rotorua, New Zealand
Stiffness loss with time was recorded for untreated radiata pine and Douglas fir framing size
lumber and preservative treated radiata pine which dad been pre-inoculated with Oligoporus
a brown rot decay fungus isolated from decaying untreated radiata pine framing.
Between stiffness measurements, samples were contained in a plastic tank located outside at
ambient temperature. Index of Condition (sensu
AWPA Standard E7-93) was assessed for all
samples at the time stiffness testing was undertaken.
Substantial loss of Index of Condition was recorded for all untreated samples before there was
any significant loss in stiffness. Decreasing order of stiffness loss (and weeks to first measurable
loss) was: untreated radiata pine sapwood (24), radiata pine heartwood (48), Douglas fir
sapwood (65), Douglas fir heartwood >106), treated radiata pine sapwood (>106). The results
indicate that for untreated Douglas fir in particular, the presence of observable decay - the main
criterion for replacement of framing when a "leaky building" is rehabilitated - may not truly
reflect residual stiffness, which would be retained when leaks were rectified and the framing
: radiata pine, Douglas fir, framing lumber, decay, stiffness
Douglas fir (Pseudotsuga menziesii
), which comprises around 5% of the New Zealand exotic
timber plantation, has been used untreated as framing timber for over 70 years. A high
proportion of this framing lumber, particularly in the South Island, has been cut from thinnings
and thus contained a substantial amount of sapwood. There were no restrictions on its use within
Following the "Leaky Building Crisis" in New Zealand which came to prominence in 2000 and
which revealed widespread decay in untreated framing due to leaks in the building envelope,
Codes and Standards which control use of timber in construction were revised. One of the major
revisions was to restrict considerably the use of untreated radiata pine and Douglas fir in
domestic buildings, in preference to a return of preservative treated framing. Their use untreated
in exterior walls was limited to single storey dwellings clad with masonry or brick veneer.
Treatment to Hazard Class H1.2 (NZS 3640:2003) was required if they were used with any other
cladding systems, such as weatherboards or EIFS.
This drew protestations from suppliers of Douglas fir framing. They argued that before the
introduction of untreated pine framing in 1997, radiata pine framing was always required to
preservative treated, although emphasis was on prevention of insect attack rather than decay,
whereas Douglas fir framing was not, irrespective of cladding system. They also argued that
most Douglas fir framing was used in "traditional" dwellings, whereas as leak problems were
mostly associated with condominium type developments, using complex Mediterranean style of
design and clad with monolithic claddings.
The producers' contention was that Douglas fir was more resistant to moisture uptake, it was
more durable than radiata pine and was stronger and stiffer. This latter distinction implied that in
the unlikely event that Douglas fir framing became decayed, it would still retain greater stiffness
than undecayed radiata pine. Although it has been shown that it is more resistant to rain-wetting
(Hedley et al.
, 2004), there was no comparative information on rates of decay of radiata pine and
Douglas fir and the effect on loss of stiffness.
MATERIALS AND METHODS
The five different treatment groups of 20 boards each were included in the trial:
Radiata pine, untreated, kiln dried sapwood.
Radiata pine, untreated, kiln dried heartwood.
Radiata pine, kiln dried H1.2 LOSP (IPBC + permethrin) treated.
Douglas fir, kiln dried untreated “sapwood”.
Douglas fir, kiln dried untreated heartwood.
The timber was all planer gauged 1000 x 90 x 45mm. Douglas fir “sapwood” samples were cut
from timber containing a high proportion of sapwood, but heartwood could not be entirely
eliminated and ranged from 0 to 75% within samples with an average of 35%. Samples had a
partially decayed feeder block, 70 x 35 x 7 mm, infected with Oligoporus placenta
attached at the centre of one 45 mm edge.
Prior to installation samples were immersed in water for two hours. They were then randomly
placed on edge, with the feeder blocks facing upward, in 1.0 m (long) x 0.8m (wide) x 0.6 m
(deep) plastic tanks (Fig 1). Layers of samples were separated by 10 x 10 mm plastic stickers.
There was water in the tanks about 20mm deep. The tanks were placed on a level site at the
Scion campus, in an area partly shaded by large trees.
Fig 1. Arrangement of samples in the exposure tank
At intervals of between 4 and 8 weeks samples were removed from the tank, weighed, assessed
for decay and mould and tested for deflection as a plank in a static bending test. Deflection
caused by a central 80 kg load was measured with a dial gauge set against the bottom face of the
sample immediately below the load. Samples were then returned to their original position in the
The surface of the samples under the decay mycelium was tested with a blunt probe to determine
whether the decay fungi were damaging the framing. The decay rating system used was similar
to AWPA Standard E7-93 (AWPA, 1999), although it specifically applied to the area on each
9 = First stages of decay or damage up to 3% of cross-section.
8 = Lightly established decay, 3-10% of cross-section.
7 = Well established decay, 10-30% of cross section.
6 = Deep established decay, 30-50% of cross section.
4 = Severe decay, nearing failure, more than 50% of the cross section.
RESULTS AND DISCUSSION
Moisture content calculations were based on sample weight (Fig 2.). Weight is reduced by
significant decay and this will have made moisture content calculations inaccurate, particularly
in the untreated radiata pine and latterly in the Douglas fir sapwood groups. They were taken
mainly to ensure that moisture contents were suitable for decay throughout the duration of the
oisture content %
Fig 2 Mean moisture content of test samples
Decay (Index of Condition) (Fig 3)
The first sample failed in the untreated radiata pine sapwood group after 20 weeks and the two
that remained after 105 weeks were in poor condition. The first radiata pine heartwood sample
failed after 48 weeks and 12 had failed at 105 weeks and there have been two failures in the
Douglas fir heartwood group. There have been no failures in the treated radiata pine group
Index of C
Fig 3 Index of Condition
From Fig 4 it is seen that the initial deflections for both Douglas fir sapwood and heartwood
were approximately half that of the pine samples. The test rig was set up in such a manner that a
deflection of 10mm occurred when the sample broke.
Untreated radiata pine sapwood decayed rapidly, although it wasn't until the mean Index of
Condition fell below 6 that deflection under load exceeded that at the start. Untreated radiata
pine heartwood had greater decay resistance and increases in mean deflection occurred more
Even though obvious decay was occurring in Douglas fir, where Indices of Condition were
between 6 and 8 after 64 weeks, no loss in stiffness was recorded in that time. Significant loss in
stiffness in Douglas fir sapwood only commenced at week 90 when Index of Condition was ~6.
Neither Douglas fir heartwood nor H1.2 treated radiata pine showed any loss of stiffness during
Fig 4. Deflection against time
Decreasing order of stiffness loss (and weeks to first measurable loss) was: untreated radiata pine
sapwood (24), radiata pine heartwood (48), Douglas fir sapwood (65), Douglas fir heartwood
>106), treated radiata pine sapwood (>106). The results indicate that for untreated Douglas fir in
particular, the presence of observable decay - the main criterion for replacement of framing when
a "leaky building" is rehabilitated - may not truly reflect residual stiffness, which would be
retained when leaks were rectified and the framing dried. Remedial treatments of the Boracol®
type could be applied as a very cost effective alternative to replacing the partly decayed framing.
AWPA. 1999. E7-93 Standard method for evaluating wood preservatives by field tests with
stakes. American Wood-Preservers' Association Book of Standards.
Hedley, M.E, Durbin, G.D., Wichmann-Hansen, L. and Knowles, L. 2004. Comparative
moisture uptake of Douglas fir and radiata pine structural lumber when exposed to rain wetting
as an indicator of relative decay resistance. IRG/WP 04-20285
NZS 3640:2003 Chemical Preservation of sawn and round timber. Standards New Zealand,
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