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Report: Pathways to Building Code compliance for farm-totara timber

June 2016
Dean Satchell
Sustainable Forest Solutions

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Douglas fir and durability

The author wishes to acknowledge the support from the organisations and individuals that have enabled this project to be undertaken. These include; the Ministry for Primary Industries (through the Sustainable Farming Fund), Tane’s Tree Trust and the New Zealand Farm-Forestry Association, (including the Indigenous Forestry Section), the Northland Totara Working Group members Paul Quinlan and David Bergin for assistance in finalising this report, Michael Hayes & Geoff Cookson, for their time and donated sample boards of timber for testing, and saw-miller, Shane Hyde, for his time and the use of his band-saw in re-sizing some of the timber samples. The time and input from Robin Curtis, CEO of NZ Sustainable Forest Products, is also acknowledged and much appreciated.
In producing this report, reasonable care has been taken regarding the accuracy of the information presented. However, no guarantee as to the truth, accuracy or validity of any of the comments, implications, recommendations, findings or conclusions are made by the author, the Northland Totara Working Group, Tane’s Tree Trust, or any other party. Therefore, neither the authors, nor any of the supporting organisations, shall not be liable for, or accept any responsibility for, any loss, damage or liability incurred as a result of direct or indirect result of any reliance by any person upon information or opinions or recommendations expressed in this work. Users of any of this information, whether contained or inferred, in or arising from this report do so at their own risk.

Untreated Douglas fir (including sapwood) is allowed for structural applications under the New Zealand Building Code under certain circumstances. This is explained below and may offer opportunities for untreated farm-totara to be used under similar circumstances if durability preformance could be demonstrated to be equivalent or better.

Exceptions for the use of untreated Douglas fir (Clause B2/AS1)

Preservative-free untreated solid Douglas fir framing may be used for roof members protected from the weather, floor members protected from the weather and not exposed to ground atmosphere, and for internal and external wall framing protected from the weather provided that the building meets all of the following requirements:

  • Is a stand alone, single household unit of no more than two storeys (as defined in NZS 3604) that is designed and constructed to NZS 3604.
  • Is situated in wind zones no greater than ‘high’ as defined in NZS 3604.
  • Has a building envelope complexity no greater than ‘medium risk’ and a deck design no greater than ‘low risk’ as defined by the risk matrix in the Acceptable Solution E2/AS1.
  • Has drained and vented cavities complying with E2/AS1 behind all claddings.
  • Uses roof and wall cladding systems and details meeting E2/AS1.
  • Has a risk matrix score of no more than 6 on any external wall face, as defined in E2/AS1.
  • Has a simple pitched roof with hips, valleys, gables or monopitches, all draining directly to external gutters.*
  • Has a roof slope of 10° or more.
  • If it has a skillion roof, the roofing material is corrugated iron or concrete, metal or clay tiles for adequate ventilation.
  • Has eaves 450 mm wide or more for single-storey houses and eaves 600 mm wide or more for 2-storey houses.

Summary of key issues from the 2010 consultation on Timber treatment changes

From summary, published March 2011 (Building Performance, MBIE)

About 75% supported the use of untreated Douglas fir for houses of low-risk design. Three key points were raised in consultation:

1. Whether untreated Douglas fir is fit for purpose.

The science and expert opinion support the use of untreated Douglas fir for low-risk buildings. The Acceptable Solution therefore allows the use of untreated Douglas fir for houses of defined low-risk design.

2. Whether allowing untreated Douglas fir creates unnecessary complexity.

The Department sought to balance simplicity, risk and the provision of a chemical-free option for consumers. Douglas fir is therefore allowed for houses of defined low-risk design.

3. Whether wider use of untreated Douglas fir should be allowed.

Given the support for simplification, the risk to internal framing from internal wet areas, and the risk of transferred moisture from external walls, the Acceptable Solution does not provide for untreated roof framing or internal framing in other than low-risk houses.

The untreated Douglas fir option does not apply to commercial buildings, because some uses for commercial buildings may have a higher moisture risk.


Background leading to the "Untreated Douglas fir exception"

There are a number of relevant articles available and information that contributed to allowing untreated Douglas fir in B2 Acceptable Solution 1:

Responding to moisture: How do Douglas fir and radiata compare?
New Zealand Tree Grower August 2007

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.

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.

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.

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.

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.

Successful durability tests for Douglas fir
New Zealand Tree Grower August 2008

Friday offcuts - 17 July 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.

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.

Douglas Fir is back - 8th April 2011

"Douglas fir – the chemical free option". Untreated Douglas fir can be used for all the framing in traditional low risk houses (e.g., houses that are one or two storeys, and have eaves and cavity walls).

DBH confined the used of Douglas fir to low risk designs primarily for reasons of simplicity. This means that alternative solutions will be needed to use untreated Douglas fir in medium and high risk buildings and in commercial buildings. Untreated Radiata is no longer allowed.

The “Alternative Solution”
For Untreated Douglas-fir (New Zealand Oregon) in Building Construction.
This document aims to provide adequate documentation to satisfy the requirements of Territorial Authorities in approving an “Alternative Solution” for Douglas-fir (New Zealand Oregon) in building construction under the verification method B2/VM1. This documentation will be progressively upgraded to strengthen the case for New Zealand Oregon (NZO) for building construction as further research results and endorsements are obtained.


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