Report: Pathways to Building Code compliance for farm-totara timber
Sustainable Forest Solutions
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.
Hazard class is defined in NZS 3640:2003 Chemical Preservation of Round and Sawn Timber as:
Describes an environment or condition categorized environments or conditions of use where timber is at particular risk of biodegradation by one or more biological agents (e.g. fungi, insects, bacteria or marine organisms).
Hazard class specifications are described in NZS 3640:2003 as:
- H1.2 applies to timber used in situations protected from the weather, but where there is a risk of moisture content conducive to decay. (6.1)
- H3.1 applies to timber used in situations above ground, exposed to the weather – generally in non- structural applications, but including cladding used as bracing if the component is painted.(6.3)
- H3.2 applies to timber used in situations above ground, exposed to weather, or protected from the weather but with a risk of moisture entrapment. This classification is for more critical end uses and includes exposed joists and decking.(6.3)
Although Hazard Class indicates level of durability, Hazard Classes relate to the level of chemical treatment required under the Building Code, as described in NZS 3640. There appears to be the expectation that each hazard class meets or exceeds the default minimum service lives for timber components but it not clear whether component's Hazard Classes offer a benchmark or threshold for in service life under the laboratory testing Verification Method described in Clause B2VM1 of the Building Code. Defining durability with Hazard Classes does not create a level playing field between Hazard Class treated timber and naturally durable timber unless the Hazard class meets but does not exceed the service life requirement for the timber component.
In situations such as where structural timber is subject to moisture ingress, the H1.2 Hazard Class does not meet the minimum service life of 50 years. This situation is therefore an exception to the requirement of 50 years service life and could be described as an extreme decay hazard, defined in NZS 3640 as:
Where the decay development is more rapid than in other ground contact situations. This can be due to soil type and conditions, accumulation of nutrients in soil, or horticultural practices such as mulching or composting affecting biological activity in the soil.
Light organic solvent preservatives (LOSP) treatments are not included in the H1.2 treatment category, with boron the only option for H1.2, although future amendments to NZS 3640 might include other options such as water-based azoles (Build 124).
To meet the requirements of H1.2 treatment, boron-based treatments with average cross-section concentrations of 0.4% mass/mass boric acid equivalent (BAE) or above are considered adequate to cope with fungal and insect risks for normal framing use and construction practices in New Zealand. H1.2 treatments offer ‘opportunity time’ for construction and detecting and repairing leaks throughout a building’s life. No treatment is capable of permanently protecting framing that remains continuously wet (Build 124).
Boron has been used commercially as a timber preservative in New Zealand since the 1950s, with no known health issues for timber users or building inhabitants (Build 124).
Current methods in use for boron impregnation have been developed for radiata pine, but it is not known how well boron would penetrate farm-totara timber to H1.2 requirements using these methods.
Hazard classes and permitted end uses for radiata pine (from Weathertight)
|Table 2: Guide to treated radiata pine applications.|
|Timber to be used for||Required treatment||Timber to be used for||Required treatment|
|External timber use|
|enclosed subfloor framing||H1.2||exposed subfloor framing||H3.2|
|veranda posts supported clear of ground||H3.2||veranda posts in ground||H5|
|deck jack studs supported clear of ground||H3.2||deck piles in ground||H5|
|deck joists/bearers||H3.2||wall framing weather exposed||H3.2|
|decking||H3.2||roof framing weather exposed||H3.2|
|cladding or exterior trims unpainted, clear finished or stained||H3.2||shingles/shakes||H3.2|
|cladding or exterior trims painted||H3.1||exterior plywood unpainted or used as bracing||H3 CCA|
|fence rails and palings||H3.2||exterior plywood painted||H3 LOSP|
|fence posts||H4||balcony barrier exposed||H3.2|
|Framing timbers (1, 2)|
|external wall framing masonry veneer cladding||H1.2||external wall framing E2/AS1 20 mm cavity cladding||H1.2|
|balcony wall framing enclosed||H1.2||cavity battens||H3.1|
|parapet framing||H1.2||interior wall framing||H1.2|
|roof framing – low slope/skillion||H1.2||enclosed cantilevered floor joists||H3.2|
|roof farming – roof space||H1.2||roof sarking timber||H1.2|
|roof sarking plywood membrane roof||H3 CCA|
|window reveals to aluminium windows||H3.1||furniture||untreated|
|Note (1)||Douglas fir may be used untreated on low-risk design buildings as defined in Amendment 7 to B2/AS1.|
|Note (2)||H1.2 boric-treated Douglas fir may be used in all framing applications where H1.2 boric-treated radiata pine is permitted.|
Preservative treatment options (from Weathertight)
Table 3 summarises the preservative type, the hazard class that can be achieved, the identification numbers and colours.
|Table 3: Preservative type and hazard class identification.|
|Generic type||Chemicals/fungicide||Identification number||Colour coding and branding for framing||Applicable hazard classes|
|Copper-based||CCA oxide (copper 23–25%, chromium 38–45%, arsenic 30–37%)||01||All|
|CCA salt (copper 23–25%, chromium 38–45%, arsenic 30–37%)||02||All|
|Copper quaternary (copper 56–67%, DDAX 33–44%)||90||H3.1 (1), H3.2, H4, H5|
|Micronized copper quaternary (4) (copper 56–67%, DDAX 33–44%)||89||H3.1 (1), H3.2, H4, H5|
|Copper azole (copper 95.8–96.4%, azole 4.2–3.6%)||58||H3.1 (1), H3.2, H4, H5|
|Micronized copper azole (4) (copper 95.8–96.4%, azole 4.2–3.6%)||88||H3.1 (1), H3.2, H4, H5|
|Boric or boron||Boron salts (0.4% retention) boric acid equivalent (BAE)||11||H1.2 pink (1) end or face mark that is a permanent ink mark, an incised mark, a burnt mark or a plastic tag stapled to the timber – every stick of timber must be marked||H1.2|
|Boron salts (0.8% retention) (2) (BAE)||11||H3.1 (edge of face branded)||H3.1|
|LOSP||CuN (copper naphthenate) (3)||57||H3.1 no added colour||H1.2, H3.1, H3.2|
|TBTO (tri-n-butyltin oxide) (3)||56||H3.1 no added colour or green||H3.1|
|TBTN (tri-n-butyltin naphthenate) (3)||62||H3.1 no added colour or green||H3.1|
|Propiconazole + tebuconazole||64||H3.1 no added colour or green||H3.1|
|Permethrin (insecticide only)||70||H1.1|
|Aqueous azoles||propiconazole + tebuconazole + permethrin||64||H1.2 green end or face branded||H1.2|
|propiconazole + tebuconazole + permethrin||64||H3.1 green end or face branded (3)||H3.1|
|Note (1)||B2/AS1 Amendment 7 has a minimum requirement of using H1.2 timber that is boric treated for enclosed framing. Treatment of framing to cantilevered floor joists and associated framing is required to be at least H3.2 and cavity battens at least H3.1. H3.1 LOSP or water-based azole treatments are not permitted for timber framing, but water-based azole at higher retention (0.04% propiconazole and 0.04% tebuconazole) is approved for framing.|
|Note (2)||H3.1 boric-treated cavity battens and external finishing timbers are required by Amendment 7 to B2/AS1 to be primed before dispatch and to have a specified type of paint coating.|
|Note (3)||B2/AS1 Amendment 7 does not allow the use of LOSP-treated timber for framing.|
|Note (4)||Micronised copper is a copper compound ground into particles that are 0.005–10 microns in size and suspended in water with the aid of a dispersant.|
From Consultation on proposals for a single hazard class for framing timber inside the building envelope (Department of Building and Housing 2010):
- Decay trials conducted by Scion demonstrate that the LOSP azole (propiconazole: tebuconazole 1:1) at a retention of 0.06% m/m as total azole, is suitable for the single hazard class. This LOSP treatment has not been proven at lower retentions.
- Decay trials conducted by Scion demonstrate that the current H1.2 LOSP iodocarb (IPBC) treatment does not provide the level of extended, longer-term durability required for the proposed new single hazard class.
- Tin-based LOSP treatments included in the current H1.2 and H3.1 hazard classes have already been voluntarily withdrawn from the framing market by the timber industry. These treatments, while effective in preventing decay, would not be acceptable for use as framing because of health, safety and environmental concerns and are specifically excluded.
Conclusion: Based on this evidence, the Department considers that the LOSP azole treatment at a retention of 0.06% m/m total azole provides protection equivalent to that of the boron treatment of 0.4 % BAE all elements (excepting cantilevered deck joists) within the building envelope.
Brush-on treatment of boron glycol (with a minimum concentration of 20% BAE applied in two brush coats approximately a half hour apart) to three or four faces of 90 x 45 mm radiata framing gave effective protection against brown rot decay where exposed to wet conditions for 3 years. Some boron preservative was lost through wetting, but the preservative was redistributed through the sample. Retention of boron if coated on three or four sides was generally above the minimum 0.4% boric acid equivalent (BAE) requirement for H1.2. (October 2013, Build 138)
References for boron impregnation of farm-totara are not available as this has not been researched to date. However, a range of treatment options are available for H1.2 boron treatment of timber, including the traditional process of dip diffusion.
Traditionally boron is applied to green sawn timber under a dip-diffusion process followed by block stacking. The treatment is allowed to diffuse for sufficient time until full penetration of sapwood has occurred (usually between two and eight weeks) before seasoning the timber. Contemporary methods for treating radiata pine require treatment of air dried timber and in a short time frame, such as pressure-diffusion or pressure treatment.
In order to assess boron penetration and retention under a contemporary pressure treatment regime, tests were undertaken at a commercial radiata treatment plant in Northland to evaluate whether standard procedure for boron-treating radiata pine would be acceptable for H1.2 treatment of farm-totara. North Sawn Lumber tested samples of timber using the fluctuation pressure process used for boron treatment of dry radiata pine. Results are available here ».