Report: Pathways to Building Code compliance for farm-totara timber
Sustainable Forest Solutions
Clause B2: 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.
The durability requirements for structural timber used in buildings are provided in clause 2 of the New Zealand Building Code.
NEW ZEALAND BUILDING CODE clause B2 covers the Acceptable Solutions (B2/AS1) and Verification Methods (B2/VM1) used to establish compliance with the durability requirements of the Building Code. The most recent edition is Amendment 8, issued on 14 August 2014.1
Verification method B2/VM1 gives options that may be accepted by the consenting authority. These include1:
- verifiable in-service history for known systems or materials – known length of service in the same environment, frequency of use without adverse results or other results that demonstrate the required durability
- laboratory testing, verifying degradation rates, testing relevance to the situation of proposed use and durability period, methods of testing and so on
- similar material comparisons – demonstrating similarity in composition, use in the same environment, required maintenance, conditions of use and so on. Where such a direct comparison is not possible, the building element shall be independently assessed to determine the degree of similarity.
Acceptable solutions from standards such as NZS 3602 and NZS 3640 that relate to timber durability are referenced by B2/AS11
B2/AS1 Table 1 (page 17) gives the nominated durability of building elements to comply with the Acceptable Solution. Structural elements like floors, walls, bracing or structural fixings, items difficult to access or replace or building elements where failure would go undetected during normal maintenance of the building are required to have a life of not less than 50 years (i.e. the life of the building).1
Radiata pine and Douglas fir framing - the current situation
To achieve a life of not less than 50 years as required for structural applications under the Building Code, radiata pine or Douglas fir treatment levels are now:
- a minimum treatment of H1.2 for radiata pine and Douglas fir solid timber framing
- remove the option of H3.1 radiata pine as a structural or framing timber
- permit H3.1 (LOSP azoles as required in NZS 3640 Table 6.2) treatment for LVL pine to satisfy the requirement of H1.2 as a minimum treatment for radiata pine and Douglas fir.
Timber framing under Clause B2 Acceptable Solution 1 B2/AS1 allowed a single hazard class for treatment of H1.2 as of 4 April 2011 (See Build 124). Since then H1.2 treatment is required for Douglas fir (There is an exception for Douglas fir) and radiata pine structural timber. However, cantilevered deck joists and associated framing must be treated to H3.2.
The relevant standards (NZS 3640 or 3602) themselves were not amended with these changes, but B2/AS1 was instead updated to modify the requirements of these standards. The changes related to only radiata pine and Douglas fir enclosed by cladding.
Allowance for other species is provided by NZS 3602, which was amended in B2/AS1 only for radiata pine and Douglas fir.
Other species continue to reference NZS 3602. Totara is not referenced in NZS 3602.
Douglas fir- the exception
The exception to H1.2 treatment was that untreated Douglas fir framing has been included for use in defined ‘low-risk’ designed houses. This was because Douglas fir was "shown to have some natural durability over that of radiata pine (though not as good as H1.2 treated framing)" (John Harper, Senior Advisor – Building Standards, Department of Building and Housing).
For structural use of other species, NZS 3602:2003 Tables 1 and 2 continue to be the only reference.
Treatment requirements for timber framing.2
Treatment requirements for framing and other timber uses.2
|Floor framing protected from weather but exposed to ground atmosphere|
|H1.2||Radiata pine Douglas fir||Jack studs, subfloor braces, bearers, wall plates, floor joists to the subfloor, blocking, walings and battens, nogs and diagonal boards.|
|H1.2||Radiata pine Douglas fir||Interior solid wood flooring for ground floors.|
|Enclosed roof framing and trusses|
|H1.2||Radiata pine Douglas fir||Sarking and framing not protected from solar-driven moisture through absorbent cladding materials.|
|H1.2||Radiata pine Douglas fir||Enclosed flat roof framing and associated roof members.|
|H1.2||Radiata pine Douglas fir||Enclosed skillion roof framing and associated roof members.|
|H1.2||Radiata pine Douglas fir||Valley boards and boards supporting flashings or box gutters, and flashings to roof penetrations and upstands to roof decks.|
|H1.2||Radiata pine Douglas fir||All roof trusses, including gable-end trusses, roof framing, ceiling and eaves framing, purlins and battens.|
|Enclosed wall framing protected from the weather|
|H1.2||Radiata pine Douglas fir||Framing and other members within or beneath a parapet.|
|H1.2||Radiata pine Douglas fir||Framing and other members within enclosed decks or balconies (see H3.2 for cantilevered decks).|
|H3.2||Radiata pine||Framing and other members within enclosed cantilevered decks (including joist trimmers, nogs and blocking).|
|H1.2||Radiata pine Douglas fir||Framing and other members supporting enclosed decks or balconies (including cantilevered decks).|
|H3.1||Radiata pine||Battens used behind cladding to form a cavity (H3.1 treatments can be either solvent-based or boron. H3.1 boron treatments supplied grey primer-painted).|
|H1.2||Radiata pine Douglas fir||All other exterior wall framing and other members including exterior and boundary joists.|
|Internal wall framing|
|H1.2||Radiata pine Douglas fir||Internal walls.|
|H1.2||Radiata pine Douglas fir||All mid-floor framing, including boundary joists, ceiling framing and ceiling battens and double top plates.|
|H1.2||Pinus species Douglas fir||Interior flooring.|
|None||Radiata pine Douglas fir||Wall framing and roof framing (including trusses) protected from the weather, in unlined and unoccupied farm buildings and outbuildings, except buildings with high internal humidity, such as saunas, spas and so on.|
|H3.2||Radiata pine||Framing exposed to the weather and above ground.|
|H4||Radiata pine||Framing such as fence posts and landscape timbers that is exposed to the weather and is in contact with the ground.|
|H5||Radiata pine||Framing such as house piles, poles and crib walling that is exposed to the weather and is in contact with the ground.|
Note 1: For structural use of other species, refer to NZS 3602:2003 Tables 1 and 2.
Note 2: For non-structural use of radiata pine, Douglas fir and other species, refer to NZS 3602:2003 Table 3.
Note 3: A higher treatment level also satisfies the level specified in this table.
Refer to NZS 3602:2003 for other framing choices, such as larch or macrocarpa.
Wet boric or boron salts timber treatments were first introduced in 1952 – before that, native timber and some of the first radiata pine framing was used untreated. From 11 September 1995, NZS 3602:1995 Timber and wood-based products for use in building allowed the use of untreated timber for framing provided it was kiln-dried and that its in-service moisture content did not exceed 18%. The change in the standard was cited in Building Code compliance document B2/AS1 on 28 February 1998. (Any use of untreated kiln-dried timber before this would have to have been consented as an Alternative Solution.)4
From 1998 to April 2004, homes were commonly constructed with untreated kiln-dried timber framing, which will readily deteriorate if it is regularly wetted. During this period, there was also limited use of H1.1 LOSP (which has no resistance to rot), H1.2 boric, H3.1 LOSP and some H3.2 CCA-treated framing.5
On 9 March 2003, the Building Industry Authority (BIA – the forerunner to both MBIE and DBH) issued BIA directive 23, which required that treated timber be used for all consents issued from 1 April 2004. Consents already issued that included use of untreated timber remained valid as long as the buildings were completed before 1 April 2005.4
In April 2004, Acceptable Solution B2/AS1 [A prescriptive design/construction solution published by the Ministry of Building, Innovation and Employment (MBIE), Building and Housing Group. Where proposed construction follows an Acceptable Solution exactly, it must be accepted as being code compliant for that specific Building Code clause.] adopted an amended version of NZS 3602, which required the use of treated timber where there was any risk of water getting into the timber frame.5
In April 2011, Amendment 7 to B2/AS1 further amended the treatment requirements of NZS 3602 to allow the use of H1.2 boron-treated radiata pine and Douglas fir framing within a closed space, except for cantilevered balcony floor joists and associated enclosed balcony wall framing where H3.2-treated timber was required. The amendment allowed the use of untreated Douglas fir in buildings with very low weathertightness risk.4
Changes to the Acceptable Solution for Building Code Clause B2 (Durability) were proposed by the Department of Building and Housing in 2010, with consultation closing in October 2010.
Proposals included adopting H1.2 as the single hazard class for timber framing inside the building envelope, except for the critical performance of cantilevered deck joists, which would need to be treated to H3.2 hazard class. Principles behind the changes included adequate protection for homeowners and Encouraging fit for purpose products.
A summary of key issues from the consultation on Timber treatment changes was released in March 2011.
The discussion document outlined how when used appropriately, timber is a versatile and durable building material. Preservative treatment of timber enhances its durability, which is especially important in situations where the timber may be subject to inadvertent wetting.
The discussion document6 stated that:
- The purpose of preservative treatment in framing is to provide protection from exposure during construction and allow time to discover and repair framing if inadvertent leaking occurs throughout the life of a building.
- Treating all timber within the house will simplify timber selection, reduce complexity and risk of mistakes, thus avoiding remedial costs where untreated timber is used in inappropriate locations.
- Research conducted over the past eight years on the durability of treated framing has confirmed that timber treated to the H1.2 retention level of boron (0.4% m/m BAE) provides satisfactory resistance to decay and the spread of fungi within the building if the framing gets wet.
- favourable results on leaching of boron from significant research within New Zealand, mainly by Scion and more recently by DBH and BRANZ,
- Boron treatments have been used in New Zealand framing for more than 50 years and there is no historical documented evidence that boron-treated wall frames would be at risk to rapid and extensive decay in buildings.
- all timber framing elements within the building envelope are protected from the weather, but at risk of occasional wetting. Improvements to design and construction practices resulting from amendments to E2/AS1 have significantly reduced the risk of moisture ingress.
In one of the longest on-going accelerated studies6 that has been published, untreated pine framing failed at 18 months whereas the boron-treated framing was almost sound after 6 years except for some superficial soft rot in the very wet sections.
In ongoing accelerated decay trials conducted by Scion, boron-treated timber continues to resist fungal attack after 7-8 years exposure6. The boron treatment was at the current H1.2 retention and the timber has been exposed to moisture and periodic wetting over the duration of the trial. Boron is not a leach-resistant preservative and some loss in boron concentration has been documented in this study. However the approved H1.2 concentration has more than allowed for any depletion over the test period and for the ongoing protection of the timber.
Additional independent research conducted by Scion and BRANZ confirms that while boron is a leachable preservative treatment, the level of treatment is effective when timber is wet for a prolonged period and under conditions favourable to fungal decay development. Even at low residual boron levels, the boron will continue to protect the timber from decay.
The toxic threshold for brown rot (the decay type most likely to develop quickly in softwood timber framing), expressed as boric acid equivalent (BAE) is 0.15 – 0.20% m/m. So the H1.2 retention of 0.40% m/m gives a 100% safety factor, ie double the toxic threshold.
The trial is of commercially available New Zealand radiata framing, treated to two levels of boron (BAE at 0.4 % m/m and 0.8% m/m) which is consistent with recognised commercial treatments. The twelve month results were analysed in March 2010, and results indicate average retentions across each batch of test samples of 0.4% m/m and 0.6% m/m respectively. The results support the adequacy of H1.2 boron for treatment of general framing timber, as outlined in the proposal.
Conclusion: Based on this evidence, the Department considers that boron as a preservative treatment at the current H1.2 retention (0.40% m/m Boric Acid Equivalent) provides effective protection from insect borers and fungal decay for all framing elements.
Building practices have improved due to regulatory changes and greater awareness of good building practices. Drained and vented cavities, better detailing of penetrations and junctions and around windows and doors, and a greater understanding of the ‘risk’ profiles of certain types of buildings to leaking have been introduced to improve weathertightness. The risk of framing getting wet is greatly reduced, as evidenced by the very much lower incidence of weathertightness failure since 2004.
The total volume of structural timber used in New Zealand housing (new houses and additions and alterations) and non-residential buildings has been estimated from the most recent BRANZ survey of trends in timber use at over 870,000 cubic metres per annum.
Whether higher hazard classes should be allowed.
The Department encourages the use of H1.2, but acknowledges that designers may have particular reasons to use higher classes in certain situations. It is therefore continuing to allow higher treatments to be used (supported by sector education).
Whether roof trusses need to be treated.
In the interests of clarity, simplicity and inventory rationalisation, the Acceptable Solution specifies that roof trusses are to be treated to the same level as all other enclosed framing (apart from cantilevered deck joists and framing which require a higher treatment).
Whether all enclosed deck framing should be H3.2, rather than only cantilevered deck joists and framing as proposed.
The risk profile of other enclosed deck framing is not as high as that of cantilevered decks. Cantilevered decks depend more critically on the strength of the timber to prevent collapse. Therefore, only cantilevered deck joists and framing are specified as H3.2.
Whether solvent-based azoles should be included as H1.2.
The proposal in the discussion document to exclude solvent-based preservatives from H1.2, was arrived at after considerable discussions with the sector, and submissions did not raise anything new. Therefore, solvent-based azoles have not been included as H1.2.
About 75% supported the use of untreated Douglas fir for houses of low-risk design.
Three key points were raised in consultation:
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.
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.
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.
B2/AS1 amendment 8 is effective from 14 February 2014 (Change to Acceptable Solution for timber preservative treatment - Codewords Issue 60 - May 2014). Amendments can be viewed and downloaded from the Acceptable Solutions and Verification Methods page. These include:
- the following as normative text: NZBC clause B2.3.1 refers to minimum durability requirements for building elements. Timber used for structural purposes is required to be durable in-service for the life of the building, being not less than 50 years unless the building has a specified intended life.
This is applicable to hazard classes H1.2, H3.2, H4, H5, and H6. Structural timber refers to timber that has been graded to characteristic strength and stiffness properties.
The minimum requirement for a H1.2 treatment for timber framing is to provide protection in-service but the preservative treatment is not designed for extended exposure to elevated moisture content.
Timber used for non-structural purposes, such as H1.1 and H3.1 is required to be durable in-service for a minimum of 5 years and 15 years respectively.
- Clause 18.104.22.168.1 is reinstated as “Complete sapwood penetration shall be achieved”.
Modifications to the previous citation of NZS 3602, in B2/AS1 amendment 7, remain unaltered.
This latest citation of B2/AS1 includes a special provision for treated LVL framing with the inclusion:
Laminated Veneer Lumber (LVL) treated using LOSP borne azoles as specified for H3.1 in NZS 3640 Table 6.2 satisfies the minimum treatment requirement of H1.2.
References to NZS 3604, as modified by references to NZS 3640 and NZS 3602 noted above.
Timber and wood-based building products
3.2.1 The following Standards form an Acceptable Solution for B2/AS1 meeting the durability requirements of timber and wood-based building elements,
a) NZS 3602 Part 1 as modified by Paragraph 3.2.2.
b) NZS 3640 as modified by Paragraph 3.2.3.
c) NZS 3604, with reference to NZS 3602 (and NZS 3640), as modified by Paragraph 3.2.1 a) and b) above.
The use of different timbers or timber treatments to those referred to in NZS 3602 are outside the scope of this Acceptable Solution. Where the use of a different timber or timber treatment is proposed, it shall be separately assessed for compliance with the Building Code. For example, if imported hard-wood is to be used to surface a deck, evidence that the timber was durable for a minimum of 15 years in the expected exposure conditions is required.
3.2.2 Modification to NZS 3602:
22.214.171.124 Level of treatment references to radiata pine and Douglas fir solid timber in Table 1 categories ‘C’, ‘D’ and ‘E’ and Table 2 category ‘B’ shall be replaced by Tables 1A and 2A below. Table 1A and Table 2A are to be read with NZS 3602 sections 108 to 111 inclusive, with the amendments in Paragraph 126.96.36.199 below.
Other references to radiata pine, Douglas fir solid timber and engineered wood products in NZS 3602, including Table 1 categories ‘A’, & ‘B’; Table 2 category ‘A’; and Table 3 are unaltered.
Laminated veneer lumber (LVL) treated using LOSP borne azoles as specified for H3.1 in NZS 3640 Table 6.2 satisfies the minimum treatment requirement of H 1.2.
- B2 Durability overview
By Tom Edhouse - 1 June 2015, Build 148. In our second article in the building control series, we look at New Zealand Building Code clause B2 Durability...
- Timber treatment has just got simpler Build 124, June 2011
- Acceptable Solutions and Verification Methods for the New Zealand Building Code Clause B2 Durability
- BRANZ Guideline, September 2013
- Branz Weathertight: How we got here
- Consultation on proposals for a single hazard class for framing timber inside the building envelope, Department of Building and Housing