Timber treatment
The Building Code clause B2 Durability sets out minimum durability requirements for building materials and components. The expected serviceable life depends on the location of the material or element in the building and its function.
- Building Code requirements
- Timber use and preservative treatment standards
- Hazard classes
- Untreated timber usage
- Preservative treatment options
Building Code requirements
The Building Code clause B2 Durability sets out minimum durability requirements for building materials and components. The expected life depends on the location of the material or element in the building and its function.
Generally, timber components must have a minimum durability of not less than:
- 5 years for components that are easy to access and replace, such as interior finishing timber
- 15 years for components that are moderately difficult to replace, such as cladding, exterior trim and exterior joinery
- the life of the building, but not less than 50 years, for structural components such as bearers, joists and studs.
Where timber may be subjected to moisture in use or is used externally, it must be:
- a naturally durable species, or
- treated with a wood preservative to make it resistant to decay fungi or wood-boring insects (borer) and render it sufficiently durable.
Untreated non-durable timber at less than 18% moisture content and protected from wetting is not particularly susceptible to borer attack or fungal decay but is not permitted for use as framing (the exception being Douglas fir used in low-risk building and meeting the requirements of B2/AS1 paragraph 3.2.2.2).
Structural timber such as these studs must typically have a 50-year durability.
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Timber use and preservative treatment standards
The requirements for timber use in buildings are defined by NZS 3602:2003 Timber and wood-based products for use in building. This standard also specifies the minimum preservative treatment levels for given end uses.Using timbers or treatments not referred to in NZS 3602:2003 is outside the scope of Acceptable Solution B2/AS1, so evidence of compliance with the Building Code would be to be provided through a different path.
The requirements of specific treatment regimes are contained in NZS 3640:2003 Chemical preservation of round and sawn timber. It contains detailed treatment specifications, the types of chemicals that may be used, the minimum preservative retention and penetration into the timber, identification of treated timber and quality control requirements.
The Ministry of Business, Innovation and Employment has sponsored free access to both of these standards. You can download NZS 3602:2003 here and NZS 3640:2003 here.
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Hazard classes
When specifying timber treatment, the most important requirement is that an appropriate treatment level is specified for the particular situation. This can be done by reference to the appropriate hazard class as defined by NZS 3640 and called up in NZS 3602 Tables 1, 2 or 3 (as modified by B2/AS1 Amendment 7).
Table 1 summarises the treatment options, and Table 2 gives the permitted end uses, primarily for radiata pine.
| Table 1: Timber treatment levels, treatment options and suitable applications. | ||||
|---|---|---|---|---|
| Timber treatment | Exposure | Comments | Timber and treatment | Typical uses |
| Untreated | Where there is no risk of dampness | Production, storage, delivery and construction and in-use conditions that will prevent exposure to external or internal moisture or conditions that would allow the moisture content to exceed 18% at any time | High-temperature kiln-dried (KD) untreated radiata pine and other appearance grade timbers | Internal stair treads, risers and handrails, internal finishing timbers, furniture |
| Production, storage, delivery and construction and in-use conditions that will prevent exposure to external or internal moisture or conditions that would allow the moisture content to exceed 18% at any time | Untreated Douglas fir | Framing in a low-risk building and meeting the requirements of B2/AS1 paragraph 3.2.2.2 | ||
| Where there is a risk of dampness | Timber with proven natural durability when exposed to the weather Not suitable for use in ground contact conditions | Western red cedar, redwood, cypress species heartwood (macrocarpa) | Weatherboards, door and window joinery, exterior trim timbers | |
| Currently no H1.1 treated timber is produced. H1.2 timber is used in the very few areas where H1.1 is still specified in the cited version of NZS 3602:2003 | Where there is no risk of dampness but borer protection is required | Production, storage, delivery and construction and in-use conditions where no risk of fungal attack exists but resistance against borer attack is desirable Internal use only, not exposed to ground atmosphere, always dry H1.1 no longer exists as an acceptable treatment in B2/AS1 | Boron treated radiata pine Copper chrome arsenate (CCA) Synthetic pyrethroids | There are very few uses for timber treated to hazard class H1.1 today |
| Treated to hazard class H1.2 | Where there is moderate risk of dampness or water | Not suitable if exposure to weather during production, storage, delivery and construction is likely to be for long periods – typically a maximum weather exposure of 3 months is recommended Internal use only where in-use moisture content will be maintained at less than 20% | Boron treated radiata pine or Douglas fir Azoles as a water-based emulsion treatment of radiata pine or Douglas fir Triadimefon + cyproconazole treated radiata pine LVL Note:
| Structural framing timber including enclosed subfloor framing but excluding piles |
| Treated to hazard class H3.1 | External use with a three-coat paint finish to protect from direct wetting of timber | External use with a three-coat paint protection system or use within a 20 mm nominal E2/AS1 drained and vented cavity Not suitable for direct exposure of timber to the weather or for stained finishes | LOSP (azole, TBTO, TBTN or CuN) CCA Copper azole Boron LOSP H3.1 azole can be used for LVL B2/AS1 does not allow the use ofH3.1 LOSP TBTO, TBTN or IPBC treated timber for framing | Fascias, weatherboards, facings and other painted trim requiring a not less than 15-year durability Exterior joinery and timber reveals for aluminium windows Timber cavity battens |
| Treated to hazard class H3.2 | External use | Suitable for exposure to weather but not in-ground contact. Not less than 15-year durability except enclosed deck framing, which requires a not less than 50-year durability | CCA Copper quaternary (including micronized copper quaternary) Copper azole (including micronized copper azole) LOSP (CuN) Amendment 7 to B2/AS1 does not allow the use of LOSP CuN for framing | Cantilevered enclosed deck joists and associated framing (joist trimmers, nogs, dwangs and blocking), decking and outdoor structures, rafters exposed to the weather, exposed subfloor framing, uncoated or stained radiata pine weatherboards and trim, fence rails and palings |
| Treated to hazard class H4 | In contact with ground or concrete | Not suitable for critical major structural components in ground contact | CCA Copper quaternary (including micronized copper quaternary) Copper azole (including micronized copper azole) | Fence posts, horizontal timbers for retaining walls |
| Treated to hazard class H5 | In contact with ground or concrete | Suitable for critical major structural components in ground contact | CCA Copper azole (including micronized copper azole) Copper quaternary (including micronized copper quaternary) | House piles and poles, crib walling, retaining wall poles |
| Treated to hazard class H6 | In contact with seawater or estuarine ground | Suitable for critical major structural components immersed in seawater or embedded in estuarine soils | CCA | Wharf piles, sea walls |
| 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 | |||
| house piles | H5 | poles and crib walling | H5 |
| 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/landscape timbers exposed to weather and in ground contact | H4 | balcony barrier exposed | H3.2 |
| Framing timbers (1, 2, 3) | |||
| 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 | roof sarking plywood membrane roof | H3 CCA |
| roof framing– roof space | H1.2 | roof sarking timber | H1.2 |
| enclosed subfloor framing | H1.2 | framing and other members within enclosed cantilevered decks | H3.2 |
| Interior timbers | |||
| window reveals to aluminium windows | H3.1 | furniture | untreated |
| plywood | untreated | finishing timbers | untreated |
| solid wood flooring | H1.2 | joinery | untreated |
| Note (1) | Douglas fir may be used untreated on low-risk buildings meeting the requirements of B2/AS1 paragraph 3.2.2.2. | ||
| Note (2) | H1.2 boric-treated Douglas fir may be used in all framing applications where H1.2 boric-treated radiata pine is permitted. | ||
| Note (3) | Refer to NZS 3602:2003 for other framing choices, such as larch or macrocarpa. | ||
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Untreated timber usage
B2/AS1 includes an option for people who want to minimise use of chemically treated timber in their home. Provided certain conditions (given in 3.2.2.2 of the Acceptable Solution) are all met, untreated solid Douglas fir framing can be used for:
- roof members protected from the weather
- floor members protected from the weather and not exposed to ground atmosphere
- internal and external wall framing protected from the weather.
Untreated radiata pine and Douglas fir can be used for framing in unlined farm buildings where framing is protected from direct wetting, not in ground contact and the building is not used for living accommodation.
For some applications, NZS 3602:2003 lists a limited number of species that are considered durable enough to meet the requirements of the Building Code without the need for treatment.
Examples of timber in which the heartwood may be used untreated are:
- externally (above ground with a not less than 15-year durability) – heartwood of larch, macrocarpa, redwood, western red cedar, vitex, kwila, selected eucalypts, beech, rimu
- internally – Douglas fir (see above), European larch, radiata pine (limited applications), heartwood of macrocarpa, rimu, matai, tawa, beech, selected eucalypts.
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Preservative treatment options
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 has a minimum requirement of using H1.2 timber that is boric treated for enclosed framing.Treatment of framing and other members within enclosed cantilevered decks must 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 B2/AS1 to be primed before dispatch and to have a specified type of paint coating. | |||
| Note (3) | B2/AS1 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. | |||
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