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

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..

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. Updated versions of both of these standards are currently being prepared.

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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 treatmentExposureCommentsTimber and treatmentTypical 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:

  • LOSP treatments (TBTO, TBTN, IPBC) are not permitted for framing
  • CuN LOSP has been removed from H1.2 and is not permitted for framing
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 of H3.1 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.

Updated: 4 August 2020