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Timber treatment

H3.1 and H3.2 treated timber - Identification and use

In 2004, Acceptable Solution B2/AS1 was amended to cite NZS 3602: 2003 Timber and Wood-based Products for Use in Building. The Standard introduced a number of changes including the introduction of two new hazard classes for treated timber - H3.1 and H3.2.

Although both H3.1 and H3.2 can be used for timber exposed to exterior conditions not in ground contact, there are important differences between them. This article is a fresh reminder of their relative uses and how to identify them on site.

Uses for H3.1

Where H3.1 is used outside it is only suitable for non-structural applications such as weatherboards, fascias etc - members requiring a minimum of 15 years durability, and it must always have paint protection. See Table 1 opposite.

Paint protection must be applied to protected faces and cut ends (before fixing) in the form of one coat of priming. The paint protection to exposed faces must be at least a primer coat and two further coats of alkyd or 100 percent acrylic (refer NZS 3602 clause 111.2) H3.1 can be used structurally (minimum 50-year durability) where it is protected from the weather but with a risk of moisture penetration such as framing in some high-risk wall locations. When used in framing, H3.1 treated timber does not require paint protection, as it is not permanently exposed to the weather.

Uses for H3.2

Where timber is used structurally outside and not in ground contact - such as for pergolas, fences, verandah posts, beams etc, use H3.2 as a minimum treatment level. H3.2 does not require painting when used externally. See Table 1 opposite.

Identification on site

Both H3.1 and H3.2 can be identified by colour coding of the timber and/or branding showing the treatment plant number, the preservative code number and the Hazard class, for example 665 62 H3.1. Branding may also carry a quality control trademark.

The requirements for colour coding and branding are set out in NZS 3640 Chemical Preservation of Round and Sawn Timber, and are summarised in the Table opposite.

For more information on the uses of treated timber and its identification, you can refer to NZS 3602 and NZS 3640, or contact the Department of Building and Housing for a copy of their booklet Timber Treatment Requirements - Notes for builders, a copy of which can also be downloaded from the Department's website, free of charge at www.dbh.govt.nz

Table giving uses and identification requirements for H3.1 and H3.2 treated timber

Uses and identification requirements for H3.1 and H3.2 treated timber
A. Uses as given in NZS 3602
Timber Cladding, minimum 15-year durability Structural framing protected from weather but with risk of moisture penetration, minimum 50-year durability Structural uses outside, not in ground contact, minimum 50-year durability
H3.1 Yes
Paint protection required, including priming of all faces and cut ends
Yes
Refer to NZS 3602 for specific applications
No
H3.2 Yes
(Exceeds minimum requirements)
Yes
(Exceeds minimum requirements)
Yes
B. Colour coding and branding for H3.1 and H3.2 treated timber to NZS 3640
H3.1 Colour coding green that is distinctly different from the green of the H3.2 preservative treatment colour, plus end branding or continuous face branding, or No colour coding but must have continuous branding repetitively along the length at 1500 mm centres on its face or one edge.
H3.2 No colour coding required. Can be identified from the distinctive 'green' of the copper-based treatment, plus end branding or continuous face branding.