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NZ Building Code · MaterialsTimber H-class treatments: matching the wood to the weather
A builder’s plain-English rundown of NZ timber hazard classes (NZS 3640) — what H1.2 through H6 mean, where each goes, and the cut rule you can’t skip.
Timber hazard classes tell you how well a piece of wood is protected against decay for the spot it’s going into — the “H” number on the end of the stamp. Get it wrong and the timber rots early, so it’s worth knowing which class belongs where before you order.
Two systems, both needed
Treated framing carries two ratings and you need to read both. They do completely different jobs:
- H-class (e.g. H1.2, H3.2) — the chemical preservative treatment that governs durability.
- Structural grade (e.g. SG6, SG8, SG10) — the strength and stiffness for load-bearing.
So a stamp reading “90×45 H1.2 SG8” is a wall stud: indoor treatment, framing strength.
The hazard classes (NZS 3640:2003)
Each class is defined by the exposure the timber will face, which in turn drives the preservative used. Working from driest to wettest:
- H1.2 – indoor, dry, protected from weather. Typically boron. Used for wall framing, roof framing inside the building, internal trim.
- H3.1 – outdoor above-ground but covered. LOSP or copper-based. Used for fascia boards and soffits.
- H3.2 – outdoor above-ground and exposed to the weather. CCA, ACQ or Tanalith E. Used for decking framing, joists and bearers above ground, weatherboards, exterior trim.
- H4 – ground contact, low decay risk. CCA. Used for fence rails, sheltered posts in ground, garden edging.
- H5 – ground contact, severe decay risk. CCA at heavy retention. Used for deck piles, fence posts, retaining wall posts, structural ground contact.
- H6 – marine, sea water immersion. CCA plus creosote. Used for marine piles and jetties (rare in residential).
The critical cut rule (NZS 3604 §6.4.3.3)
Cutting treated timber exposes untreated heartwood, which drops the hazard class right where the water gets in. Two rules keep the protection intact:
- No cuts within 150mm of finished ground level on H5 piles.
- All cuts in H3.2, H4 and H5 must be brush-treated with copper naphthenate (or equivalent — TBTO/TBTN for H5) to maintain the hazard class.
Plain-English guide, not advice. This page helps you understand and navigate the rules — it is general information, not design, engineering or consent advice, and it does not reproduce the copyrighted tables of NZS 3604 or any Standard. Always check the current Standard or Acceptable Solution and your BCA, and use a suitably qualified LBP, engineer or QS where it matters.
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Common questions
What’s the difference between H-class and structural grade?
They’re two separate ratings on the same piece of timber. H-class (like H1.2 or H3.2) is the chemical preservative treatment for durability; structural grade (like SG6, SG8, SG10) is the strength and stiffness for load-bearing. A stamp reading “90×45 H1.2 SG8” means a wall stud — indoor treatment, framing strength.
Which H-class do I use for a deck?
It depends on the part. Decking framing, joists and bearers above ground are H3.2 (outdoor above-ground, exposed to weather), while deck piles going into the ground are H5 (ground contact, severe decay risk).
Do I need to treat cut ends of treated timber?
Yes. All cuts in H3.2, H4 and H5 must be brush-treated with copper naphthenate (or an equivalent — TBTO or TBTN for H5) to maintain the hazard class, because cutting exposes untreated timber.
Can I cut an H5 pile near the ground?
No. Under NZS 3604 §6.4.3.3, there are to be no cuts within 150mm of finished ground level on H5 piles.
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