NZ TIMBER EDITORIAL GUIDE
Timber Treatment Levels NZ
An H-class describes the biological hazard a preservative treatment is intended to resist. It is not a structural grade, strength rating, moisture reading or promise that every product is suitable for every detail in that exposure.
Read the H-class as an exposure boundary
| Class | Broad exposure represented | Typical questions before purchase |
|---|---|---|
| H1.1 | protected interior, low decay risk | Is this exact indoor use covered by the producer statement? |
| H1.2 | protected interior framing where borer and decay protection is required | Is the timber enclosed, dry in service and correctly identified? |
| H2 | protected interior use with a higher hazard than H1 | Which H2 subclass and regional/product limitation applies? |
| H3.1 / H3.2 | exterior above ground | Is the member weather-exposed, coated, structural, and able to drain and dry? |
| H4 | ground contact or severe exterior wetting | Is it actually in soil, embedded, or persistently wet? |
| H5 | severe ground contact or fresh-water exposure | Does the engineered use require the higher hazard class? |
| H6 | marine-water exposure | Is the product specifically produced and documented for marine use? |
The table is an orientation, not a substitute for NZS 3640, AS/NZS 1604.1, an Acceptable Solution, consent documents or a treatment producer's product statement. Subclasses matter. So do species, preservative, penetration pattern, retention and end use.
Treatment, grade and size answer different questions
- Treatment class addresses biological hazard in a stated exposure.
- SG8 or another structural grade describes verified structural properties; treatment does not create grade.
- A dimension such as 100 x 100 mm describes section size, not capacity or durability.
- Species affects treatability, movement, machining and natural durability.
- A coating can support weather protection but does not turn an untreated product into H3.2 timber.
A useful order line therefore keeps each attribute visible: species, treatment class and preservative, structural grade where relevant, finished or nominal section, length, profile and producer identification. 'Treated 4x2' is not enough for a design-sensitive substitution.
Identification is part of the product
New Zealand treated timber is identified through branding, labels, packet marks or documentation appropriate to the product. Keep the identifier until inspection and record it with delivery photographs. If cutting removes an identifying mark, the pack documentation and order record become more important.
- Match the order to the plans or producer's documented intended use.
- Check the H-class and preservative, not just the colour of the timber.
- Confirm structural grade separately.
- Ask how field cuts, holes and notches must be treated.
- Retain invoices, producer literature and treatment identifiers with project records.
Colour is unreliable. Some preservatives are colourless, and green or brown dye does not prove a hazard class. Weathering and surface coatings can also change appearance.
Four decisions people often collapse into one
| Situation | Treatment decision | Separate decision still required |
|---|---|---|
| Interior wall framing | H1.2 may be relevant under the applicable compliance pathway | grade, moisture, framing design and producer identification |
| Exterior deck joist | commonly H3.2 above ground | SG grade, joist span, fixings, ventilation and detailing |
| Fence post in soil | commonly H4 ground-contact product | post section, spacing, embedment and wind/load design |
| Retaining member | often H5 for severe ground contact depending on system | engineered wall design, drainage and consent status |
These are common New Zealand purchasing patterns, not universal permissions. The final specification comes from the current compliance pathway, designer and product documentation.
Preservatives, handling and end of life
The hazard class is only one part of treatment identification. Copper-based systems, boron treatments and other preservatives have different permitted uses, appearance, corrosion implications and handling directions. Check the producer's safety data and intended-use literature. Wear appropriate protection when cutting, control dust, wash before eating and keep scraps away from children, animals and waterways.
MBIE advises against burning treated timber because combustion can release hazardous substances and concentrate chemicals in ash. Do not put treated offcuts into a household fire, barbecue or garden incinerator. Use the local council or waste operator's current disposal route, and separate identifiable treated material from clean untreated offcuts.
- Ask which preservative system was used and whether the timber was treated dry or wet.
- Confirm compatible nails, screws, bolts, brackets and flashings.
- Protect cut ends only with the product and method authorised for that treatment system.
- Never use smell, colour or age to declare a demolition offcut untreated.
- Record the product before a coating or installation hides its marks.
A purchase-order pattern that survives substitutions
Write: 'Radiata pine, H3.2 [preservative/product], SG8, 140 x 45 mm supplied section, 4.8 m, producer identified' rather than 'treated 6x2'. If the seller cannot supply one attribute, the gap becomes visible before dispatch. The responsible designer can then assess a proposed alternative without reverse-engineering a vague order.
| Supplier proposal | Question to return |
|---|---|
| same size, different H-class | Does the documented end use permit that exposure class? |
| same H-class, no grade shown | Where is structural grade verified? |
| same grade, different species | Does design/product documentation allow the species and treatment penetration? |
| higher H-class | Are preservative, handling, appearance and intended-use constraints still acceptable? |