H3 2 vs h4 treated timber which treatment level do you need compared
Quick Answer
Use H3.2 treated timber for outdoor use above ground, where the wood can get wet from weather but is not in regular ground contact. Use H4 for timber that touches the ground or is set into it, such as fence posts, retaining edges and some landscaping work. If you choose too low a treatment level, the timber can decay early even if the wood species and design seem suitable.
Overview
H3.2 and H4 are hazard treatment levels used to match timber to the decay and insect risk in service. The practical difference is simple: H3.2 is generally for outside, above-ground situations, while H4 is for in-ground or ground-contact use. That means decking framing, pergola parts above ground and exterior cladding battens may suit H3.2 if the product is specified for that job, whereas posts, piles, sleepers and other timber touching soil usually need H4 or a higher treatment level depending on the application. The most common mistake is choosing by appearance or price instead of by exposure. Timber near the ground but not actually in it can still need careful checking, especially where water can sit, splashing is heavy or ventilation is poor. Treatment level is only one part of durability: correct fasteners, drainage, end-grain sealing where required by the manufacturer, and cutting treatment on site all affect service life. Always check the timber stamp, merchant description and the relevant manufacturer guidance for the exact intended use. If the timber is structural, or if the project affects a deck, retaining structure, load-bearing frame or anything needing consent or inspection, confirm the required treatment level with your local building authority, designer or supplier before buying.
Who this is for
Homeowners, DIYers and renovators choosing treated timber for fences, decks, garden structures and general outdoor building work.
What you’ll need
- project plan or sketch showing where each timber piece will sit
- supplier product description or treatment stamp details
- manufacturer installation guidance for the timber product
- local building authority or consent information if the work is structural
Before you start
Work out exactly whether each timber member will be above ground, in ground contact, or in a damp splash zone. Check whether it is structural or non-structural, because treatment level and timber grade can both matter. Also check compatibility of fixings and connectors with treated timber, as some treatments require more corrosion-resistant hardware.
Step-by-step
- 1
Define where the timber will actually sit
List each part of the project: posts, rails, joists, bearers, battens, sleepers, trims and any buried or soil-touching components. Mark whether each piece is fully above ground, close to the ground, or in direct soil contact.
Why: The treatment level is chosen by exposure to moisture, decay and insects in service, not by the general type of project.
- 2
Choose H3.2 for outdoor above-ground use
Select H3.2 when the timber is outside and exposed to weather but not intended to be in contact with the ground. Confirm the product is suitable for the exact role, especially if it is structural framing or a component in a deck or pergola.
Why: H3.2 is designed for a lower hazard environment than timber buried in soil or constantly damp from ground contact.
- 3
Choose H4 for ground-contact use
Select H4 when the timber will touch soil, be embedded in the ground, or be used where sustained wetting and decay risk are significantly higher. This commonly applies to posts, some landscaping timbers and other in-ground members.
Why: Ground contact creates a much harsher environment for timber, so a higher treatment level is needed for durability.
- 4
Check the timber stamp and seller description
Before purchase and again before installation, find the treatment marking on the timber or packaging and compare it with the invoice or product listing. If it is not clearly marked, ask the supplier for written confirmation.
Why: Mixed stock, similar-looking pieces and vague labels are common causes of the wrong treatment being installed.
- 5
Review details that increase moisture risk
Look for areas where water can pool, soil can build up, vegetation can trap moisture or ventilation is poor. If your design leaves timber damp for long periods, ask the supplier, designer or building professional whether a higher hazard treatment or a design change is better.
Why: A borderline location can perform more like ground-contact service if moisture is not able to dry out.
- 6
Use compatible fixings and protect cut surfaces as required
Follow the timber and fastener manufacturer guidance for nails, screws, brackets and any treatment of cuts, drill holes or notches made on site. Keep timber separated from soil unless it is specifically intended for contact.
Why: Even correctly treated timber can fail early if corrosion damages fixings or untreated exposed wood fibres are left vulnerable.
- 7
Get expert confirmation for structural or consented work
If the timber is load-bearing, part of a deck support system, retaining structure or another regulated build, check the exact requirement with your local building authority, designer, engineer or merchant technical team before starting.
Why: Treatment level is only one part of compliance and safety; structural class, span, fixing details and consent rules may also apply.
Why this works
Timber treatment hazard classes match preservative protection to the severity of the environment the wood will face. Above-ground exterior timber generally dries more easily than in-ground timber, so it can use a lower hazard treatment than wood exposed to constant soil moisture and decay organisms.
Common mistakes to avoid
- Using H3.2 for fence or pergola posts simply because the rest of the structure is above ground
- Assuming all outdoor treated timber is interchangeable without checking the hazard class stamp
- Ignoring splash zones, trapped moisture and poor drainage near the ground
- Choosing treatment level correctly but using unsuitable fixings that corrode in treated timber
- Not checking whether structural timber also needs a particular grade or compliance documentation
Troubleshooting
You already bought timber and are not sure whether it is H3.2 or H4
Check the stamp, bundle label or supplier paperwork. If still unclear, ask the merchant for written confirmation before installation.
A design includes timber very close to soil but not buried
Improve drainage and clearance if possible, and confirm with the supplier or building professional whether H3.2 remains suitable for that exposure.
Cut ends are exposed after trimming posts or framing
Follow the product manufacturer guidance for protecting cut ends and penetrations, especially on exterior work.
Fasteners are staining or showing early corrosion
Check that the fixings are compatible with the timber treatment and site exposure, then replace with the specified corrosion-resistant type if needed.
Compare your options
H3.2 treated timber
Best for: Exterior above-ground timber exposed to weather but not intended for ground contact
Pros: Suitable for many common outdoor applications; usually easier to source for general exterior framing and joinery uses; typically avoids paying for more treatment than needed
Cons: Not suitable where timber is buried or in direct ground contact; can be wrongly chosen for posts and landscaping
H4 treated timber
Best for: Ground-contact and in-ground uses such as posts and other timber exposed to sustained moisture from soil
Pros: Higher level of protection for harsh exposure; better suited to buried or soil-contact situations
Cons: Can cost more than H3.2; may be unnecessary for fully above-ground components; still requires correct detailing and compatible hardware
| Option | Best for | Pros | Cons |
|---|---|---|---|
| H3.2 treated timber | Exterior above-ground timber exposed to weather but not intended for ground contact | Suitable for many common outdoor applications; usually easier to source for general exterior framing and joinery uses; typically avoids paying for more treatment than needed | Not suitable where timber is buried or in direct ground contact; can be wrongly chosen for posts and landscaping |
| H4 treated timber | Ground-contact and in-ground uses such as posts and other timber exposed to sustained moisture from soil | Higher level of protection for harsh exposure; better suited to buried or soil-contact situations | Can cost more than H3.2; may be unnecessary for fully above-ground components; still requires correct detailing and compatible hardware |
Alternatives
- Redesign the project so timber stays clear of soil, allowing an above-ground treatment level where appropriate
- Use non-timber supports or post anchors to keep structural timber out of the ground where the design and regulations allow
- Consider naturally durable or alternative materials if treatment suitability, appearance or maintenance is a concern
Pro tips
- Buy all timber for one stage of the job at the same time and keep labels until installation is complete
- Separate posts, rails and framing on your materials list so you do not accidentally order one treatment class for everything
- Ask the supplier to identify structural grade and hazard class on the quote, not just 'treated pine'
- Keep timber stored off the ground and covered before use so it does not pick up unnecessary moisture or contamination
Safety notes
- Wear gloves, eye protection and a dust mask or respirator suitable for wood dust when cutting or drilling treated timber
- Do not burn treated timber offcuts or sawdust
- Wash hands after handling treated wood, especially before eating
- Follow disposal advice from your local authority for treated timber waste
Legal & regulatory notes
Treatment requirements can depend on local building rules, the structural role of the timber and the exact product standard claimed by the supplier. For regulated building work, decks, retaining structures or structural framing, confirm requirements with your local building authority and the product documentation before installation.
What this guide does not cover: This guide compares H3.2 and H4 in practical terms only. It does not replace project-specific engineering, consent advice, manufacturer instructions, or full treatment-class tables for every timber use.
Cost considerations
H4 timber often costs more than H3.2 because it is treated for a harsher service environment. Using H4 everywhere may increase material cost unnecessarily, but using H3.2 where H4 is needed can be far more expensive later if parts fail and need replacing.
Prefer to hire a professional?
If this job is beyond a confident DIY — or the work needs to be done by a licensed professional — you can find a verified carpenter or joiner through these directories of real, verified trade businesses. Compare local tradespeople, read their details and request quotes before you commit.
Frequently asked questions
Can I use H4 instead of H3.2?+
Often yes from a durability point of view, but it may be unnecessary and more expensive for above-ground parts. You should still check structural suitability, appearance, fixings and any project-specific requirements.
Is H3.2 suitable for fence posts?+
Not if the posts will be set into the ground. In-ground posts generally need H4 or another treatment level specified for that use.
What if the timber is outside but under a roof?+
Being under cover does not automatically remove the need for exterior treatment. Consider whether wind-driven rain, condensation, splashback or poor ventilation can still keep the timber damp, and check the manufacturer's intended use guidance.
Does treatment level tell me whether the timber is structural?+
No. Treatment level relates to durability in service, while structural suitability depends on grading, design and compliance information.
Do I need to treat cut ends?+
Follow the timber product manufacturer's instructions. Site cuts and holes can expose wood fibres, so many products require additional protection on cut surfaces.
Sources & references
Guidance on this page is traced to documented sources. Last checked 25 September 2026.
- BRANZ · industry
Supports practical building guidance on selecting timber treatment for exposure conditions and checking compliance details for New Zealand construction.
- Building Performance New Zealand · government
Supports the need to check local building requirements and compliance expectations for structural or consent-related building work.
The core H3.2 versus H4 distinction is stable, but product approvals, treatment systems and local compliance expectations can change over time.