Article: H3 vs H4 Treated Pine: Differences, Sizes & Uses Explained
H3 vs H4 Treated Pine: Differences, Sizes & Uses Explained
The short answer: H3 treated pine is rated for outdoor timber that stays above ground and dries out between wettings — deck joists, bearers, pergola frames and wall framing exposed to the weather. H4 treated pine is rated for timber that goes into the ground — fence posts, garden edging and landscaping sleepers. If the piece sits in soil or concrete, you need H4 or higher. If it's off the ground and can drain and dry, H3 is the right (and cheaper) choice.
THQ has supplied decking and subframe timber to Australian builders and homeowners for 15+ years. Below is the practical version: what each hazard class means, which sizes go where, and the mistakes we see most often on site.
What do the H-ratings on treated pine mean?
The "H" stands for hazard class under the Australian Standard for preservative-treated timber (AS 1604). Each class tells you the exposure the timber has been treated to handle. The higher the number, the harsher the conditions.
| Hazard class | Exposure | Typical uses |
|---|---|---|
| H1 | Inside, above ground, dry. Protected from insects other than termites. | Interior joinery, furniture |
| H2 | Inside, above ground, dry. Protected from termites and borers. | Internal wall and roof framing |
| H3 | Outside, above ground, exposed to weather but able to dry out. Protected from decay and termites. | Deck joists and bearers, pergola framing, fascias, outdoor framing |
| H4 | Outside, in-ground contact, non-critical. | Fence posts, garden edging, landscaping sleepers |
| H5 | In-ground or in fresh water, where failure is critical. | House stumps, retaining walls, building poles |
| H6 | Prolonged immersion in sea water. | Marine piles, jetties |
H3 vs H4 treated pine: the key differences
- Ground contact. H3 is not made for soil contact. H4 is. That is the single deciding factor.
- Treatment level. H4 carries a higher preservative retention to handle constant moisture and soil organisms.
- Cost. H3 costs less per metre. On a deck subframe, using H3 for everything that sits above ground keeps the budget down without cutting corners.
- Typical sections. H3 is the standard for structural framing sizes like 70x45, 90x45 and 140x45. H4 is more often sold as round posts, sleepers and edging.
Which do you need for a deck subframe?
For a standard composite deck, the joists and bearers sit above ground on stumps or post supports, so H3 is the correct rating for the frame. The parts that go into the ground — the stumps or posts — need H4/H5 timber, or galvanised steel stirrups and stumps set in concrete so the H3 timber stays clear of the soil.
Two rules we give every customer:
- Keep H3 out of the dirt. If a joist or bearer will touch soil, sit it up on a stirrup or switch to an in-ground rated member.
- Let the frame breathe. Leave space under the deck for airflow so the timber can dry out after rain. A frame that stays wet works against any treatment.
For the full build sequence, read our composite decking installation guide. For most THQ composite boards, joists go at 400mm centres for a standard layout and 300mm centres for a diagonal layout — always check the guide for your board profile.
Treated pine sizes and what they're used for
Sizes are written as width x thickness in millimetres. These are the H3 sections THQ stocks and where each one usually goes:
| Size | Lengths at THQ | Common use |
|---|---|---|
| 70x45 H3 | 3.6m | Decking joists on low, well-supported decks; noggins and blocking; light outdoor framing |
| 90x45 H3 | 3.6m and 5.4m | The all-rounder: deck joists, pergola frames, outdoor stud framing |
| 140x45 H3 | 3.6m and 5.4m | Deck bearers, pergola beams, longer-spanning members |
The 5.4m lengths mean fewer joins across a long frame, which makes for a stiffer, flatter deck. You can see every size and length in one place in our H3 treated pine framing range.
What about spans?
How far a joist or bearer can span depends on the timber's stress grade, the member size, the spacing and the load. Spans for timber framing in Australia come from the span tables in AS 1684 (Residential timber-framed construction). Don't size a frame by guesswork. Check the tables, or get your builder or engineer to sign it off, especially on raised decks or anything that needs a building permit.
Mistakes we see on site
- Burying H3. H3 posts set in soil or concrete are the most common reason a subframe rots early.
- Not sealing cut ends. Cutting exposes untreated timber in the core. Brush a preservative end-sealer on every cut you make.
- Undersized joists. Using 70x45 where the span really calls for 90x45 gives a bouncy deck and stresses the board fixings.
- No airflow. Enclosing the sides of a low deck traps moisture against the frame.
More on avoiding these: common composite decking mistakes and how to avoid them. Weighing up pine against other timbers? See treated pine vs hardwood.
FAQs
Can H3 treated pine go in the ground?
No. H3 is rated for above-ground outdoor use only. Any timber in contact with soil needs H4 or higher, depending on whether failure would be structural.
Is H3 treated pine termite resistant?
Yes. H3 treatment protects against termites and decay in above-ground outdoor conditions. Keep it out of the ground and seal cut ends to keep that protection intact.
Is H3 or H4 better for decking joists?
H3 is the right choice for joists and bearers that sit above ground on stumps or stirrups. H4 only makes sense where a member is in contact with the ground.
What size treated pine do I need for deck joists?
90x45 is the most common deck joist size, with 140x45 used for bearers and longer spans. The exact size depends on span, spacing and load. Check the AS 1684 span tables or ask your builder.
Can you paint or stain H3 treated pine?
Yes, once the timber has dried out. Use an exterior-grade primer and paint, or a stain made for treated timber.
Does THQ sell H4 treated pine?
THQ stocks H3 treated pine framing in 70x45, 90x45 and 140x45. For in-ground stumps and posts, use H4/H5 rated posts or galvanised steel stirrups and stumps.
Get your subframe timber and decking from one supplier
THQ stocks H3 treated pine alongside our composite decking range, so you can order the frame and the boards in one delivery. Pick up from our Victorian warehouses in Somerton, Cranbourne West, Pakenham and Truganina, or from Carole Park in Brisbane. We offer trade pricing for builders, free decking samples and fast delivery across our service areas.
Call 1800 084 700 or email sales@thq.com.au for a quote on your subframe and decking.