The timber under a deck has to suit its exposure. Business Queensland's summary of AS 1604 describes H3 as suitable where timber is kept off the ground but exposed to weather or periodic wetting, and H4 and H5 for timber in contact with the ground. H3 pine suits deck framing kept off the ground; it is not for ground contact. Member sizes come from AS 1684 or your engineer, not from this guide.
Above the ground
- H3 is described for timber kept off the ground but exposed to weather or periodic wetting. WoodSolutions' preservative treatments and use classes page lists decking among typical H3 applications.
- H1 and H2 are described for timber completely protected from weather and wetting, so they are not for an exposed deck.
In or on the ground
- H4 is for timber in contact with the ground or continually damp. WoodSolutions describes it for non-critical uses such as landscaping and fence posts, and not for critical structural elements.
- For any structural element in ground contact, such as posts or stumps, WoodSolutions calls for H5 or a Class 1 durable hardwood.
Our H3 pine therefore does not suit framing in contact with the ground. How close to the ground framing can sit, and the ventilation, drainage and termite-inspection access it needs, depend on your site and plans: confirm them with your designer, certifier or engineer. See H3 vs H4 treated pine.
Sizes, joints and fixings
- Sizes: bearer, joist and post sizes come from the span tables in AS 1684 for your deck height, grade and loads. Tiled decks, spas and hot tubs need a structural engineer.
- Joist width: where boards abut over a joist, the joist needs enough width to fix both board ends without splitting. Take joist sizes from AS 1684 or your engineer, and fixing positions from the installation instructions for the boards you choose.
- Fixings: use fixings that suit the treatment in your timber. WoodSolutions recommends hot-dip galvanised or 304/316 stainless steel fixings for CCA- and ACQ-treated timber. The treatment of the timber you buy is confirmed on your quote and delivery paperwork.
Where our products fit
We list H3 treated pine framing in 70 x 35, 70 x 45 and 90 x 35 (per linear metre), and treated 90 x 45 and H3 140 x 45 (by the board). We do not list hardwood framing, or pine treated to H4 or H5; those can be quoted. For what the grades mean, see MGP10, MGP12 and F7.
FAQ
Can I use your 42 mm Merbau sections as joists?
Their grade and any structural rating are confirmed at quote stage, not assumed. Ask before you plan a structural use.
Do you give joist sizes or spans?
No. Use AS 1684 or your engineer.
Talk to us
Send your framing list and the treatment your plans call for through request a quote.
Sydney Timber Supply team
Hazard class references from Business Queensland's summary of AS 1604 and WoodSolutions' preservative treatments and use classes page. Reviewed November 2026.