H3 or H4 treated pine under a composite deck? Where each one goes

H3 for the parts of the frame that get rained on and dry out again. H4 for the parts that stay damp. That is the whole rule. The trap is thinking about where the timber sits rather than whether it ever dries. A joist well off the ground in the open is H3 territory. The same joist a finger's width off a wet slab behind a retaining wall is in H4 conditions whatever the stamp on it says. Composite boards make this matter more, because they are meant to last a long time and lifting them to replace a rotten frame is a big job.

What the H numbers actually promise

Treated pine in Australia is graded by hazard class under the preservation standard, AS 1604, from H1 for timber inside and protected from the weather up to H6 for marine work. The class sets how much preservative goes into the timber and what it is expected to survive. It is about decay and insects. Span tables are set out by stress grade, such as the F7 treated pine in Timber Queensland's deck tables, so check the stress grade as well as the H class.

For a deck frame, two classes matter.

• H3 is for timber outside and above the ground that gets wet now and then and is allowed to dry after each wetting. Decking, deck framing, pergola beams and fascias are typical uses.
• H4 is for timber in the ground or continuously damp, in jobs that are not critical, such as fence posts, garden walls and landscaping.

That second definition is the one to read twice. WoodSolutions says H4 is "not intended for structural reliability" and that any structural element in ground contact needs H5 or a Class 1 durable hardwood¹. Timber Queensland's deck data sheet is just as plain: posts in the ground and any framing on the ground get H5, and H3 is for framing more than 150 mm clear of it³. Its outdoor data sheet lists deck posts among the H5 jobs. I'd keep H4 posts out of concrete holes, because that is a step below what the timber bodies recommend for a deck post. It is not that H4 posts fall over next year. The people who wrote the classes did not design H4 for the job.

Where each class goes on a composite deck frame

Bearers and joists on a deck with real clearance underneath are H3. Posts are where the choice bites, and my answer is to avoid the choice. Put the posts on hot-dip galvanised stirrups set in concrete footings, with a 75 mm gap between the concrete and the bottom of the post so termites have to show themselves³. A post on a stirrup is out of the ground and able to dry, and the outdoor data sheet lists "posts not in ground" among its H3 uses². If a post has to go into the ground, use H5, put drainage gravel under it and slope the fill away from it, as the data sheet on decks describes³.

The grey area is the low deck, such as one built over an old patio or a slab at the back door. One board maker asks for at least 40 mm of air under the boards where water drains freely and 90 mm where it can pool⁵. A second maker's chart asks for more clearance over soil than over concrete and says ventilation under the deck is essential⁷. Those figures are about the boards. The frame under them has its own problem, because a joist that close to the ground in still air dries slowly between showers.

WoodSolutions draws the line at 400 mm: framing closer to the ground than that should be H4 or a Class 1 hardwood¹. Jack Norton of the Timber Preservers Association of Australia wrote in January 2026 that a deck exposed to the ground with poor ventilation turns an H3 application into an H4 situation, and his rule for timber that can't dry was to "step up the treatment"⁴.

So for a low composite deck my advice is H4 joists and bearers as the minimum, with drainage and a gap built in as well. H4 does not make a wet frame dry. It slows the rot down.

The parts of the frame that rot first

The places to protect are the tops and the ends. Rain gets into the timber at every screw hole in a joist top, and WoodSolutions wants drill holes and cut ends in treated timber re-coated on site¹. At the ends, a cut can expose heartwood. Treatment goes into the sapwood and barely into the heartwood. The standards also limit untreated heartwood in treated framing to about a fifth of the cross-section².

A Victorian owner reviewing NewTechWood on ProductReview, in a review the site showed as two years old in October 2026, wrote that the joists under his timber deck "were starting to rot at the top from screw holes" after fourteen years¹¹. That and the yearly oiling were why he and his wife changed to composite boards. Hidden clips still screw into the joist tops, so I'd tape and seal a composite frame just as carefully.

These jobs are small, and they protect the parts that fail first.

• Seal every cut end and every notch with an end-grain preservative before the piece goes in. WoodSolutions says to reseal cut ends immediately after cutting.
• Tape the tops of the joists. One board maker suggests its self-adhesive tape on timber frames to prevent moisture damage⁸, and WoodSolutions recommends flashing strips or damp-proof membranes over exposed joists¹.
• Use hot-dip galvanised or stainless fixings. WoodSolutions lists hot-dip galvanised or 304 or 316 stainless steel for fixings in CCA- or ACQ-treated timber¹, and Timber Queensland's data sheet on decks rules out zinc-plated bolts, screws and nails³.

In March 2023 a Whirlpool member planned a low composite deck over a concrete pool area, on H3 pine joists sitting on pedestals a couple of centimetres off the slab⁹. One reply warned against letting the timber rest directly on the concrete, because "it will rot with near 100% certainty within 10 years"⁹. The same member called the plan "a massive compromise" with "almost zero airflow" and suggested a designed low-deck system with an aluminium base, or engineered H4 joists, instead⁹. Others suggested joist protection tape between the joists and boards, and spraying the cut ends with a sealer.

In a different thread in March 2019, another member described his own low frame: H3 joists sitting 80 to 150 mm above the concrete on adjustable pedestals¹⁰. He painted the frame and covered the joists with a protection strip. That is the right instinct. Treatment class, clearance and detailing all have to work together.

Does the class change the centres?

No. The hazard class is about rot and insects. Composite boards need closer joists than hardwood decking regardless. Timber Queensland's data sheet lets 19 mm hardwood decking span 500 mm³, while one composite maker gives 450 mm for houses and 400 mm for stairs and commercial decks⁵. The plastic in composite also creeps: one maker's engineering notes assume long-term sag under a heavy point load of up to four times the short-term figure⁶. Build the frame at the board's centres in whatever class the exposure calls for. The joist spacing guide covers the centres in detail, and steel vs timber covers when to skip pine altogether.

One more check I'd make at the yard: the H class and the stress grade are separate marks, so make sure the H4 you buy carries the stress grade your span table assumes.

Before you order

Measure the clearance from the soil or slab to the underside of the joists at the lowest point of the deck. If it's well over 400 mm and the air moves, I'd order H3 bearers and joists. Put the posts on stirrups and spend the saving on joist tape and a tin of end sealer. If the frame will sit low or against a wall that can't be flashed, order H4 bearers and joists as the minimum. Then fix the drainage so the treatment isn't doing all the work. Keep posts on stirrups wherever a stirrup will do, and make any post that goes into the ground H5³.

The framing explainer has the classes side by side, and the frame-building guide has the order to build in. For a deck over a slab or close to the dirt, the concrete guide and the ground-level guide cover drainage. If you're thinking of reusing an old frame, read composite decking on an existing deck frame first. The deck designer will draw a concept plan at the right joist centres for the boards you choose.