Treated pine is the workhorse of Australian outdoor construction. It’s cost-effective, dimensionally consistent, available in long lengths, and — when specified to the right hazard class — will outlast most projects you’ll put it into.

For Melbourne builders and chippies working through wet winters and hot, dry summers, it’s the default choice for everything from sub-floor framing on a deck to a 1.8m paling fence down a side boundary.

This guide cuts through the basics and gets into the detail that matters on site: choosing the right treatment grade, sizing members correctly, picking compatible fixings, and avoiding the small mistakes that cost callbacks. We’ll focus on the two project types that make up the bulk of outdoor treated pine work — decking and pergolas, and fencing and gates — and finish with the handling, cutting and finishing practices that keep your jobs warranty-clean.

Understanding hazard classes: H3 vs H4 (and why it matters)

Every piece of treated pine sold in Australia is rated under AS 1604 — Specification for preservative treatment, which assigns a hazard class (H1 through H6) based on the timber’s intended service environment. For outdoor work, two classes do the heavy lifting:

  • H3 — outdoor, above ground. Suitable for decking boards, joists and bearers (where ventilated and not in ground contact), pergola posts above a damp-proof course, fence rails and palings.
  • H4 — outdoor, in-ground contact. Required for fence posts, retaining wall sleepers, pergola posts set directly into concrete footings, and anything where the timber will be exposed to soil moisture.
Cross-section diagram of a treated pine paling fence showing H3 timber above ground (palings, rails, visible post) and H4 timber below ground (post set in concrete footing)

H3 above ground, H4 in the ground — getting this wrong is the most common reason treated pine fails early.

A common site mistake is dropping an H3 post straight into a concrete footing. It will fail prematurely — sometimes within five to seven years. If it’s going in the ground, it needs to be H4. For more on the hazard class system and chemistry options (CCA, ACQ, copper azole, MCA), WoodSolutions and the Timber Preservers Association of Australia both have free technical resources worth bookmarking.

Always check the brand or stamp on the end of the stick. If it’s not legible, ask your supplier for confirmation before you fix it.

Decking and pergolas: specifying treated pine that lasts

For deck framing, the standard call-up is H3 KD M10/F7 — kiln-dried, machine stress-graded to mgp10 OR VISUAL GRADED TO F7.

Westall Timber’s online range covers the common sub-floor sizes builders are pulling for residential decks:

Joist spacing depends on your decking board thickness. As a working rule:

  • 19mm boards → 450mm centres maximum
  • 22mm or 23mm boards → 500–600mm centres acceptable
Plan view and section detail of treated pine deck framing showing 90x45 H3 joists at 450mm centres, 140x45 H3 bearers, and 19mm decking boards.

Standard treated pine deck framing — 90×45 joists at 450mm centres on 140×45 bearers.

Refer to AS 1684 (Residential Timber-Framed Construction) for span tables and verify against the deck board manufacturer’s specification — particularly important when mixing treated pine sub-floor with hardwood or composite deck boards.

For decking boards themselves, treated pine is the budget option and looks tidy when oiled, though many builders specify hardwood (merbau, spotted gum, blackbutt) on the wear surface for longevity and feel underfoot. There’s a good comparison in our decking timber guide if you’re presenting options to a client.

Pergola work is generally H3 territory for rafters, beams and posts above ground. If posts are set into the ground or concrete, step up to H4. For exposed rafters and beams that catch the weather, H3 is rated for the application, but Melbourne’s wet winters punish poorly-detailed end-grain. Detail your tops with a slight fall, and flash anything that sits flat.

A practical tip: gap deck boards at 3–5mm depending on the moisture content of the timber when fixed. Pine moves a lot as it dries. Boards laid tight on a damp summer afternoon will lift and cup by the following winter.

Fencing and gates: getting the structure right

A standard 1.8m paling fence in treated pine is one of the most common jobs across Melbourne. Done properly, it’ll sit straight for 15–20 years. Here’s the typical build-up:

  • Posts: For Pergoals H4-treated pine, 125×75 or 100×75, set 600mm into concrete footings (deeper for gate posts and corners). H4 is non-negotiable here. The shop’s online catalogue focuses on H3, so for in-ground posts, call the yard direct or request a quote.
  • Posts: For fences are usually Cypress pine/ hardwood or Redgum 125 x 75, which are all naturally durable timbers for this application.
  • Rails: H3 treated pine, typically 75×50 rough sawn — strong enough for paling fixings without splitting.
  • Palings: H3 treated pine. Westall stocks both 1.8m × 100×12 palings, 1.8m × 150×12 palings, and the 2.1m × 100×12 paling for higher-spec fences. Also have 1.5 x 110x 12 & 1.5 x 150 x 12.
  • Plynth: H3 Treated 150×25 rough sawn used on the bottom of the fence where the palings sit on
  • Capping (optional): 90 x 45 hooked capping for pergolas makes a clean top rail or capping board that protects paling end-grain.

Gates need to be over-engineered relative to the fence around them — they swing, slam and sag. A few rules:

  • Use double posts or a single 90×90 (or larger) H4 post on the hinge side.A standard 115 x 115 should be used on anything wider than a 900mm pedestrian gate.
  • Concrete the hinge-side post deeper than the line posts — minimum 750mm for a standard gate, deeper for vehicle gates.
  • Build the gate frame from H3 rough-sawn pine, glued and screwed, with a diagonal brace running from the bottom hinge corner up to the top latch corner.
  • Allow a 10–15mm gap under the gate to clear seasonal ground movement and lawn growth.

For taller fences (over 1.8m) or where wind exposure is significant, step the post size up and check local council requirements — some Melbourne councils have height limits and setback rules that catch tradies out on boundary fences.

Fixings, brackets and corrosion: don’t mix metals

Modern treated pine preservatives — particularly the copper-based chemistries (ACQ, MCA, copper azole) that have largely replaced CCA in residential applications — are corrosive to mild steel. This is the single biggest source of premature fence and deck failures, and it’s entirely preventable.

Minimum specification for fixings into treated pine:

  • Hot-dipped galvanised to AS/NZS 4680, Class 3 coating or better
  • Type 304 stainless steel for exposed deck screws and visible fixings
  • Type 316 stainless for coastal sites or pool surrounds

Don’t use electroplated screws, ungalvanised nails, or mixed metals (e.g. a galvanised bracket with stainless bolts can set up a galvanic cell and accelerate corrosion). Stick to one specification across the assembly.

Westall stocks decking and structural screws, galvanised brackets and post anchors, and the structural adhesives and sealants needed for gluing and sealing penetrations. For more on tie-downs and bracing, the Timber Queensland technical data sheets are a useful reference for span and connection details.

Site practice: cutting, handling, finishing

A few habits that separate clean jobs from callback jobs:

  • Re-treat cut ends. Every time you cut a length, you expose untreated heartwood. Brush or spray cut ends with an end-grain preservative — particularly critical on posts cut to height and joists trimmed to fit.
  • Store off the ground, stickered between layers, under cover if possible. Treated pine arrives green and heavy; bundles sitting on wet ground will pick up moisture and twist.
  • Wear PPE when cutting. Dust masks, eye protection, gloves. Wash hands before eating. Don’t burn off-cuts.
  • Let decks weather before finishing. Treated pine needs 4–6 weeks of weathering before a stain or decking oil will adhere properly. Brush off mill glaze with a deck cleaner before the first coat. Treated Pine needs to be sealed before installing to help protect against warping and twisting
  • Re-oil decks every 12–18 months in Melbourne’s climate. Worth telling the client up front — it’s not a fit-and-forget product.

Frequently asked questions

What’s the difference between H3 and H4 treated pine?

H3 is rated for outdoor use above ground — decking boards, joists, bearers, pergola beams, fence rails and palings. H4 is rated for in-ground contact, so it’s required for fence posts, sleepers, and any post set into a concrete footing or directly into soil. Putting H3 in the ground is one of the most common reasons treated pine fails early.

How long does treated pine last outdoors?

Properly specified and installed, H3 above-ground treated pine will give 15–25 years of service in Melbourne’s climate. H4 in-ground posts typically last 25+ years. Lifespan drops sharply if the hazard class is wrong for the application, if cut ends aren’t re-treated, or if incompatible fixings cause early corrosion at connection points.

Can you put treated pine straight into concrete?

Only if it’s H4. The concrete itself isn’t the issue — it’s the moisture wicking up through the footing and the soil contact below it. H3 posts set in concrete will rot at the soil line within a few years. Use H4 minimum, and detail the top of the footing with a slight crown to shed water away from the post.

What screws should I use for treated pine?

Hot-dipped galvanised (Class 3 coating to AS/NZS 4680) or Type 304 stainless steel as a minimum. For coastal or pool environments, step up to Type 316 stainless. Avoid electroplated screws and don’t mix metals across a single connection — galvanic corrosion will eat the assembly from the inside.

Does treated pine need to be sealed or oiled?

Treated Pine needs to be sealed before installing to help protect against warping and twisting.

It doesn’t need to be sealed for the preservative treatment to work — the chemistry handles rot and termites. But for aesthetics and surface protection, treated pine decks should be oiled every 12–18 months in Melbourne. Wait 4–6 weeks after install before the first coat so mill glaze and surface moisture can clear.

How deep should a treated pine fence post be set?

For a standard 1.8m paling fence, 600mm into concrete is the working minimum. Gate posts and corner posts should go deeper — 750mm or more — and be set in larger-diameter footings. For fences taller than 1.8m or in exposed wind locations, increase post size and depth accordingly.

Sourcing treated pine through Westall Timber

Westall Timber operates a 4,000sqm drive-through yard in Springvale with Victoria-wide delivery and a Phone & Collect service for tradies who want to ring through an order and pick up on the way to site. The online shop covers the common treated pine sizes and lengths in the Timber collection, with click-and-collect typically ready within 24 hours.

For larger structural sizes, H4 posts, or anything not listed online, request a quote or call the yard directly on (03) 9546 5622 — the team can spec the job, confirm stock, and book delivery to site across Melbourne metro and regional Victoria.

Get a FREE Quote from Westall Timber

Get the grade right, fix it with compatible hardware, and treated pine will repay you with decades of service. Cut corners on either, and you’ll be back inside two years.

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