Key takeaways
- H3 timber should be used for above-ground outdoor applications, while H4 is required anywhere framing sits close to or in contact with the ground.
- Most deck failures begin in the subframe, particularly around joists where moisture tends to collect and airflow is limited.
- Small installation details like sealing cut ends, using joist tape, and choosing the right fasteners can significantly extend the life of a deck.
- Close-to-ground-level decks need extra attention regarding ventilation, board spacing, and moisture management to reduce movement and premature decay.
- A long-lasting deck comes down to using the right products in the right locations and not rushing the build
Thinking of building a new deck? Here’s what to keep in mind
Plenty of guides on how to build a deck will show you how to lay boards and pour footings. Few explain why some decks last decades while others start failing far earlier than expected.
Rather than walking through every step of a deck build, this guide focuses on the part that matters most and the part that catches most people out: timber selection, treatment levels, and the installation details that tend to get skipped.
Australia is one of the toughest environments in the world for outdoor timber. Coastal humidity, termite pressure, punishing UV and heavy rain cycles mean the difference between a deck that lasts and one that doesn’t often comes down to decisions made at the hardware store, not on the tools.
Whether you’re planning a deck or halfway through a build, this guide breaks down what those decisions are, why they matter, and exactly what to use where.
Get to know the decking materials you’ll be using
Choosing timber for a deck sounds pretty straightforward until you’re standing in front of rows of products that all look fairly similar but are designed for completely different conditions.
The problem is that outdoor timber isn’t a general-purpose product. Each treatment level is engineered for a specific set of conditions, and using the wrong one, even slightly outside its intended environment, can take years off the life of a deck.
Australia’s timber treatment standard, AS/NZS 1604.1:2021, specifies how timber products should be classified and used across different environments. For deck builders, the two main treatment classifications used are H3 and H4. Understanding what each one is designed for and where it should be used within a build is one of the most important parts of building a deck properly.
The most important part is to choose the right timber
For people building their own deck, most problems usually come back to using the wrong treatment level in the wrong location.
H3: Above-ground applications
H3 timber is designed for exterior, above-ground applications where timber is periodically wet but has an opportunity to dry out between wetting cycles.
It’s the right choice for:
- Decking boards
- Handrails
- Pergola framing
- Subframe members sitting well clear of the ground (at least 150mm above finished ground level).
Woodhouse Weatherproof® LOSP-treated radiata pine, treated to H3, is a popular and well-proven option for these applications, carrying a 25-year warranty against fungal decay and termite attack when correctly installed.
H4: On or near the ground
H4 timber is built for harsher environments. Within 150mm of ground level, rising soil moisture is constant, and the drying window that H3 relies on doesn’t exist. That’s why AS/NZS 1604.1:2021 requires H4 as the minimum treatment for any framing installed within that zone. Use it for:
- Posts and stumps
- Bearers and joists in low decks
- Any subframe member within 150mm of ground level
Woodhouse TerraFrame® is an MGP10-graded structural timber treated to H4 with Micronized Copper Azole (MCA). It’s designed specifically for in-ground and on-ground applications and carries a 50-year guarantee against fungal decay and termite attack.
The rule to remember is simple: H3 above ground, H4 within 150mm of it, and nothing untreated outdoors, regardless of how sheltered the area appears.
Understand your subframe
If timber choice is the most important decision, the subframe is the most important place to get it right. It’s out of sight once the deck is built, it’s where moisture tends to collect, and it’s almost always where problems begin.
A standard deck subframe has three main components:
- Posts or stumps: the vertical supports that transfer loads into the ground
- Bearers: horizontal members that span across the posts and carry the joists
- Joists: the members that sit on the bearers and directly support the decking boards
For a visual breakdown of how these components connect, refer to the Woodhouse TerraFrame Installation Guide.
Why joists are the common failure points in DIY decks
Joists sit in the part of the subframe where moisture accumulates most. Once decay sets in, it moves fast, and because joists are hidden under the decking boards, it’s usually well advanced before anyone notices. A soft spot underfoot is often the first sign, and by then the damage is rarely limited to one board.
Using correctly treated timber at the framing stage, H4 where it’s needed, is the most effective thing you can do to prevent this.
Spacing
For most residential decks, joists are spaced at 450mm centre-to-centre. If you’re using composite decking, check the manufacturer’s specs first as some products require 400mm, and may have additional requirements around blocking and double joisting.
Ventilation
Good ventilation beneath a deck helps reduce trapped moisture and can extend the life of the subframe. Where fascia boards are used, they should be designed and installed in a way that still allows adequate airflow around the perimeter of the deck.
Low decks
If your deck sits within 400mm of the ground, moisture and airflow become much bigger considerations. Narrower decking boards are often preferred in low-clearance builds because they tend to handle movement more effectively than wider boards in damp conditions. Pre-coating all faces and edges before installation can also help reduce uneven moisture absorption and improve long-term stability.
Decking boards should generally be installed with at least a 5mm gap, unless manufacturer specifications state otherwise.”
For low-to-ground construction methods and alternatives to conventional joist-over-bearer framing, Timber Queensland’s Technical Data Sheet 13 is a free and practical reference.
Installation details that protect your investment
Good timber selection gets you most of the way there. What follows are the installation details that protect that investment once the build is underway. They’re small things individually, but skipping any of them is how moisture finds its way in.
Seal every cut
Every time you cut treated timber, you expose raw, untreated wood. That exposed surface needs to be sealed immediately with a remedial in-can preservative before it’s fixed into position. A product like Dulux® Timber End Sealer is appropriate. Don’t rip-saw or substantially resurface treated timber either, as this removes the treatment layer and will void your warranty.
Use joist tape
Applying a good-quality joist tape to the tops of bearers and joists discharges water away from the framing and meaningfully extends the service life of the subframe. Woodhouse recommends VapourSeal® sealing tape for use with TerraFrame products.
Get your fixings right
Use hot-dip galvanised nails and fasteners throughout the build. In coastal areas or around swimming pools, stainless steel is the better option due to the higher corrosion risk. Electroplated fasteners aren’t designed for treated timber in wet outdoor conditions and can deteriorate long before the timber itself.
Keep the timber dry before you install it
TerraFrame is kiln-dried to around 12% moisture content. Installing timber at or close to this level minimises the initial shrinkage and movement that happens as it settles into its environment. Keep products dry and covered on site until they’re ready to go in.
Leave the pest inspection gap
A minimum 40mm clearance between the deck framing and any adjoining structure needs to stay clear throughout the build and after it. This is the pest inspection zone. It allows termite monitoring to happen and perimeter treatment systems on the house to be accessed and retreated when needed.
What to check before you sign off
Good deck construction comes down to the details most people never see once the job is finished. Before sign-off, these are the things worth double-checking.
- Is all subframe timber within 150mm of ground level treated to H4 or above?
- Have all cuts and penetrations been sealed with a remedial preservative?
- Is there adequate cross-flow ventilation underneath the deck?
- Are fasteners hot-dip galvanised, or stainless steel if coastal?
- Is the 40mm pest inspection gap in place between framing and adjoining structures?
- Have decking boards been pre-coated and gapped at a minimum of 5mm?
- Does the structure comply with AS 1684.2:2021 and any applicable local council requirements?
Stronger deck builds begin with the right foundations
A long-lasting deck rarely comes down to one big decision. More often, it’s the combination of smaller choices made throughout the build, from selecting the right treatment level and protecting cut ends through to allowing proper airflow beneath the structure.
Australian conditions are hard on outdoor timber, but when the right products are used in the right locations, and the installation details aren’t rushed, a deck can perform well for decades.
If you’re planning a new deck or preparing for an upcoming build, Woodhouse offers a range of treated structural and outdoor timber products designed for Australian conditions, including H3 Weatherproof® and H4 TerraFrame®. Always refer to the relevant installation guides, Australian Standards, and local council requirements before construction begins.