Deck joist spacing in Australia: 400, 450 or 500 mm?
Short answer
Under AS 1684, joists for 19 mm hardwood decking (standard or medium feature grade) can be up to 500 mm apart. For treated pine, it's 400 mm for 19 mm boards and 450 mm for 22 mm boards. Lay boards at 45° and you need to go closer, about 350 mm for 19 mm hardwood. Composite boards follow the manufacturer's instructions, usually around 400 mm, closer for diagonal layouts. Most builders simply use 450 mm centres for standard timber decks.
Joist spacing is the distance between the centres of your joists. Get it wrong and boards bounce, cup and crack, and screws work loose. Get it right and the deck feels solid for decades.
Joist spacing is about what your decking boards can span. It’s a different question from how far each joist can span between bearers, which is covered further down.
Maximum joist spacing by decking board
These come from the Australian Standard for timber framing, AS 1684, as summarised by WoodSolutions:
| Decking board | Thickness | Max joist spacing |
|---|---|---|
| Hardwood, standard or medium feature grade | 19 mm | 500 mm |
| Cypress, grade 1 | 19 mm | 400 mm |
| Cypress, grade 1 | 21 mm | 450 mm |
| Treated pine (softwood), standard grade | 19 mm | 400 mm |
| Treated pine (softwood), standard grade | 22 mm | 450 mm |
Composite and PVC boards aren’t covered by AS 1684. Follow the manufacturer’s installation guide. Many specify around 400 mm centres for standard layouts and closer spacing (around 300 mm) for diagonal boards, stairs or heavy use.
Laying boards at 45° or in a pattern
Angled boards span further between joists than square-laid boards do, so the joists have to come closer together. For 19 mm standard grade hardwood at 45°, space joists at 350 mm to keep the board span within 500 mm. Herringbone, chevron and picture-frame patterns usually need extra joists or noggins where boards meet.
What size joists, and how far can they span?
How far a joist can span between bearers depends on its size, stress grade (for example MGP10 pine or F17 hardwood), the spacing, and the deck’s height. Don’t guess this one:
- For decks up to 1 m off the ground, AS 1684 has dedicated deck span tables (Tables 5 and 6). Above 1 m, other tables apply, and in most places so does council approval.
- Timber suppliers like Hyne publish free span tables for their products. Use the table for the exact timber you’re buying.
- Use joists at least 45 mm wide (treated pine or seasoned hardwood) so screws near board ends don’t split them.
The other numbers that matter
- Gap between boards: about 3 to 4 mm is ideal long term, adjusted for how wet or dry the boards are when laid.
- Gap at the house wall: leave a gap between the deck frame and the wall, and about 10 mm between the boards and the wall, so water drains and termites can be spotted.
- Posts: sit them on galvanised stirrups at least 75 mm above the ground.
- Low decks: aim for at least 400 mm of space under the deck for airflow and inspection. Closer to the ground, use more durable timber and consider screwing down the first boards so they can be lifted.
Is a deck DIY?
A low, simple deck is one of the most popular DIY builds in Australia. Higher decks, decks attached to the house, and anything over about 1 m usually need approval and engineering. See do you need council approval for a deck in NSW and the full timber deck materials list.
Frequently asked questions
Is 600 mm joist spacing OK for decking? Not for standard timber decking. AS 1684 tops out at 500 mm for 19 mm hardwood, and less for pine. Wider spacing makes boards bounce and crack.
What joist spacing do I need for composite decking? Whatever the manufacturer specifies, usually around 400 mm, closer for diagonal boards. Using wider spacing can void the warranty.
Can I use 450 mm spacing for everything? For square-laid hardwood and 22 mm pine, yes. For 19 mm pine boards, you need 400 mm. For 45° boards, go closer.
How far can a 140x45 joist span? It depends on its stress grade, the spacing and the deck height. Check the span table for the exact product you’re buying.
Sources
Every rule on this page is taken from these official sources. If a rule has changed, please tell us and we’ll fix it.
This guide is general information, not legal or professional advice. Rules differ between states, councils and individual properties, and they change. Confirm with your state regulator or council before starting work, and if in doubt, get a licensed tradesperson.