By Sam Alex, civil engineer · About the author
A deck is one of the most satisfying DIY projects, and one of the easiest to get wrong where it matters: the parts you can't see. Here is how I'd plan one, working from the ground up, with a full material take-off for a 4.8 m × 3.6 m deck.
*Check with your council or a private certifier before you start. Many decks are exempt from approval if they're low and small, but height, size, closeness to boundaries and attachment to the house all matter.*
How a deck is built
From the ground up: 1. Footings and posts (or stumps): carry the load into the ground 2. Bearers: large beams sitting on the posts 3. Joists: smaller beams sitting across the bearers, usually at 450 mm centres 4. Decking boards: fixed across the joists
Each layer's size and spacing depends on the one above it and on the span. In Australia, timber sizes come from AS 1684 span tables or the supplier's own span tables. Composite and aluminium framing systems have their own. Don't size framing by eye.
Step 1: Lay out the deck
For our 4.8 m × 3.6 m deck, run the boards along the 4.8 m length, so 4.8 m boards finish in one piece with no joins. That means the joists run across the 3.6 m direction, and the bearers run along the 4.8 m direction.
Step 2: Footings and bearers
With three bearers (at the front, middle and back), each joist spans about 1.8 m. Each bearer needs posts along its length at the spacing the span tables allow for your bearer size. For example, four posts per bearer gives 12 footings in total. Footing size and depth depend on the soil and the loads. On reactive clay or sloping ground, get engineering advice.
Step 3: Joists
At 450 mm centres along 4.8 m: 4.8 ÷ 0.45 = 10.7 spaces, so 11 spaces and 12 joists, each 3.6 m long. Add a double joist or blocking wherever boards need extra support, such as at steps or a picture-frame border.
Many composite boards need joists at 400 mm or closer. Check the board manufacturer's requirements before setting the joist spacing. It changes the whole frame.
Step 4: Decking boards
With 140 mm boards and a 5 mm gap, each board covers 0.145 m of width: 1. Rows: 3.6 ÷ 0.145 = 24.8, so 25 rows 2. One 4.8 m board per row: 25 boards 3. Add about 10% for defects and cuts: 28 boards
Our decking calculator gives the same answer from the area: 17.28 m² ÷ (0.145 × 4.8) × 1.1 = 27.3, rounded up to 28.
Step 5: Fixings
- Board screws: two per board at every joist: 25 rows × 12 joists × 2 = 600 screws, plus spares. Use stainless steel near the coast.
- Joist hangers or skew nails, depending on the design
- Bearer-to-post connections: brackets or bolts as specified
- Hidden fixing clips if you're using a clip system instead of face screws
Complete take-off (typical, for planning only)
| Item | Quantity |
|---|---|
| Footings / posts | about 12 (depends on bearer span) |
| Bearers | 3 × 4.8 m |
| Joists | 12 × 3.6 m |
| Decking boards (140 mm) | 28 × 4.8 m |
| Decking screws | about 600 + spares |
| Oil or stain (timber decks) | two coats × 17.3 m², about 2.5–3.5 L depending on product |
Your span tables may change the member sizes and post spacing.
Things that make a deck last
- Ventilation: leave airflow under low decks. Trapped moisture rots timber and bows boards.
- Board gaps: 3–5 mm for hardwood, or as the manufacturer specifies for composites.
- Joist protection tape on the top of joists reduces rot at the screw holes.
- Pre-drill hardwood near board ends to stop splitting.
- Finish timber decks before or soon after laying, and re-oil every 6–12 months.
Height and safety
Decks more than about 1 m above the ground generally need balustrades under the National Construction Code, with minimum heights and limits on gaps (to stop children climbing or slipping through). Stairs must meet the riser and going limits in our stair calculator guide.
Frequently asked questions
How many decking boards do I need for a 4.8 m × 3.6 m deck?
About 25 boards of 4.8 m at 140 mm wide with 5 mm gaps, so order about 28 with spares.
What joist spacing should I use?
Commonly 450 mm for 19 mm hardwood decking. Many composites need 400 mm or less. Follow the board manufacturer.
Do I need council approval for a deck?
It depends on height, size, location and your state's rules. Check with your council or a private certifier before building.