A deck looks simple, but its material list has more lines than most small jobs. Work from the ground up, and price each layer before moving to the next, so nothing is missed.
1. Layout and design decisions
Before counting anything, settle the basics: deck size and height, whether it's attached to the house with a ledger or freestanding, joist direction and spacing, beam positions, stairs and railings. Height matters most. A deck more than about 1 m above the ground usually needs a balustrade, and taller decks need bracing and bigger footings. In Australia, small low decks are often exempt development (in NSW, for example, up to 25 m² and no more than 1 m above the ground, with other conditions), but bigger or higher decks need approval from the council or a private certifier, so check your state's rules before you quote.
2. Footings and posts
Lay out the beam lines and post spacing first. Every post needs a footing: a concrete pad footing dug to firm ground (often 450–600 mm deep, deeper in reactive clay, as set by the site classification), with the post on a galvanised stirrup or post anchor. Count footings, work out the concrete per footing, and add post bases or anchors. The pier calculator gives the concrete for any number of footings.
3. Beams and joists
Joists = deck length ÷ joist spacing + 1. Add rim and end joists, and a ledger if the deck is attached to the house. Count hangers (two per joist at a ledger or flush beam) and ties where joists sit on beams. Beams are usually two or three plies of joist-size timber, so count those pieces too.
Joist and beam sizes and spans must come from AS 1684 span tables, the timber maker's span tables or a structural engineer, with council or certifier approval where needed. A calculator can count parts, but it can't size them. Composite decking often needs closer joist spacing, especially for diagonal boards.
4. Deck boards
Work in rows, not area. Rows = (deck width + gap) ÷ (board width + gap). Then choose a board length that suits the deck: a 5.4 m deck with 5.4 m boards wastes almost nothing, while a slightly longer deck forces joins in every row. Where joins are needed, stagger them and land them over doubled joists or blocking. Picture-frame borders and breaker boards add extra boards and extra framing, so count them separately.
5. Fixings
Two screws per board at every joist. On a 4.8 × 3.6 m deck with 86 mm boards on 450 mm joist centres, that's around 960 screws. Hidden clip systems need one clip per joist crossing. Use stainless or coated fixings rated for treated timber, and the system made for composite boards.
6. Stairs and railings
Count stringers (one every 450 mm or so), treads and risers from the stair calculator. Railing posts, top and bottom rails, and balusters spaced under the 125 mm rule are a big part of the cost on raised decks. Railing posts must be through-bolted or fixed with approved hardware to resist the guard loads. A post screwed into the rim joist alone isn't enough.
7. The small stuff that adds up
- Joist tape on top of joists and beams
- Flashing and flashing tape for the ledger, so water can't get behind it
- Structural screws or bolts for the ledger
- End-grain preserver for cut ends of treated timber
- Weed membrane and gravel under low decks
- Stain or oil for the first coat, if included
Waste
10% for boards and framing is typical. Go higher for diagonal boards, picture-frame borders, angled corners or curved edges.
A worked example
A 4.8 × 3.6 m attached deck in H3 treated pine, joists at 450 mm centres: 12 joists plus rims, a ledger and one bearer on four H4 posts. Four 300 mm × 600 mm deep footings (about 0.17 m³, roughly 18 bags of 20 kg). Boards: 40 rows of 86 mm board (with 5 mm gaps) at 4.8 m, 44 boards with waste. Screws: about 960, or two boxes of 500.
Use the deck framing, deck board and baluster calculators, adding each to the same job, to build the whole quote in minutes.
Common questions
Composite or timber?
Composite costs more up front but needs no staining and lasts longer. Timber is cheaper and easier to work, but needs regular maintenance. Price both if the client is unsure.
How long does a deck take to build?
A simple ground-level deck of this size is often two to four days for a small crew. Raised decks with stairs and railings take longer.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check the NCC, Australian Standards and your building surveyor or certifier for your project.