Deck Building Guide: Estimating Boards, Joists, Screws & Cost

A deck looks simple — a flat platform of boards — but a complete material list has several layers: the footings and posts that carry the deck to the ground, the beams and joists that form the frame, the deck boards you walk on, the fixings that hold everything together, and the finishing pieces like fascia, stairs, and railing. This guide focuses on the part most people want to price first: the deck boards, the joists under them, and the screws that fasten them. Get those right and you have the bulk of the material cost and the most error-prone counting out of the way.

The deck board and joist calculator turns your deck size, board width, gap, board length, and joist spacing into a board count, the total lineal length of decking, the number of joists, and an approximate screw count. It works in metric or imperial, which matters because decking is sold by the lineal metre in Australia and much of Europe and by the lineal foot or the board in North America. Understanding how each figure is derived lets you adapt the estimate to your own layout.

Deck boards are laid in parallel courses across the deck. The key number is the board coverage: the width of one board plus the gap you leave beside it. A 140 millimetre board with a 5 millimetre gap covers 145 millimetres of deck. Divide the deck width by that coverage and round up, and you have the number of courses. Then divide the deck length by the length of the boards you buy and round up to find how many boards make up each course. Multiply the two and you have the board count before waste.

Board direction changes everything. Boards run along the length you enter, so if you decide to run them the other way, swap the length and width — the course count, the joist count, and the offcut pattern all change with them. Running boards along the longer dimension usually means fewer butt joints and a cleaner look, but it can mean more joist material. Diagonal decking looks great but wastes more board at the angled cuts and needs closer joist spacing, so add extra to the waste allowance for a diagonal layout.

The gap between boards is not just cosmetic — it lets water drain and gives timber room to move. Pressure-treated softwood that is installed wet or 'green' will shrink as it dries, so a 3 to 5 millimetre gap (about 1/8 to 3/16 inch) at installation opens up to a comfortable spacing later. Kiln-dried hardwood is already at a stable moisture content and can be spaced a little wider because it will expand rather than shrink. Get this wrong and you either trap water and debris between tight boards or end up with gaps wide enough to catch a heel.

Composite and PVC decking behave differently again. They move mainly with temperature rather than moisture, and manufacturers publish specific gap requirements along the board length and, importantly, larger gaps at the board ends where two lengths meet. These gaps are part of the warranty, so follow the fixing guide for your specific product rather than a generic rule. In the calculator, set the gap to match your material, and remember that a larger gap means slightly fewer boards because each course covers more width.

Joists run at right angles to the deck boards and support them. The board that spans between joists has to be stiff enough not to sag underfoot, so the spacing depends on the decking material and how it is laid. Timber decking laid square to the joists is commonly supported at 400 to 450 millimetres (16 to 18 inches) on centre. Composite decking is often specified at 300 to 400 millimetres because it is less stiff, and any decking laid diagonally needs to come in to around 300 millimetres because the effective span across the diagonal is longer.

The calculator estimates the number of joists as the deck length divided by the spacing, plus one for the starting joist, and multiplies by the deck width to give the total lineal metres of joist material. That figure covers the common joists only. The beams (bearers) that the joists sit on, the ledger board where the deck meets the house, joist hangers, blocking between joists, and the posts and footings are all separate — and the beam and post sizing in particular is a structural matter. Use an approved span table or an engineer to size the frame; the calculator handles the counting once the spacing is set.

Screws are easy to under-order. The rule of thumb is two screws at every point where a board crosses a joist, so a deck with 28 board courses and 13 joists needs roughly 28 × 13 × 2 = 728 screws before you account for butt joints, where two board ends meet on a shared joist and each end takes its own pair. Hidden fastener systems for grooved composite boards change the count again — they use clips at each joist instead of face screws — so check your system's coverage per box. Always buy screws by the box and keep a spare box; nothing stalls a deck build like a mid-afternoon run to the store.

Beyond boards, joists, and screws, the pieces that quietly add up are the fascia boards that hide the frame edge, the stair stringers and treads, the railing posts, balusters and top rail, the flashing at the ledger, and the joist hangers and structural screws or bolts. If your deck sits close to the ground you may also need weed matting and a gravel bed underneath — the gravel and aggregate calculator will size that. And if the deck adjoins a framed wall or you are building a privacy screen, the wall framing stud calculator helps count the studs and plates for that.

Deck cost is dominated by three choices: the decking material, the size, and the height and complexity. Material spans a wide range — pressure-treated softwood is the budget option, hardwoods and thermally modified timber sit in the middle, and premium capped composite is at the top, though composite saves on long-term maintenance. Because the calculator gives you the lineal metres or feet of decking, you can multiply directly by a per-metre or per-foot price from your supplier to compare materials on a like-for-like basis for your exact deck.

Size scales the whole material list, but height and complexity scale the labour and the substructure. A low ground-level deck needs modest footings and no railing; a raised deck needs taller posts, bigger beams, more bracing, a code-compliant railing, and stairs, all of which add material and hours. Rather than trusting a single per-square-metre rule, price the parts you can count — decking, joists, screws — from the calculator, then get a builder's quote for the substructure and finishing. Our guide on how to check a contractor's quote explains how to compare quotes line by line so you are not caught by a low headline number that leaves out the railing or the footings.

Start with an accurate deck size and decide the board direction, because that sets everything else. Enter the board width and gap for your chosen material, the length of the boards your supplier stocks, and the joist spacing from the decking manufacturer's span table. Read off the board count, the lineal length of decking, the joist count, and the screw estimate. Add a waste allowance — 10 percent for a simple rectangular deck, more for diagonal or picture-frame layouts — and you have a defensible material list for the walking surface and its immediate support.

Then extend the list to the parts the calculator does not count: beams, posts, footings, hangers, fascia, stairs, and railing. Price the decking by multiplying the lineal figure by your supplier's rate, and treat the substructure and finishing either as a separate takeoff or as a builder's quoted item. Keep every assumption written next to its quantity so that if you change the board or the layout, you can update just the affected line. That discipline — count what you can, quote what you cannot, and record every assumption — is what turns a rough guess into a deck estimate you can actually build to.