How to Plan a Deck: Size, Layout and Materials
A well-planned deck is designed around the materials. Choosing dimensions that suit stock board lengths and joist spacing saves money and gives a cleaner result. This guide covers the planning decisions before you buy anything.
Start with the site
- Check property lines, setbacks and whether a permit is required — most attached decks and many raised decks need one.
- Call your utility locating service before digging footings (811 in the US).
- Note the height of the door threshold; the finished deck surface usually sits a little below it.
- Look at sun, shade, drainage and views to decide size and shape.
Size the deck around stock lengths
Deck boards come in fixed lengths — 12, 16 and 20 ft in North America; 3.6, 4.8 and 5.4 m elsewhere. A deck that is 16 ft along the house uses 16 ft boards with no joints. A 17 ft deck needs a joint in every row, landing on a joist, and wastes most of a second board. If you can, choose a length that matches a stock size, or slightly less to allow for trimming the ends.
The other dimension — perpendicular to the boards — sets the number of rows. Each row is the board width plus the gap. For 5½ in boards with a 3/16 in gap, 12 ft of depth is 144 ÷ 5.6875 = 25.3, so 26 rows.
Board direction and patterns
Boards usually run parallel to the house. Diagonal decking looks striking but needs closer joist spacing (often 12 in rather than 16 in), more cutting and around 15% waste. Picture-frame borders and breaker boards add extra blocking. Decide the pattern before you design the framing.
Joist spacing and framing
| Decking | Typical maximum joist spacing |
|---|---|
| 5/4 × 6 wood, perpendicular | 16 in (400 mm) |
| 2 × 6 wood, perpendicular | 24 in (600 mm) |
| Composite, perpendicular | 16 in, sometimes 12 in — see manufacturer |
| Any decking, diagonal 45° | 12 in (300 mm) typical |
Joist size, span, beam size, post spacing and footing depth are structural. In the US the American Wood Council's DCA 6 guide gives prescriptive tables for many simple decks; elsewhere, follow local building regulations. Our deck board calculator counts joists for budgeting but does not size them.
Footings
Footings carry the deck load to the ground. Depth is set by frost depth and soil; size by load and post spacing. Many DIY decks use poured concrete piers in tube forms or precast footings. The concrete per footing is π × radius² × depth — use the bag calculator to convert it to bags.
Building the materials list
- Deck boards, screws or hidden clips — from the deck board calculator.
- Joists, rim joists and beams by size and length.
- Ledger board, flashing and structural fasteners if attached to the house.
- Posts, post bases and footing concrete.
- Joist hangers, hurricane ties and structural screws.
- Stairs: stringers, treads and risers — from the stair calculator.
- Railing posts, rails and balusters.
Worked example: a 16 × 12 ft deck
- Boards run along the 16 ft house wall; 12 ft deep.
- Rows: 144 in ÷ (5.5 + 0.1875) in = 25.3 → 26 rows of 16 ft boards.
- With 10% waste: 29 boards.
- Joists at 16 in centers across 16 ft: 192 ÷ 16 = 12 spaces → 13 joists, each a little under 12 ft long.
- Face screws: 26 rows × 13 joists × 2 = 676 screws (buy a bucket of 750 or 1,000).
The deck board calculator reproduces these numbers and switches to metric board lengths automatically.
Budget and timeline
Decking materials are often a little under half of a professionally built deck's price, but for DIY the materials are most of the cost — together with tool hire (post-hole auger, mitre saw, impact driver) and permit fees. Get material prices for boards, framing and fasteners separately; composite boards may cost two to three times as much as treated softwood but need much less maintenance over the deck's life.
A straightforward ground-level deck can be a long weekend for two people once the footings are in; raised and attached decks take longer because of the ledger, posts, stairs and railings, and the inspection schedule if you need a permit. Allow drying time for concrete footings before loading them.
Common planning mistakes
- Designing a length that misses stock board lengths by a few inches.
- Forgetting the stair run when placing the deck near a path or boundary.
- Using 6 in instead of 5½ in for board rows.
Wood or composite?
Pressure-treated softwood is the cheapest to buy but needs regular cleaning and sealing. Hardwoods are durable and expensive. Composite and PVC boards cost more upfront, need less maintenance and have specific gap, fastener and joist spacing requirements — always follow the manufacturer's installation guide or the warranty may be void.
Calculators for this guide
- Deck Board Calculator — Work out how many deck boards, joists, screws or hidden clips you need for a deck, using real board widths, gaps and stock lengths.
- Concrete Bag Calculator — How many bags of concrete do you need? Compare 40, 60 and 80 lb or 20, 25 and 30 kg bags, total weight and cost — with the math shown.
- Board Foot Calculator — Convert lumber dimensions to board feet and cubic meters, and price hardwood or dimensional lumber by the board foot.
- Stair Calculator — Work out the number of steps, riser height, tread depth, total run, stringer length and angle — and check them against common code limits.
More guides
Estimates only. This guide explains general methods. Verify quantities with your supplier and follow local building codes; for structural or safety-critical work consult a licensed professional. Editorial policy · Report an error.