How to build a wood deck: step-by-step guide

1. Plan and check the rules

Decide the size, height and board direction, and check local building rules. Many areas require a permit for decks attached to a house or raised above a certain height, and all raised decks need guards (railings). Attaching a deck to a house through a ledger board requires correct flashing and fixings and must never be done on brick veneer or hollow masonry; a freestanding deck avoids these risks.

The Deck calculator gives the number of decking boards, joists and screws for your size.

2. Choose materials and fasteners

Use pressure-treated timber rated for ground contact for posts and anything near the soil, and naturally durable or treated wood, or composite, for the decking. Treated wood corrodes ordinary steel, so use hot-dip galvanized or stainless steel screws, bolts and connectors. Near the sea, stainless steel is the safer choice.

3. Lay out and build the footings

Mark the outline with stakes and string, and square it by checking that the diagonals are equal. Dig footing holes at the post positions, typically no more than 1.5–2 m apart along each beam. Footings must reach firm, undisturbed soil and go below the local frost depth so that the deck does not heave in winter.

Pour concrete footings, and set metal post bases into them so that the posts stand clear of the concrete and do not rot. The Concrete calculator helps work out the concrete for the footings.

4. Set posts and beams

Fix the posts to the bases, plumb them, and cut them to height with a level line. Bolt the beams to the posts or seat them in post caps. The beams carry the joists, so their size and span must suit the load; follow a span table for your timber size and species.

5. Install the joists

Lay the joists across the beams at 40 cm centers, or as close as the decking manufacturer requires; many composite boards need 30–40 cm. Fix them with joist hangers or hurricane ties, and add a rim joist around the edge. Check that the tops of the joists are flat, planing or shimming where needed. Joist tape on the tops keeps water out of the screw holes and extends the life of the frame.

6. Lay the decking boards

Start with a straight board along the most visible edge. Leave a gap of about 5 mm between boards for drainage and movement; a spacer makes this consistent. Fix each board to every joist with two screws placed about 2 cm from the edges, and pre-drill near board ends to avoid splitting. Stagger the end joints and make sure each one lands on a joist, or on a doubled joist.

Let the boards overhang and trim them all at once with a circular saw along a chalk line for a clean edge.

7. Add steps and guards

Even a low deck often needs a step or two. Keep every riser the same height and every tread the same depth; the Stair calculator works out a comfortable rise and run. Raised decks need guards of the height required by your local code, with gaps narrow enough that a child cannot get through.

8. Finish and maintain

Many treated or new hardwood boards should weather for a few weeks before oiling; follow the board supplier’s advice. Apply a deck oil or stain, and renew it every year or two. Keep the gaps clear of leaves and dirt, and clean off algae, which makes boards slippery when wet.

Common mistakes

• Setting posts directly in concrete or soil, where they rot quickly.
• Using ordinary screws in treated timber, which corrode and lose grip.
• Joists spaced too far apart for the boards, which makes the deck bouncy.
• Laying boards tight together, so water sits on the deck and the boards cup.
• Attaching a deck to the house without proper flashing and fixings, which can cause rot or failure.

Frequently asked questions

Sources

• AWC DCA 6 — Prescriptive Residential Wood Deck Construction Guide
• Wikipedia — Deck (building)