How do I lay decking boards?

A deck is a floor built outdoors, which means the two things that matter are drainage and ventilation. Boards laid tight together with no fall and no airflow underneath rot in a few seasons whatever the timber grade.

Safety

What you'll need

• Decking boards, treated softwood or composite
• Joists — 150mm x 47mm treated, at 400mm centres
• Decking screws, stainless or coated, twice the board thickness plus 30mm
• Weed membrane and gravel for underneath
• A circular saw, drill and spacers

Step by step

1. Prepare the ground before any timber goes down.Clear vegetation, lay a permeable weed membrane and a layer of gravel. It stops growth pushing up through the deck and helps the ground drain.
2. Set the frame level, with a fall away from the house.A fall of about 1:100 across the deck — around 10mm per metre — takes water away. If the deck runs alongside a wall, the fall must go away from it, and the deck surface must stay at least 150mm below the damp-proof course.
3. Space the joists at 400mm centres.400mm for standard 32mm softwood boards laid square to the joists. Drop to 300mm centres for boards laid diagonally, for thinner boards, or where the deck will carry a hot tub or heavy planters — check the board manufacturer's span table.
4. Leave a ventilation gap under the deck.At least 50mm of clear air below the joists, and keep the perimeter open or vented rather than boxed in solid. A sealed void holds moisture and the joists rot from below.
5. Lay the first board straight and to a line.The first board sets every board after it. Snap a chalk line or run a string, and check the board is straight before fixing — decking timber is rarely perfectly straight.
6. Use spacers for a consistent 5-6mm gap.Cut spacers, or use proprietary ones, between every board. The gap lets water through and allows the board to swell in wet weather. Softwood laid wet needs a smaller gap; laid dry, a larger one.
7. Fix twice per joist, back from the edge.Two screws per board at each joist, 20mm in from the board edges, pre-drilled in dense timber. Screwing too close to the edge splits the board, and one screw per joist lets the board cup.
8. Stagger the end joints over joists.Board ends must land on a joist, and adjacent joints should not line up. Add a double joist or a noggin wherever ends meet.
9. Trim the edges after laying.Run boards long and cut the whole edge in one pass with a circular saw against a straightedge. It is far more accurate than cutting each board to length first.
10. Treat every cut end and finish the deck.End-grain preserver on every cut. Then leave new treated timber to weather for a few weeks before oiling or staining, or the finish will not take.

Tips

• Buy boards a few days early and stack them flat, off the ground and covered, so they acclimatise before laying.
• Grooved boards do not provide meaningful grip and hold dirt. Lay them smooth side up unless the manufacturer says otherwise.
• Hidden fixing clips give a clean surface but are slower and make individual board replacement harder. Screws are more practical for most decks.

Common mistakes

• Laying boards tight together — There is nowhere for water to drain and no room for the boards to swell. They lift, cup and trap debris, and the deck rots from the top down.
• Boxing in the deck perimeter solid — It stops airflow under the deck and the joists sit in still humid air. Use slatted or vented skirting so the void breathes.

If it doesn't work

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