What joist spacing and fall does decking need?

Almost every complaint about decking — bouncy, slippery, rotten, standing water — traces back to the frame rather than the boards. The frame is the part that is invisible once it is built and impossible to fix afterwards.

Safety

What you'll need

• Treated joist timber
• Decking boards
• Joist hangers and coach screws
• Stainless or coated decking screws
• Spirit level and long straight edge
• End-grain preservative
• Weed membrane and gravel

Step by step

1. Check the regulations before designing.Raised decking may need planning permission, and decking attached to the house or above a certain height may fall under building regulations. Check first, not after.
2. Set the finished height against the damp-proof course.The deck surface must be at least 150mm below the DPC — two brick courses. This constrains the whole build and has to be decided at the start.
3. Prepare the ground.Clear vegetation, lay weed membrane and a layer of gravel underneath. A deck built over bare soil grows weeds through it and holds moisture against the frame.
4. Support the frame properly.Concrete pads or posts on concrete footings, spaced so no beam spans more than the timber section can carry. Timber must not sit directly on soil anywhere.
5. Set joists at 400mm centres.This is the standard for boards laid perpendicular to the joists. Wider spacing is the main cause of a deck that flexes underfoot.
6. Reduce to 300mm centres for diagonal boards.A diagonal board spans further between joists than a perpendicular one, so the centres have to close up to keep the same support.
7. Build in the fall.About 1 in 100, or 10mm per metre, running away from the house. Set it in the frame heights, not by planing the boards.
8. Fit noggins between joists.Short blocks between the joists at intervals stop them twisting and stiffen the whole frame considerably. They cost very little and make a large difference to how the deck feels.
9. Treat every cut end.Cutting tanalised timber exposes untreated wood exactly where water sits. End-grain preservative on every cut, without exception.
10. Leave about 5mm between boards.For drainage and for timber movement. Boards butted tight trap water and debris and the deck stays wet.
11. Use the right fixings.Stainless or properly coated decking screws, two per board per joist, pre-drilled near board ends to prevent splitting. Ordinary screws corrode in treated timber.
12. Allow air movement underneath.A deck sealed all round on all four sides rots from below. Leave a ventilated gap, or use slatted rather than solid skirting.

Tips

• 400mm centres straight, 300mm centres diagonal. Everything else follows from that.
• Build the fall into the frame — you cannot add it afterwards.
• Noggins between joists are cheap and are the difference between solid and bouncy.

Common mistakes

• Spacing joists at 600mm because that is what floor joists use — Decking boards are much thinner than floorboards and span far less. At 600mm centres the deck flexes noticeably and the boards cup and split.
• Building the deck level and hoping the gaps drain it — Water sits on a level deck, in the gaps and on the boards, and the timber stays wet. A fall of 10mm per metre away from the house is what keeps it dry.

If it doesn't work

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