Why have my decking boards cupped or curled?

Timber shrinks more across the growth rings than along them, so a board whose two faces are at different moisture contents curls. On a deck, the underside is the wet face: it faces a shaded, damp void with no air movement and no sun, and it never dries. The top face gets sun and wind and dries hard. The board curls up at the edges, water sits in the trough, and the process accelerates.

Safety

What you'll need

• Screwdriver and a drill
• Exterior-rated decking screws
• Replacement boards (optional)
• Torch for inspecting underneath
• Membrane and gravel for the void (optional)
• End-grain preservative

Step by step

1. Look at which way the boards have curled.Edges up and centre down means the underside is wetter, which is the usual case and points at the ventilation. Edges down and centre up — crowning — is rarer and usually means the top face is being kept wet, by shade, standing debris or a covering.
2. Get underneath and check the void.You want at least 50mm of clear air under the joists and airflow across it from at least two sides. Solid skirting all round, a deck built tight to the ground, or a void filled with leaves and debris all stop the underside drying.
3. Check how the boards are fixed.Two fixings per board per joist, 20mm in from each edge. A single central fixing lets the board rotate around it and lift at both edges, which produces exactly the same appearance as moisture cupping.
4. Open the void up.Replace solid skirting with slatted or vented skirting, clear the debris out, and if the ground under the deck is bare soil, put down a permeable membrane and 50mm of gravel. Reducing the humidity under the deck is the actual fix.
5. Re-fix the boards that are only lifting.A board that has lifted through poor fixing rather than permanent distortion can often be pulled back down with a second screw at each joist, pre-drilled. Do this in dry weather when the board is at its flattest.
6. Replace the boards that have taken a set.Once a board has cupped badly and dried in that shape, screwing it down either fails or splits it. Replace it, and check the joist underneath while the gap is open.
7. Fit replacements the right way up and let them acclimatise.Stack new boards flat and covered for a few days before laying. Follow the manufacturer's orientation guidance if there is any; where there is none, lay them the way the deck was laid so they weather consistently.
8. Clear the board gaps as a routine.Blocked gaps hold water on the joists and stop air passing up through the deck. A thin blade or a jet along every gap once a year does more than it sounds.

Tips

• A cupped board holds water, and water on a deck is slipperiness plus rot. Cupping is worth fixing for the safety reason before the cosmetic one.
• If you have to replace scattered boards on a weathered deck, take the replacements from a less visible area and put the new ones there — a new board next to a silvered one is very obvious for the first two years.
• Board gap spacers are cheap and worth using on a replacement so the new board sits with the same gap as its neighbours.
• Treating the underside of new boards before laying is a job you only get one chance at, and it is the face that matters most.

Common mistakes

• Screwing a cupped board down flat — The board is fighting you with the full force of differential shrinkage. It either splits at the fixings or pulls the screws out over the following months.
• Boxing in the deck to make it look tidy — It is the single most reliable way to cause cupping, rot and a mouldy void. Ventilated skirting looks almost identical and lets the deck breathe.
• One screw per joist — It is quicker and it lets every board rotate around the fixing line. Two screws per joist is not a preference, it is what keeps the board flat.

If it doesn't work

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