How do I replace a rotten decking board?

One rotten board is almost always a symptom: water has been sitting somewhere it should not, and the joist under that board is likely to be wet too. Replacing the board without checking underneath means doing it again next year.

Safety

Step by step

1. Find the extent of the damage first.Push a screwdriver into the board along its length and into the joist below through the gap. Soft timber goes. Map it before cutting anything.
2. Locate the joists.The existing screw lines show where the joists are. Mark the joist centres on the neighbouring boards so you can cut over the middle of a joist.
3. Remove the screws you can.Old decking screws often shear. An impact driver and a fresh bit gets most out; the rest will be cut through.
4. Cut the board over a joist centre.A multi-tool or a circular saw set to the board thickness, plunging carefully. Set the depth precisely — cutting into the joist below weakens it.
5. Lever the section out from the middle.Work from the middle towards the ends, protecting the neighbouring boards with a scrap of timber under the crowbar.
6. Inspect and treat the joist underneath.This is the point of the exercise. If the joist top is soft, cut out and sister a new length alongside. If it is sound but damp, let it dry and brush on preservative before covering it.
7. Find and fix the water source.A leaking downpipe, a planter sitting on the deck, a blocked gap, or a low spot holding water. The new board will go the same way if the cause is still there.
8. Cut and fit the new board.Ends landing centrally on joists, the same 5-6mm gap either side, two screws per joist 20mm from the edges, pre-drilled at the ends. Treat every cut face with end-grain preserver first.
9. Weather the new board before finishing.New treated timber will not take stain properly and will look obviously new. Leave it a few weeks, then apply the same finish as the rest of the deck.

Tips

• Keep a couple of spare boards from the original build. Decking profiles change and an exact match later is unlikely.
• If several boards in one area have gone, take up the whole area and deal with the joists properly rather than patching board by board.
• A board that is sound but stained can often be turned over and refitted, giving a clean face for nothing.

Common mistakes

• Replacing the board without checking the joist — The water that rotted the board went past it into the joist. A new board on a rotten joist is unsafe and short-lived.
• Cutting the new board short so it does not land on a joist — The end has no support, flexes underfoot and lifts. Both ends of every board must bear on a joist centre.

If it doesn't work

Questions people ask