Replace a rotten deck board

Replace a single rotten deck board safely, checking for hidden cables first, then refit with rust-proof screws.

DK-01Check the board and buy a matchPress and flex the suspect board to confirm it's soft, spongy or crumbling rather than just weathered. Measure its length, width and thickness so you can buy a matching treated softwood or composite board. Check a few boards either side too — rot often spreads along a run.

1 each · Replacement deck board (treated softwood, 32mm x 120mm x 2400mm)

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DK-02Gather your toolsYou'll need a flat bar or crowbar, a drill/driver, a saw to trim the new board, a tape measure, and a cable and pipe detector. Put on gloves and a dust mask — old decking is often mouldy or treated with chemicals you don't want to breathe in.

1 each · Flat bar/crowbar

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1 pair · Work gloves

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1 each · Dust mask

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DK-03Check for cables and pipes before you lift or drill anythingGarden lighting, pond pumps and hot tub supplies often run under decking, so sweep the area with a cable and pipe detector before prising up boards or driving in screws. Detectors can miss cables that are deep or run in conduit, so still work carefully and stop immediately if you feel resistance or see a cable. If there's any doubt, isolate the relevant circuit at the consumer unit first, and know where your outdoor water isolation valve or stopcock is in case you nick a pipe.

1 each · Cable and pipe detector

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DK-04Remove the rotten boardSlide the flat bar under the board near a joist and lever gently upwards, working along its length. Pull or unscrew any fixings left behind rather than yanking the board free, as old screws often shear. Lift the board clear and set it aside for disposal.

DK-05Check and treat the joists underneathPress a screwdriver into the exposed joist top — if it sinks in easily, the joist is also rotten and needs proper repair before you go further. If the timber is sound, brush off debris and apply a wood preservative to the exposed top of the joist to protect it going forward.

1 each · Wood preservative (500ml)

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DK-06Measure and cut the new boardMeasure the gap exactly, including the small expansion gaps you'll leave at each end (around 5mm). Mark the new board and cut it square using your circular saw, supporting both ends of the cut so it doesn't drop and splinter.

DK-07Treat the cut endsFreshly cut ends have no factory treatment, so they rot fastest if left bare. Brush wood preservative onto both cut ends and let it soak in and dry fully before fitting, following the drying time on the tin.

1 each · Wood preservative (500ml)

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DK-08Fit the new boardLay the board in place and use 5mm spacers at each end and against neighbouring boards to keep gaps even for drainage. Pre-drill pilot holes near the ends to stop the board splitting, then fix with stainless steel decking screws driven straight down into the centre of each joist below.

1 box · Stainless steel decking screws (5mm x 50mm, box of 200)

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1 pack · Timber spacers (5mm)

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DK-09Check the finishRun a hand over the board to check it sits flush with its neighbours, with no screw heads standing proud. Check the gaps are even along the full length, and tighten any screw that hasn't seated properly.

DK-10Tidy up and dispose of the old boardSweep away sawdust and off-cuts, and bag the rotten board for disposal — treated timber usually can't go in a home compost or wood burner, so check your council's guidance for treated wood waste. Wipe down and dry your tools before putting them away.

1 each · Heavy-duty rubble/waste bag

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