Replace a rotten shed floor — HomeGraft

Replace a rotten shed floor by lifting old boards, treating or renewing bearers, and fitting new weatherproof timber panels.

SF-01Clear the shed and check the damageEmpty the shed completely so you can see the whole floor. Press down with your foot across every board to find soft, spongy or crumbling spots, paying close attention to where the floor meets the walls, since rain splashback rots this area first. Decide whether the whole floor needs replacing or just the worst sections.

1 pair · Work gloves

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1 each · Torch

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SF-02Prop the shed walls before lifting anythingMany sheds rely on the floor to hold the walls square. Before you lift a single board, wedge lengths of timber diagonally into the corners inside, or brace across the doorway, so the walls can't splay outwards once the floor is gone. If your shed has a strong timber base frame that supports the walls on its own, you can skip this.

4 each · Timber batten 3x2, 2.4m (for props)

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SF-03Lift the old floorWork board by board with a crowbar, prising up nails or screws as you go rather than levering hard on one spot. Pull out every old nail from the boards and bearers, or hammer them flat, so nothing catches your hands or damages the new floor later. Bag up the rotten timber as you clear it.

1 each · Crowbar

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1 pack of 10 · Heavy-duty rubble sacks

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SF-04Check the bearers and treat or replace themThe bearers, the timber battens the floor rests on, rot fastest because they sit closest to the ground. Push a screwdriver point into each one; if it sinks in easily, the timber is rotten and must be swapped for new treated timber of the same size. Sound bearers just need a fresh coat of wood preservative before you build on top of them.

3 each · Treated timber bearer 47x100mm, 2.4m

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1 each · Wood preservative, 5L

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1 each · Paintbrush

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SF-05Level the base if it has sunk or crackedIf your shed sits on paving slabs or a timber grid that has sunk, sagged or cracked, sort this out before fitting new bearers on top. Reset or add slabs on a bed of sharp sand, checking level across both the width and length with a spirit level. A floor built on an uneven base will flex and crack again quickly.

4 each · Paving slab 600x600mm

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1 each · Sharp sand, 25kg bag

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1 each · Spirit level, 600mm

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SF-06Cut the new floor panelsUse marine plywood or OSB3 at least 18mm thick; anything thinner flexes underfoot and won't last outdoors. Measure the shed's internal footprint and mark your sheet, then cut with a circular saw, supporting both sides of the cut so the sheet doesn't snap before you finish. Leave a small 5mm gap at each edge so the panel doesn't grip the walls and trap moisture.

1 each · Marine plywood or OSB3, 18mm, 2440x1220mm sheet

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1 each · Tape measure

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SF-07Seal the cut edges and undersideCut edges soak up water fastest, so this is where rot starts again if you skip this step. Paint every cut edge and the whole underside of each panel with wood preservative or exterior floor paint before fitting, and let it dry fully before moving on.

1 each · Wood preservative, 5L

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1 each · Paintbrush

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SF-08Fix the new floor downScrew each panel into the bearers every 300mm along the edges and across the middle, using 50mm decking screws, and countersink each screw slightly so nothing catches on feet or stored items. If your shed has a mains power supply run to it, cables can run under the floor in places you don't expect, so if you're at all unsure what's beneath a fixing point, isolate the circuit at the consumer unit before drilling or screwing there.

1 box · Decking screws, 50mm, box of 200

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SF-09Refit edge trim and give the floor a final coatIf your shed has timber edge trim hiding the join between floor and walls, refit it now or replace it if it was rotten too. Once everything is fixed down, give the whole floor a final coat of exterior floor paint or preservative and leave it to dry fully before you walk on it.

1 each · Timber edge trim, 2.4m

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1 each · Exterior floor paint, 2.5L

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SF-10Tidy up and move back inSweep out the shed and put your tools away. Stack the old rotten timber ready for disposal; treated wood often can't go in with general garden waste, so check your local council's rules before you get rid of it. Once the floor is fully dry, move your things back in.

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