Total Sheds
Maintenance Care · 6 min read
Soft spots in your shed floor? How to cut out the rot, sister the joists and fit new boards — plus how to fix the ground moisture that caused it.
Repair Guide
Soft boards underfoot are fixable in an afternoon — but they're also the shed telling you about the ground beneath it. Here's the proper repair, and the cure.
A rotten floor board is one of the most fixable faults a shed can have — a saw, exterior-grade ply and a tin of preservative will see it right in an afternoon. But before you cut anything, understand this: a rotten floor is almost never the problem. It's the symptom. Replace the boards without fixing what's underneath and you buy two or three years, not twenty. Fix the cause as well, and the repair lasts as long as the shed.
Nine times out of ten, a rotten floor is a ground-moisture problem. A shed sat directly on earth, or on slabs that hold water, has damp air trapped underneath with nowhere to go. The underside never dries, and rot works up through the boards until you feel it as a soft spot. Even tanalised, pressure-treated timber gives in eventually if it stands in permanent damp.
Before you buy any timber, crouch down and look under the shed. No air gap beneath the floor — no bearers, no daylight visible underneath — and you've found your cause. Our animated base guide shows what a proper base looks like; the shed base article weighs the options. Sort the airflow alongside the repair, or you're patching a hull that's still taking on water.
Isolated rot — one soft patch with sound timber around it — is a straightforward cut-and-replace. Work in order.
Clear everything out and probe the floor with a flat-head screwdriver. Sound timber pushes back; rot gives way with barely any pressure — if it's soft, it's gone. Mark a line at least 30cm beyond the last soft spot in every direction, because rot travels further than it shows.
Tip: Probe the whole floor while you're down there — ten minutes now saves a second repair later.
Set a circular saw to the exact depth of the boards — no deeper, or you'll score the joists — and cut back to the centreline of the nearest joists or bearers so the new board has somewhere to land. Square off the corners with a hand saw.
Careful: Sweep for screws and nails along your cut line before the saw goes in.
With the boards out, probe every joist you can reach. If one is soft, sister a new length of tanalised timber alongside it — bedded on exterior adhesive and screwed through every 300mm or so — so the new floor lands on sound wood.
Careful: Soft joists mean sistering, not patching. A board fixed over a rotten joist has nothing to hold onto, and the repair fails from below.
Cut the new section from 18mm exterior-grade ply, or matching tongue-and-groove boards if you want the repair invisible. Then treat every face — and every cut edge — with wood preservative before it goes anywhere near the floor.
Tip: Cut edges and end grain drink moisture fastest — and they're the surfaces everyone forgets.
Screw the new section to the joists with exterior-grade screws — two per joist crossing — leaving a 2–3mm expansion gap at the edges so the board can move with the seasons.
Give the repair and the surrounding boards a full coat of preservative, worked into the joints. You want one continuous protected surface, not a treated island in an untreated floor.
If yours is a dip-treated budget shed already showing rot in the walls or frame, be honest about where the money goes. A £60 floor repair on a building that's failing around it isn't thrift — it's a deposit on the next repair. At that point, price a shed made properly: FSC-certified timber on tanalised framing, backed by a 15-year anti-rot guarantee. Have a look at what we build and do the sums. Sometimes the repair still wins, and that's fine too.
A bearer base lifts the floor clear of the ground so air moves beneath it. Airflow is the cheapest rot prevention there is.
Keep the surrounding ground clear and below floor level, so rain drains away from the shed rather than under it.
Water from a failed roof runs down inside the walls and pools on the floor. Check the covering every autumn, while a leak is small.
Fuel, paint and fence treatment reach the floor eventually. Stand them on trays so a slow leak stays a wipe-up, not a soak-in.
Once a year, empty a corner, probe the boards and look underneath. Rot caught at one board never becomes a re-floor.
Two of those deserve proper reading: our felt vs EPDM guide compares roof coverings straight, and everything for the treating — preservative, paint, timber base kits, EPDM kits — is in the aftercare shop.
Please don't. It feels quick, but you're sealing damp, rotten timber under a lid — the rot keeps working, the ply has nothing solid to screw into, and within a season or two the whole sandwich is soft. Cut the rot out, always.
18mm exterior-grade is the standard for shed floors and matches most factory-fitted boards. Match the existing thickness so the floor stays level; thinner than 15mm flexes between joists and works fixings loose.
Probe them like the boards — top, sides and ends, especially nearest the ground. Soft at one point, sister it with new tanalised timber. Spongy along the whole length, or crumbling at the ends, and you're re-flooring, not repairing.
The base, almost certainly. A young floor doesn't rot from age; it rots from sitting on bare earth or water-holding slabs with no air underneath. Fix the base, or the new boards will follow the originals on the same schedule.
Not all treatment is equal. Budget sheds are usually dip-treated — a thin surface coat that weathers off within a couple of years. Pressure treatment forces preservative deep into the timber and lasts decades. Even so, nothing beats permanent standing water forever: treatment buys time, airflow buys decades.
Gather the lot before the first cut — a half-open floor shouldn't wait a week for screws.
Preservative, exterior paint, timber base kits and EPDM roof kits — everything that keeps a repaired floor dry, from the workshop that builds the sheds.
Shop aftercareReady to put it into practice? Every building is made in our West Midlands workshop with free mainland delivery.
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