How do I repair a shed wall that has rotted at the bottom?

Cutting out the decayed bottom rail and cladding without dropping the wall, splicing in treated timber, and fixing the splash and ground contact that caused it.

Difficulty: intermediate
Time: 5 hr
Read: 5 min
Safety: warning

Short answer

Prop the wall, cut the rotten section out square with sound timber either side, splice in a treated replacement of the same section with a long scarf or a bolted lap, and replace the bottom of the cladding with a drip detail clear of the ground. Then fix the cause — ground contact, splash-back or a missing gutter — or you will do it again.

Shed walls rot in a band from the ground up to about 200mm, and always for the same reasons: the bottom rail is sitting on or near wet ground, and rain coming off an ungutted roof splashes up the cladding. The repair itself is ordinary carpentry. What makes it worth doing properly is that the same shed will be fine for another twenty years if you deal with the water, and will be back in the same state in four if you do not.

Safety

The bottom rail of a shed wall usually carries the wall above it and often the roof load through the corner posts. Do not cut a section out of a bottom rail without propping the wall above first. Wear a mask when cutting out rotten timber — decayed wood carries mould spores — and treat cut ends of new preservative-treated timber, which is chemically aggressive on skin.

What you'll need

Treated timber to match the bottom rail section
Cladding to match
Circular saw or a multi-tool for the cut-out
Props — adjustable props or stout timber and wedges
Coach screws or structural screws, exterior rated
End-grain preservative
Chisel and a hammer
Concrete or plastic bearer pads (optional)

Where a shed wall rots and how the bottom rail is spliced

Step by step

Find out how far the rot actually goes.Probe with a screwdriver upward and along in 100mm steps until you reach timber that resists firmly. Rot runs further along the grain than you expect, particularly in the bottom rail. Mark the sound points.
Work out what is holding the wall up.In most sheds the corner posts carry the roof and the bottom rail ties the wall together and holds the cladding. If the rot is in a corner post as well as the rail, this stops being a patch and becomes a rebuild of that corner.
Prop the wall before cutting anything.A prop or a stout timber under the wall plate at each side of the section you are removing, wedged down onto a solid base. Take the weight, do not lift the wall.
Remove the cladding above the repair.Enough courses to work on the rail and to let you fix the new cladding with a proper lap. Number the boards as they come off if they are shiplap or featheredge and you intend to reuse them.
Cut the rotten rail out square, 150mm into sound timber.Two vertical cuts, square and clean. Cutting back to the edge of visible rot leaves the decayed fibres that were about to fail. Give yourself the extra.
Splice in the new rail.Either a long tapered scarf joint glued and screwed, or the simpler and stronger option: butt the new piece in and bolt a treated splice plate across the joint on the inside face, extending 200mm each side. Use exterior-rated structural fixings.
Lift the whole rail clear of the ground.This is the part that decides whether the repair lasts. Bed the new rail on concrete or plastic pads, or on a strip of DPC over a slab, so no timber touches soil and air can move under it.
Treat every cut end and every new face.Sawn ends of treated timber are untreated wood in the worst possible position. End-grain preservative on all of them, and two coats on the underside of the rail before it goes down.
Re-clad with a drip at the bottom.The lowest cladding board should finish clear of the ground with its bottom edge cut at an angle or a small drip batten fitted, so water leaves the board rather than tracking back under it onto the rail.
Deal with the water before you put the tools away.Gutter the roof, take the ground away from the wall with a gravel margin, and cut back any planting against the cladding. The repair fails without this.

Tips

Photograph the wall construction before you take any cladding off. Shed framing varies and reassembling from memory takes twice as long.
If the shed sits on slabs, this is the moment to add a strip of DPC between slab and rail. It costs almost nothing and it is the difference between a repair and a permanent fix.
Buy the replacement timber a few days ahead and let it sit outside under cover. New treated timber is wet and shrinks noticeably as it dries.
A gravel margin 200mm wide around the shed stops splash-back and keeps vegetation off the cladding — the cheapest single improvement you can make.

Common mistakes

Filling the rot with wood filler or resin — Filler restores the surface, not the strength, and it seals damp decayed timber in where it carries on rotting. Structural timber gets cut out and replaced.
Cutting the rail out without propping — The wall above drops onto the saw, pinches the blade and the cladding racks. Even a small shed wall carries enough load to make a mess of this.
Putting the new rail back on the same wet ground — Treated timber resists decay; it does not survive permanent ground contact indefinitely. Getting the rail clear of the ground is the whole point of the exercise.

If it doesn't work

The rot goes into the corner post as well

Cause: Water has been standing at the corner, which is where two walls' worth of run-off meets. — Fix: A corner post repair means taking two walls' cladding back and propping both. It is doable but it is a day's work, and on a cheap shed a new shed is often the better answer. Price both honestly.

The floor is soft where it meets the rotten wall

Cause: The same water reached the floor bearers, which sit at the same level as the rail. — Fix: Do the floor at the same time. Access to bearers is never easier than with the wall open.

The wall was rotten again in three years

Cause: The cause was never fixed — the rail is still in ground contact, or an ungutted roof is still splashing the cladding. — Fix: Lift the rail onto pads with a DPC strip, gutter the roof, and take the ground back from the wall with a gravel margin. The carpentry is the easy half of this job.

Questions people ask

Is it worth repairing or should I replace the shed?

Repair if the frame, roof and floor are otherwise sound and the rot is confined to one wall's bottom rail — that is a few hours and a small amount of timber. Replace if rot has reached two or more corner posts, or if the floor and the walls have gone together, because at that point you are rebuilding the shed in situ at higher cost than buying one.

Can I just replace the cladding and leave the rail?

Only if the rail probes sound throughout. New cladding fixed to a rotten rail has nothing to hold it, and the screws will pull out within a season.

What timber should I use?

Preservative-treated softwood to the same section, ideally treated to a use class rated for ground contact or close to it. The treatment class matters more here than the species.