How do I level a shed that has sunk on one side?

A shed that has dropped at one corner tells you almost immediately by the door: it stops latching, then it fouls the frame, then it will not shut at all. The tempting fix is to plane the door, which hides the problem for a season while the frame keeps racking. The structure is only ever as square as what it sits on, so the base is the thing to correct.

Safety

What you'll need

• Trolley jack or bottle jack, 2 tonne or more
• Solid packing — concrete blocks, paving slabs, hardwood blocks
• Long spirit level or a level and a straightedge
• Screwdriver for probing timber
• Spade and a hand tamper
• Sharp sand or MOT type 1 (optional)
• Concrete bearer pads (optional)

Step by step

1. Empty the shed completely before you touch anything.The contents are usually a good proportion of the weight, they are concentrated along one wall, and they will slide when the floor tilts. This is not an optional step.
2. Establish how far out it is, and in which direction.A long level across the floor in both directions, and along each wall plate. Note the numbers. A shed dropped at one corner behaves differently from one that has tipped along a whole side.
3. Probe every bearer along its whole length with a screwdriver.Push hard. Sound treated timber resists; rotten timber takes the blade with no effort. If the bearer has gone soft the shed has not sunk at all — it has collapsed onto the ground, and jacking it up without replacing the bearer just repeats the failure.
4. Look at the ground under the low corner.Slabs tipped or sunk, a bearer bedded into soft soil, a rut where water runs, roots. If you cannot see, dig a small inspection hole beside the corner. Settlement almost always has a visible cause.
5. Get the jack onto something solid, under a bearer, not under the floor.A slab or a stout board under the jack to spread the load, and the jack head bearing directly on a bearer or the frame rail — never on the floor deck, which will simply punch through.
6. Lift in small stages and pack as you go.20 to 30mm at a time, then slide solid packing under the bearer beside the jack and lower onto it. Repeat. The shed is supported by the packing at all times, never by the jack, and never by nothing.
7. Watch the whole structure while you lift, not just the corner.Doors and windows tell you what the frame is doing. If a door starts to bind hard or a cladding joint opens up, you are twisting the shed rather than lifting it. Come back down and lift a longer length rather than one point.
8. Fix what caused it before you lower it back.Dig out the soft ground, compact sharp sand or MOT type 1 in thin layers, and bed a slab or a concrete pad under the bearer. Packing a shed up on bricks over soft ground buys one wet season.
9. Re-check the level and the door, then reload.Level across both diagonals, then open and close the door before anything goes back inside. Get it right while the shed is empty.

Tips

• Take photographs from the same two positions before and after. Judging by eye whether a shed has come back to square is unreliable.
• Concrete bearer pads sold for sheds are cheap and are far better than bricks, which are not frost-resistant when laid flat and soak up water from below.
• If the shed sits on a timber frame on gravel, the frame is the thing to level, not the shed. Lift the shed clear on blocks, correct the frame, put it back.
• Do the levelling in dry weather. Compacting wet clay produces a base that dries out and shrinks away again.

Common mistakes

• Packing under the floor deck rather than the bearer — The deck is a thin sheet spanning between bearers. A point load under it lifts a bulge in the floor, does nothing for the structure, and eventually punches through.
• Lifting the whole side in one movement — It twists the frame. Cladding joints open, glazing cracks and the door goes permanently out of true. Lift a little, pack, move along, repeat.
• Using bricks laid flat as packing — Common bricks are not frost-resistant and are not intended to be a foundation. They spall, crumble at the arris and let the shed down again in a couple of winters.

If it doesn't work

Checklist

• Shed emptied
• Every bearer probed for rot
• Level readings taken and written down
• Jack on a spread load, bearing on a bearer
• Solid packing under the shed at all times
• Cause of the settlement dug out and corrected
• Door checked before reloading

Questions people ask