A garden shed built on an unlevel base will have doors that stick or won't close, walls that slowly twist out of square, and a floor that rocks underfoot. Getting the base level before building — or re-levelling an existing shed — prevents all of these problems and extends the life of the structure significantly.
Tools and materials for levelling a shed base
A spade or rake, gravel or compacted hardcore, timber or concrete paving slabs for the base, timber shims if needed, and your phone with Spirit Level Online open in the browser.
Why a level base matters so much
Sheds are rectangular structures — they rely on all four corners being at the same height to stay square. Even 20–30 mm of difference across a 2.4 m shed base is enough for the frame to rack (twist out of square), which causes doors to bind and walls to gap at the corners. Unlike a house, a shed has no deep foundation to correct for ground movement — the base is everything.
Step-by-step: levelling a new shed base
Clear and compact the ground. Remove turf, topsoil, and any soft material from the shed footprint. Soft ground under a base will settle unevenly over time. Compact the subsoil firmly.
Lay a gravel drainage layer. A 75–100 mm layer of compacted gravel or hardcore allows water to drain away from under the shed, preventing the base from shifting with moisture. Rake it roughly level by eye first.
Place the base slabs or bearers. Position your concrete paving slabs, railway sleepers, or timber bearers on the gravel. Start with one corner and work outward.
Check the first slab for level. Place your phone flat on the first slab and open Spirit Level Online in Surface Mode. Adjust the gravel beneath until the reading shows 0.0° in both directions.
Bring each subsequent slab to the same level. Add or remove gravel beneath each slab until it matches the first. Check across multiple slabs by placing your phone flat on a long straight board spanning two slabs at once.
Check diagonals. Once all slabs are individually level, place the board diagonally across opposite corners and check the reading. Both diagonals should read the same — this confirms the base is not only level but also flat.
Check again after placing the shed floor. Once the shed floor panel is down, check it with the spirit level before erecting the walls. Any flex in the floor can introduce a new slope not visible from the slabs alone.
Tip: Check level again after winter. Frost heave can shift a shed base by several centimetres over winter, especially on clay soils. Check the base level each spring and re-shim if needed. Catching a small shift early prevents the frame from racking.
Re-levelling an existing shed
If your shed door has started sticking or the floor rocks, the base has probably shifted. Place your phone flat on the shed floor and check the level. If it's off, you'll need to jack one side slightly, add shims or packing under the base, and lower it back down. A car jack or bottle jack placed under the floor beam works well for this — lift just enough to slide shims in, no more.
For all levelling jobs outdoors and indoors — shed bases, patio slabs, fence posts, and more — Spirit Level Online works directly in your browser with no app download required.