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Why the base matters more than you think

A wooden shed is only ever as good as what sits beneath it. Get the base right and your building will stay square, dry and usable for decades. Get it wrong and you will spend the next few years fighting sticking doors, damp patches and a floor that seems to move with the seasons.

Wood is a natural material and it behaves accordingly. It expands and contracts with the weather, and it will happily soak up moisture from any surface it touches. A shed resting on soft, uneven ground will twist and settle, and once the frame is out of square, no amount of adjusting hinges will put it right.

The good news is that there are only a few realistic options for a domestic garden, and each suits a particular set of circumstances. The trick is matching the base to your soil, your shed and how much effort you fancy putting in.

Ground preparation and drainage come first

Whatever base you choose, the ground beneath it needs to be firm and free-draining. Start by clearing the area of turf and topsoil, then dig down far enough to accommodate your chosen base plus a compacted sub-base of around 75–100mm of MOT Type 1 hardcore. Rake it level, then compact it properly with a plate vibrator — a hand tamper will do for a tiny shed, but hire the machine if you can, because a genuinely firm sub-base is what stops everything above it from sinking.

You want a small fall across the site, roughly 1:60, so that water runs away from the shed rather than pooling underneath it. This matters more than most people expect. A perfectly flat base on heavy clay soil in a wet winter is simply a shallow pond with a shed on top.

Also dig out a strip of around 300mm all the way around the base and fill it with gravel. This French drain effect catches surface water before it reaches the shed and makes a real difference on poorly drained ground.

Concrete: the heavyweight option

A poured concrete slab is the most robust base you can build, and it is the one I would choose for a large shed, a workshop or any building that will carry serious weight inside it — a workbench, stored tools, a ride-on mower.

A typical slab for a garden shed is 100mm thick, with a single sheet of reinforcing mesh set in the middle if the shed is over about 2.4 metres square. Timber the edges to form a level box, pour in one continuous go, and work the surface with a float until it is smooth. Sealing the edges with a strip of damp-proof membrane beneath the slab helps stop moisture rising through the concrete.

Concrete needs around 48 hours to become firm enough to walk on and up to a week before you place the shed, depending on the weather. It is hard work and it is not cheap, but it will not move. One caution: do not let a concrete slab extend beyond the shed walls and slope it slightly away from the centre, otherwise rainwater will run under the building instead of away from it.

Paving slabs: a practical middle ground

Paving slabs are the most popular choice for a standard garden shed, and with good reason. They are quicker and cheaper than concrete, straightforward to lay, and easy to remove or adjust if you change your mind.

For a typical 6x4 or 8x6 shed, use slabs of at least 50mm thickness and lay them on a full mortar bed or a well-compacted sharp sand and cement mix, not just sand. Bed them so the tops sit perfectly flush — a 2 metre straight edge or a long spirit level laid across the whole footprint is the only reliable way to check. Any slab that rocks when you press a corner needs lifting and re-bedding.

Keep the whole pad about 50mm larger than the shed footprint on every side to shed water clear of the timber, and arrange the slabs so their joints do not run directly under the middle of the floor bearers. If you are using the base as a patio later, slabs are the obvious way to go.

Timber bearers: quick, cheap and surprisingly effective

Many modern sheds are supplied with a wooden floor already built on timber joists, and these can simply be set on pressure-treated bearers laid across a levelled, compacted gravel or hardcore pad. Use bearers of at least 50mm x 75mm, and lay them so the shed's own joists run across them rather than along them.

The main advantage is ventilation. Air can circulate under the floor and keep the underside of the shed dry. The main disadvantage is longevity — timber in contact with damp ground will eventually rot, even when pressure-treated. To reduce the risk, sit the bearers on concrete blocks or paving slabs rather than directly on soil, keep them clear of the ground, and space them no more than 600mm apart so the floor does not sag between them.

Choosing between them

• Small shed on well-drained soil: paving slabs will serve you perfectly well.
• Large shed, workshop or heavy contents: a concrete slab is worth the extra effort.
• Bare-bones budget or a temporary building: timber bearers on a gravel pad, provided you accept they will need replacing eventually.
• Sloping or awkward site: concrete, or slabs on a stepped and properly built up sub-base — never just pack one edge with bricks.

A final thought: check whether the building will need planning permission before you start digging, particularly on a corner plot or in a front garden. It is far easier to sort out paperwork beforehand than to move a shed you have just installed.