Garden Building Bases: A Complete Guide to Timber, Slab & Concrete Foundations

Getting the base right is the single most important part of installing a new shed, summerhouse, log cabin or garden room. Get it wrong and you risk a floor that rots, a door that won't shut, or a building that slowly sinks and twists out of square. Get it right and your building will stay solid and level for decades.

This guide covers the three main base types used for garden buildings in the UK — timber bearers, paving slabs and concrete — including which one suits which building, and step-by-step instructions for building each one yourself.

What every garden building base needs, regardless of size

Whichever base type you choose, there are two rules that never change:

1. It must be flat, level and square, with no risk of subsidence later on. The entire floor area needs to be supported — for a log cabin, that means the foundation bearer running the full perimeter. If you're adding a floor, that also needs full support.
2. It should match the building's footprint as closely as possible. A base larger than the building means rainfall hits solid paving or concrete and splashes back up against the walls — you can see this on our own show site at Woodmeadow Garden Centre in Northampton, where the display buildings sit on a larger concrete and slab area and need regular cleaning as a result. A base cut to the exact footprint lets rain drain straight into the surrounding grass or gravel instead, and avoids low spots where water can pool under the building and rot the floor over time.

It's also worth considering a damp proof membrane in the sub-base, particularly for buildings that will be used for storage or as a living space.

Which base type suits your building?

Base type	Best for	Typical cost	Permanence
Timber bearers	Small to medium sheds, summerhouses	Lowest cost	Easiest to remove or relocate
Paving slabs	Most sheds and summerhouses, some larger buildings if the groundwork is done well	Mid-range	Can be lifted and moved with minimal disruption
Concrete	Log cabins, large summerhouses, wooden garages	Highest cost	Permanent — very difficult to remove once laid

If you're not sure which is right for your building, get in touch and we'll advise based on the size and weight you're working with.

Timber bearer shed base: step-by-step

The simplest and cheapest option, and a good fit for smaller sheds and summerhouses.

1. Clear the site of overhanging branches, hedges and bushes.
2. Mark out the area with pegs and string, matching the building's footprint as closely as possible.
3. Clear vegetation and dig down 3–6 inches, depending on the bearer size you're using.
4. Level the ground with a spirit level and compact it thoroughly, especially where soil has been disturbed.
5. Consider laying a weed suppression membrane before positioning your bearers.
6. Lay the bearers, checking level across both planes. For extra stability, bed them on bricks or slabs.

For our own Taylors-manufactured sheds, we run bearers along both ends and then space them roughly a foot apart, so the whole floor is supported along its length — we'll advise on the right configuration for your building. Pressure-treated timber in 6"x2", 4"x2" or 3"x2" works well; some people use reclaimed sleepers, though these are harder to get level.

Paving slab shed base: step-by-step

A popular middle-ground option — solid support, and can be lifted later with minimal disruption.

There are two common approaches:

• Full coverage — slabs across the entire footprint. Usual for log cabins.
• Spaced slabs — laid at right angles to the floor joists roughly every 2 ft, using fewer slabs. If your building has a two-piece floor, make sure there's a central row of slabs supporting the join — ask us if you're not sure how your building is configured.

Steps:

1. Mark out and clear vegetation from the site. Check the area is square by measuring both diagonals — they should match exactly.
2. Compact the ground and get it as level as possible.
3. Lay a couple of inches of dry sand/cement mix (an alternative to a hardcore-and-wet-cement base) and compact it, checking level front-to-back and side-to-side.
4. Starting in a corner, bed your first slab level in all directions — this sets the reference point for the whole base. Adjust with a rubber mallet or piece of wood, adding or removing mix as needed.
5. Work outward a row at a time, measuring from the first slab throughout.

Every slab needs to be level across the full area, or the shed floor above it won't be properly supported.

Concrete shed base: step-by-step

The strongest option, and the one we recommend for log cabins, larger and corner or octagonal summerhouses, and wooden garages. The main downside is that it's very difficult to remove or adjust once laid.

Depth guide:

• Standard sheds/summerhouses (e.g. 10' x 14'): 3 inches
• Log cabins and wooden garages: 4 inches, on compacted, finely broken hardcore

Ideally, aim for roughly half the base depth above ground level.

Steps:

1. Clear the site as above, then peg out the area at each corner and roughly every 2 ft along the sides, using the outside dimension of the building.
2. Fix shuttering (ideally 2" thick) to the pegs at the depth of your finished base. Make sure corners are completely square, the top of the shuttering is level in both directions, and pegs don't protrude above it.
3. For large bases — we've built bespoke garage bases up to 45' x 30' — it's worth ordering ready-mix from a concrete lorry. For smaller bases, mixing it yourself is usually more economical.

Mixing your own concrete:

Use a consistent measure — a 3-gallon bucket works well. A good general ratio is 1 part cement to 5 parts "all-in 20mm" ballast (a gravel-based aggregate available from any builders' merchant, usually sold in 40kg bags — 1.25 bags per cubic foot of concrete needed). As a rule of thumb, calculate your required volume, add a third for compaction, then round up — it's far easier to have a little extra than to run short.

As a guide, an 8' x 6' shed base needs roughly 20 bags of all-in 20mm ballast to 4 bags of cement (the 1:5 ratio).

Mix gradually with water until the concrete is a uniform colour — wet enough to be workable, not so wet that it weakens the mix.

Laying and finishing:

1. Lay concrete in layers, tamping each one into the corners and against the shuttering.
2. Once full, drag a straight beam across the top of the shuttering in a sawing motion to level the surface.
3. Once it's gone off slightly, smooth the surface with a wooden or plastic float (or hire a finishing machine for larger areas).
4. Leave the base to set for 3–4 days before removing the shuttering, then finish the edges with pea shingle or turf.

A few extra things to watch for:

• Winter pours — protect fresh concrete from frost with plastic sheeting, as frost damage can be significant.
• Large bases — introduce expansion joints in anything above roughly 12' x 12'.

FAQs

Do I need a base for a garden building? Yes. Placing a shed, summerhouse or log cabin directly on grass or soil leads to damp, rot and an uneven floor within a few years. Every building needs a flat, level, well-supported base.

What's the best base for a log cabin? Concrete, in almost all cases. Log cabins are significantly heavier than standard sheds, and a concrete base gives the even, permanent support needed to stop the structure settling or twisting out of square.

Can I put a shed base straight onto grass? No — the ground needs to be cleared, levelled and compacted first, with an appropriate sub-base (hardcore, sand/cement or bearers) underneath, whichever base type you choose.

How big should a shed base be? As close to the building's exact footprint as possible. A base that's larger causes rain to splash back up against the walls; one that's smaller leaves parts of the floor unsupported.

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Unsure which base is right for your building? Contact our team or visit us at Woodmeadow Garden Centre, Kettering Road, Northampton — we're happy to advise, or you may prefer to hire a professional installer for larger concrete bases.