Technical Installation · 6 min read

Shed Foundation Options Explained: Which Base Is Best?

The four shed foundation options that work, the ones that don't, and how to choose the right base for your garden building — from the makers.

Groundwork, done right

Foundation options for garden buildings, explained

Concrete, slabs, timber bearers or plastic grids — every base that works, and how to pick yours without overthinking it.

We've been making garden buildings in our West Midlands workshop since 1995, and one thing has never changed: the base decides how the building ages. Every foundation that works — there are only four worth your time — must deliver three things. Firm, so the floor doesn't flex. Level, so the doors swing true and the frame stays square. Dry, so the timber above isn't drinking from the ground.

Get those three right and the rest is simply choosing your trade-offs: money against effort, permanence against flexibility. Here's the honest version of each option — for the groundwork itself, start with our shed base knowledge hub.

The four that work

Concrete slab

The permanent option. Poured concrete over compacted hardcore — the most effort and cost, and the strongest base you can build.

Paving slabs

Quicker and good value. Slabs on a sharp sand and cement bed suit small-to-mid sheds, as long as every slab sits dead flat.

Timber base kit

A pressure-treated bearer frame that lifts the floor for airflow. Quick to fit, and movable if you ever re-plan the garden.

Plastic grid system

Light, permeable and fast. Interlocking grids on a levelled, membraned sub-base — ideal where digging isn't welcome.

The four options in detail

1. Concrete slab — the permanent option

The full job: 75–100mm of concrete over 75mm of compacted hardcore, held in timber formwork and left to cure for 48 hours or more before anything sits on it. It asks the most of you — digging, mixing, waiting — and repays every bit. For large or heavy buildings, workshops, and anything staying put for decades, nothing else comes close. The honest downside is commitment: concrete doesn't move house when you re-plan the garden. Pouring one yourself? Our shed base preparation guide walks the job through.

2. Paving slabs — the sensible middle

Slabs on a sharp sand and cement bed, over ground that's been levelled and compacted first. Quicker than concrete, good value, ideal for small-to-mid sheds. One rule matters above all: every slab flat, none rocking. A slab that shifts under your boot will shift under a building.

3. Pressure-treated timber base kit — the one we make

A tanalised bearer frame between the ground and your floor, lifting the building so air moves underneath — the kindest thing you can do for a timber floor. Quick to fit on reasonably level ground, it pairs well with slabs or firm ground, and if you ever re-plan the garden it moves with the building. We make these in the same workshop as our buildings, from the same pressure-treated timber — you'll find them in the shop.

4. Plastic grid systems — light, fast, permeable

Interlocking grids over a levelled sub-base of sharp sand or gravel, with a weed membrane beneath. Get that sub-base right and grids are excellent: light to carry, fast to lay, and permeable, so rain drains through rather than pooling. They earn their keep wherever digging is unwelcome — many allotments ban concrete outright, which is why grids feature heavily in our guide to the best shed for an allotment.

Concrete, or something lighter?

Signs you want concrete

The building is large, heavy or working hard — a workshop, not a tool store
You're certain of the spot and want it there for decades
Machinery, benches or heavy racking are going inside
You'd rather do one hard weekend than revisit the base again

Signs a lighter base wins

It's a small-to-mid shed and well-laid slabs will carry it comfortably
You might re-plan the garden — a timber base kit moves with the building
Digging or concrete isn't allowed, as on many allotment plots
Budget and time are tight, and the building doesn't demand more

What doesn't work — and the rule that saves every base

Some things look like a base and aren't. Bare earth and grass hold moisture against the floor and sink unevenly within a season. Decking is built for people, their weight spread across the boards — not for the point loads of a building's bearers — and unless engineered for the job it will bow and twist. And any base smaller than the building's footprint leaves the frame overhanging thin air; cover the full footprint, edge to edge. Still choosing? The Build Studio shows the exact footprint as you configure your building.

Then the rule that applies whatever you build: finish the base slightly proud of the surrounding ground, so rain runs away rather than towards the building, and get it level to within 10mm across the footprint. And have it ready before delivery day — the base can't go down while the lorry waits. See what to expect on our delivery page; delivery is free across the UK mainland.

Whichever you choose

Five steps that apply to every base, concrete slab to grid on gravel.

1
Mark out the footprint
Peg out the full footprint with string lines, plus a little working room around it.
Tip: Measure both diagonals — when they match, your rectangle is square.
2
Level the ground
Strip turf and topsoil, dig to firm ground, and compact what's left. Every base is only as good as what it sits on.
3
Build the base
Follow the method for your choice — pour and cure, bed the slabs, fix the bearer frame, or lay membrane and grids.
Careful: Concrete needs at least 48 hours to cure before anything sits on it. Don't rush this.
4
Check twice
Spirit level in both directions and corner to corner. Within 10mm across the footprint, nothing rocking, everything firm.
5
Book delivery
With the base finished and cured, book your date knowing the building can go straight down the day it arrives.

Foundation questions, answered straight

What's the cheapest shed base that's still adequate?

For most small-to-mid sheds, paving slabs on a sharp sand and cement bed — modest materials, one careful day's work, a base that lasts. Plastic grids run it close where the ground suits them. The genuinely cheapest base is the one you only build once.

Can I put a shed on my decking?

Almost always no. Decking is designed for people, whose weight spreads across the boards. A garden building loads everything through a handful of bearers — point loads decking wasn't engineered to carry, so it bows and twists under a building.

Does a shed base need planning permission?

The base itself doesn't, and most garden buildings fall under permitted development. Rules tighten in conservation areas, near listed buildings and for larger or taller buildings — check with your local authority if any apply. On allotments the rules come from your tenancy agreement, not the council; many plots ban concrete, which is where grids come in.

How level is level enough?

Within 10mm across the whole footprint. That sounds fussy until you remember the frame amplifies every error: a base 20mm out means doors that catch and a floor that creaks. Check with a spirit level in both directions and corner to corner before calling it done.

Grid or slabs for an allotment shed?

Usually grids: light enough to carry down a narrow path, permeable so the plot keeps draining, laid without concrete, and removable when a tenancy ends — everything allotment committees tend to insist on. Check your plot rules first, but where concrete is banned, a grid on a membraned sand or gravel sub-base is the answer.

The pre-delivery base checklist

Run through this the week before your building arrives — every line a yes, or the base isn't ready.

Firm — no flex, sink or rocking anywhere on the base
Level — within 10mm across the full footprint, checked corner to corner
Dry — slightly proud of the surrounding ground so rain runs away
Full footprint covered, with no edge of the frame left overhanging
Concrete fully cured — 48 hours minimum before the building goes on
Clear access from the road to the base for the delivery crew

Watch the six steps, animated

Our animated base guide walks the whole job through, one clear stage at a time — marking out, levelling, building, checking.

Watch the base guide

Ready to put it into practice? Every building is made in our West Midlands workshop with free mainland delivery.

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