How to Pour a Monolithic Slab: Shed & Garden Building Bases (UK)
Published 31 July 2026 · Browse all tools
If you're putting down a base for a shed, a garden room or a small garage, a monolithic slab is often the simplest way to do it: the floor and the footing are poured together as one lump of concrete rather than as two separate jobs. It's quick, it's strong, and it's easy to get the concrete quantity badly wrong because the thickened edge hides extra volume. This guide walks through when a monolithic slab makes sense in the UK, the depths and mix to use, and how to get the amount right. The free Monolithic Concrete Slab Calculator does the volume; this covers everything around it.
What a monolithic slab actually is
A monolithic slab combines the main floor slab with an integral thickened perimeter edge, poured in one operation so there's no cold joint between the footing and the floor. The result is a single continuous piece of concrete — the "monolithic" part just means one pour, one unit.
The traditional two-stage approach pours strip footings first, lets them cure, then places a separate floor slab on top. A monolithic slab does both in one go. In UK practice you'll also hear "thickened-edge slab" as a straightforward description of what it is; "raft" is used for larger versions that spread load across softer ground.
One pour, one continuous piece — the thickened edge is a footing and the slab is the floor, cast together.
When it's the right choice — and when it isn't
A monolithic slab suits: sheds, garden rooms and home offices, summerhouses, small detached garages, on reasonably level, well-drained ground. The common thread is a light structure, flat site and soil that drains rather than holds water.
It's less suitable on sloping sites, soft or made-up ground, ground near trees on shrinkable clay (which heaves and settles with moisture), or anywhere frost heave is a real risk. Those situations need a properly designed foundation — either a deeper strip footing or a fully engineered raft — rather than a DIY pour.
A monolithic slab loves flat, firm, well-drained ground. On anything soft, sloping or clay-heavy near trees, get a foundation designed.
Depths and the sub-base underneath
For a light garden shed, a 100 mm slab body over a compacted sub-base is typical. For a garden room or home office carrying real weight — furniture, people, equipment — 100–150 mm is common. The thickened perimeter edge is usually taken down to 300 mm total depth or more, and 300 mm wide, to carry the wall load and stiffen the perimeter.
Under all of it: a compacted MOT Type 1 crushed stone sub-base, commonly 100–150 mm thick. The sub-base is what actually stops the slab settling and cracking — it spreads load, drains water away from the underside and gives the concrete a stable, even base to cure against. BS EN 13285 covers the Type 1 spec. For anything habitable — a garden room you'll work in rather than just store things — a DPM (damp-proof membrane) between the sub-base and the concrete is worth including.
In practice, most UK groundworkers pour a garden-room base like this straight off a well-compacted Type 1 sub-base with a DPM and a light mesh, and let the thickened edge do the structural work.
The slab is only as good as the compacted Type 1 under it — skimping on the sub-base is the top cause of a cracked home-poured base.
The mix, and why ready-mix usually wins
Mix guidance: a general-purpose C20 / GEN3 mix suits most domestic garden bases — it's widely available and gives consistent results for light loads. For a garage floor or anything carrying heavier loads, a C25 / ST4 mix is a common step up and gives more margin on strength and surface durability.
Below about a cubic metre, bagged concrete and a hired mixer is workable. Above that, ready-mix from a UK supplier beats hand-mixing on consistency and time — and consistency matters more for a slab than for most jobs, because uneven hydration shows up as surface variation and differential shrinkage cracking. The part that catches DIY pours out is ordering: ready-mix comes in fixed load bands, so a 2.24 m³ job means paying for the next band up, and it's cheaper to design the base to the band than to bin the excess.
On anything over about a cubic metre, order ready-mix — the consistency is worth more than the saving on bags.
Worked example — a 4 m × 3 m garden room base in Nottingham
A garden room base in a Nottingham back garden: slab 4 m × 3 m, 100 mm thick body, thickened edge 300 mm wide and 300 mm total depth all round.
- Slab body: 4 × 3 × 0.1 = 1.20 m³
- Edge extra depth below slab: 300 − 100 = 200 mm (0.2 m). Perimeter: (2 × 4) + (2 × 3) = 14 m. Edge volume: 14 × 0.3 × 0.2 = 0.84 m³
- Total before waste: 1.20 + 0.84 = 2.04 m³
- With 10% waste: 2.24 m³ — the number to tell the ready-mix plant
The thickened edge added 0.84 m³ — 40% on top of the flat-slab figure. That's the volume people eyeball and forget. Local ready-mix plants will usually quote per cubic metre delivered and add a part-load charge below about 3 m³, so it's worth ringing two or three before you book. At 2026 Midlands ready-mix prices, budget roughly £120–£160 per m³ for a standard C20 mix delivered, so around £300–£400 in concrete for this base before the sub-base and DPM.
The mistake that cracks these slabs
Most competing guides on this topic give the volume formula and stop there. The failure in practice isn't usually the concrete — it's the ground prep. A monolithic slab poured on loose spoil, an uncompacted sub-base, or ground that holds water will crack at the thickened edge first, because that's where the load concentrates.
Compaction and drainage do more for the slab's life than an extra 25 mm of concrete thickness ever will. If water can't drain away from the underside of the slab, freeze-thaw cycles and ground movement work on it year after year until it fails. Compact the sub-base properly — hire a plate compactor if you're doing it yourself — and make sure the ground around the slab can drain away from it, not towards it.
Spend the effort on compaction and drainage before the pour — no amount of extra concrete rescues a slab sitting on soft ground.
Get your quantity right
The Monolithic Concrete Slab Calculator handles both parts of the volume automatically. Enter the slab size, body thickness, edge width and edge depth and it returns the total volume including the thickened edge, plus a waste allowance — the number to hand to the ready-mix plant or use to count bags.
For the sub-base underneath, the Gravel Under Concrete Slab Calculator works out Type 1 tonnage and bulk bags. For general slab and footing volumes, the Concrete Volume Calculator covers other shapes.
Work out the slab and the edge together — never price a monolithic base off the flat-slab volume alone.