Concrete for Shed and Garage Bases
Most shed bases need less than a cubic metre of concrete — which is why drum suppliers quote them grudgingly and we built our whole model around them. Right mix, right depth, from 0.5m³ with no part-load penalty.
Which Concrete for a Shed or Garage Base?
For an unreinforced shed, workshop or garage base, the mix is C16/20 at S2 consistence — what builders call GEN3. We batch an equivalent designed mix to the strength and durability requirements of BS 8500 / BS EN 206. It's plenty for a garden building; don't pay for C35 when C20 will do. Our C20 concrete page has the grade in full.
One rule to hold onto: GEN-type mixes are for unreinforced work only. Anything with embedded metal counts as reinforced. So if your garage floor design includes mesh — heavier loads, a workshop with serious kit — the mix steps up to C28/35 (see C35 concrete). The steel handles the tension; the higher grade is what a reinforced floor slab is specified at. If you're not sure whether your base needs steel at all, that's a design question for whoever's building it — but once the answer is "yes", the grade changes with it.
How Thick Should the Base Be?
- Shed base: around 100mm — same guidance as paths.
- Garage floor: around 150mm — same territory as driveways.
Depth is where the volume (and the money) lives: going from 100mm to 150mm adds half as much concrete again, so match the depth to the building. A lawnmower store doesn't need a garage floor under it.
Base Prep Basics
We're the concrete, not the groundworks — but a good pour starts before the truck arrives, so here's the practical version. Dig out to firm ground and get a compacted sub-base down: the slab is only as good as what it sits on. Set formwork to your finished level and check it twice — the concrete will faithfully reproduce every error in it. Have your levelling tools ready, because at roughly a minute per cubic metre of discharge, a shed base is poured almost as soon as it's started.
If mesh is in your design: A142 suits a ~100mm domestic slab, needs 50mm of cover, and must sit on chairs — not get "pulled up" into the wet concrete mid-pour, which is the classic failure. Prefer fewer trip hazards? Polypropylene micro-fibres control early-age shrinkage cracking and toughen the surface — but they're not structural and don't replace specified steel. We supply fibre-dosed mixes on request.
And one safety line worth taking seriously: wet cement burns skin chemically, and you may not feel it happening. Waterproof gloves and boots, never kneel in fresh concrete, and wash splashes off straight away.
How Much Concrete? Worked Examples
Length × width × depth, all in metres (100mm = 0.1m):
- 8ft × 6ft shed on a 2.5m × 2m slab at 100mm: 2.5 × 2 × 0.1 = 0.5m³ — exactly our minimum load.
- Larger shed or workshop, 3m × 2.4m at 100mm: 0.72m³ — order 0.75m³.
- Single garage floor, 5m × 3m at 150mm: 2.25m³ — order around 2.5m³ with contingency.
Check your own dimensions with our concrete calculator — it converts the depths, adds the contingency and suggests the mix. And because we batch on site, the truck carries spare material anyway: you pay for the metered volume that goes into your formwork, nothing more.
The Small-Load Advantage
Here's the awkward truth about shed bases: a typical one is well under 1m³, and the drum-mix industry isn't built for that. Drum suppliers charge for the empty truck capacity — typically £15–£40 for every empty cubic metre, depending on supplier, or flat small-load fees of £50 and up. A 0.7m³ base on a 6m³ truck is mostly fee.
Our volumetric model was built for exactly this job: a 0.5m³ minimum, no part-load penalty, ever — the truck batches your half-cube fresh on site and the meter records it. Mixing by hand for a base this size is a long day with a shovel; getting it batched to spec takes minutes. See our mini mix and small load concrete page for how the small-pour service works.
Curing: the Cheap Step People Skip
Keeping young concrete moist and warm is what develops its surface durability — and industry guidance is blunt about the cost of skipping it: poorly cured slabs can lose up to half their surface durability, showing up later as dusting and crazing. The first 24 hours matter most.
The routine: keep the slab covered and damp for at least 3–4 days, ideally a week — polythene sheeting or damp hessian kept wet both work. Light foot traffic after 24–48 hours; cars after about a week to be safe; full design strength arrives at 28 days. Never water-cure in frost.
Shed and Garage Base Concrete Prices
| Mix | Use | Guide price per m³ |
|---|---|---|
| C20 (C16/20, GEN3-equivalent), S2 | Unreinforced shed, workshop & garage bases | from £105 |
| C35 (C28/35), S2 | Reinforced garage floors | from £120 |
All prices per m³, +VAT (add 20%), guide prices — confirmed when you book. So the 0.5m³ shed base above is roughly £53 +VAT of concrete, and the 2.25m³ garage floor around £236 +VAT — metered as it pours. Full pricing lives in our ready mix concrete prices guide.
Shed & Garage Base FAQs
What concrete mix do I need for a shed base?
C16/20 — what builders call GEN3 — is the mix for unreinforced shed, workshop and garage bases. We batch an equivalent designed mix to BS 8500 / BS EN 206 at S2 consistence. Don't pay for a stronger grade than a garden building needs.
How thick should a concrete shed base or garage base be?
Around 100mm for a shed base and around 150mm for a garage floor, both on a compacted sub-base. Depth is where the volume lives — going from 100mm to 150mm adds half as much concrete again.
How much concrete do I need for an 8ft × 6ft shed base?
A 2.5m × 2m slab at 100mm deep is 2.5 × 2 × 0.1 = 0.5m³ — exactly our minimum load. A 3m × 2.4m base at 100mm is 0.72m³, so order 0.75m³. Check your own dimensions with the concrete calculator — and because we batch on site, you only pay for what's poured.
Can I order less than 1m³ of concrete?
Yes — our minimum is 0.5m³, with no part-load penalty, ever. Drum suppliers typically charge £15–£40 for every empty cubic metre on the truck, or flat small-load fees of £50 and up, which is why tiny pours get expensive quotes elsewhere. Small bases are exactly what our volumetric model is built for.
When can I build on the base or park in the garage?
Light foot traffic after 24–48 hours. Cars after about a week to be safe. Heavy vehicles wait 28 days, when concrete reaches its design strength. Keep the slab covered and damp for at least 3–4 days, ideally a week — and never water-cure in frost.
Does a garage base need mesh or fibres?
An unreinforced C16/20 base suits most sheds and single garages. If your design includes steel, the slab counts as reinforced and the mix steps up to C28/35. A142 mesh suits a ~100mm domestic slab, needs 50mm cover and must sit on chairs — not be pulled up mid-pour. Polypropylene micro-fibres control early-age shrinkage cracking and toughen the surface, but they're not structural and don't replace specified steel. We supply fibre-dosed mixes on request.
Half a Cube or a Whole Garage Floor — Same Phone Call.
Tell us the base size and what's going on it. We'll confirm the mix, the volume and the price in minutes — and only ever charge for what's poured.