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Groundworks · concrete or slabs · sub base
What do I need for a shed base?
The base a shed or garden building stands on - a concrete slab or paving slabs - on a compacted Type 1 sub base: the concrete or the slabs, the bedding, the sub base, and for concrete the membrane and the formwork, with a budget.
Measure it
Every box has its own unit next to it.
Change the unit beside any box and the number converts. The (i) beside each question says what the box wants and where the figure usually comes from. Anything marked per cent is a percentage of the quantity above it.
Check the rates
Rates and assumptions used
These are the figures the calculator works to. Change any of them and the order list updates — your merchant’s pack sizes and your own waste allowances beat the defaults every time.
Order list
The orange figure is the one you order to. Highlighted rows are the other numbers you order to. The small print under each row is the sum that produced it, so you can check it against your own take-off. Amber rows are warnings, not quantities.
How this one works
- Base area =
length × width- the shed’s own floor, or a little less so rain runs off - Concrete =
area × slab depth × (1 + waste %); slabs =area ÷ one slab × (1 + cutting %) - Type 1 =
area × compacted depth × 2.1 t/m³ × (1 + waste %) - Membrane =
(length + 0.6) × (width + 0.6)- 300 mm up each side; formwork = the base’s perimeter
The figures the calculator works to. Every one can be changed in step 2, and your merchant’s pack sizes and your own waste allowances beat these defaults every time.
| Rate | Default | Why |
|---|---|---|
| Type 1 density | 2.1 t/m³ | Compacted. |
| Bedding density | 1.6 t/m³ | Sharp sand, loose. |
| Bulk bag weight | 0.85 t |
A worked example
With the defaults the page opens on:
- How long is the base? 3 metres
- How wide is it? 2.4 metres
- Concrete or paving slabs? A concrete slab
- How thick is the concrete? 100 millimetres
- Which slab size? 600 × 600
- How deep is the bedding? 40 millimetres
- How deep is the Type 1, compacted? 100 millimetres
- How much spare? 5 per cent
- What does concrete cost a m³? £140.00
- What does one slab cost? £8.00
Concrete slab
- Concrete: 0.76 m³ — 100 mm thick +5%
- Damp-proof membrane: 10.8 m² — 300 mm up each side
- Formwork: 10.8 m — round the edge, at the slab’s depth
Base
- Base area: 7.2 m² — L × W
- Type 1 sub base: 1.59 t — 100 mm × 2.1 t/m³ +5%
- Type 1 bulk bags: 2 bags — at 0.85 t a bag
Budget
- Concrete cost: £106 — excludes Type 1, the membrane, formwork, delivery and VAT
Change any figure above and every line recalculates; the small print under each row is the sum that produced it.
Questions people ask
How much concrete for a shed base?
Length × width × the slab’s thickness, plus a little spare. A 3 m × 2.4 m base at 100 mm with 5% spare takes 0.76 m³ - about £106 at £140 a m³.
How many slabs for a shed base?
The base’s area plus cutting spare, ÷ one slab. The same 7.2 m² in 600 × 600 slabs at 5% spare is 21 slabs, on 0.46 t of bedding at 40 mm.
How much Type 1 under a shed base?
Area × compacted depth × 2.1 t/m³, plus spare: 1.59 t - two bulk bags - at 100 mm for 7.2 m². The right depth depends on your ground; this works out the quantity.
Should the base be bigger than the shed?
The same size as the shed’s floor, or a little smaller, so rain runs off the edge rather than onto the base under the walls.
Calculators for the same job
Doing this for a whole job rather than one wall? SiteLedger keeps the valuations, supplier statements, invoices and variations against the project they belong to.
Read before you order. These work out quantities, not structural design — spans, lintels, rebar and foundation depths need an engineer and Building Control sign-off. Waste allowances are defaults, not promises: awkward cuts, long runs and bad weather all eat more than 5%. Where a merchant sells whole packs, the figure is rounded up to whole packs.