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Project guide

How much concrete do I need for a shed base?

Make the base a little bigger than the shed, work out the volume, and convert it to bags of cement and ballast for a 1:5 mix.

Updated Based on manufacturer guidance, sources below

The short answer

Make the base 100mm longer and 100mm wider than the shed, and 100mm thick. Volume = length x width x depth, plus 10%. A 1:5 mix of cement to all-in ballast takes about 12 x 25kg bags of cement and 1.7 bulk bags of ballast per m³. An 8 x 6ft (2.4 x 1.8m) shed needs about 0.52 m³: 7 bags of cement and 1 bulk bag of ballast.

The formula

  • Base = (shed length + 0.1m) x (shed width + 0.1m)
  • Concrete (m³) = base area x thickness x 1.1
  • Cement (25kg bags) = m³ x 12; all-in ballast (bulk bags) = m³ x 1.7
  • Hardcore (MOT Type 1) at 100mm = base area x 0.1 x 2.1 tonnes

Worked example

An 8 x 6ft shed (2.4 x 1.8m) on a 100mm base:

  • Base: 2.5 x 1.9m = 4.75 m²
  • Concrete: 4.75 x 0.1 x 1.1 = 0.52 m³
  • Cement: 0.52 x 12 = 6.3, so 7 bags
  • Ballast: 0.52 x 1.7 = 0.9, so 1 bulk bag
  • Hardcore: about 1 tonne of MOT Type 1 underneath

Work it out for your job

Change the measurements and the quantities update straight away.

Base size
2.50 x 1.90m
Concrete
0.52 m³ (including 10%)
MOT Type 1 hardcore
about 1 tonne at 100mm

Your list

  • 7Cement 25kg bags (1:5 mix)
  • 1All-in ballast, bulk bags
  • 4Formwork boards 25 x 150mm x 3.0m
  • 3Battens for 13 pegs
  • 1Screws to fix formwork to pegs
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Materials checklist

  • Cement25kg bags, mixed 1 part cement to 5 parts all-in ballast.
  • All-in ballastSand and gravel already mixed, in bulk bags.
  • Formwork boards25mm timber boards to hold the concrete, set level.
  • Battens for pegsPegs every metre round the outside of the formwork.
  • ScrewsTo fix the formwork to the pegs.
  • MOT Type 1 hardcoreAbout 100mm, compacted, under the slab.
  • Damp-proof membrane1200 gauge polythene over a thin layer of sand.

You'll also need

  • Wheelbarrow, shovel, rake, spirit level and tamper, or hire a cement mixer
  • A long straight timber to tamp and level the concrete
  • Plastic sheeting to cover the slab in hot weather
  • Gloves, goggles, long sleeves and boots: wet cement burns skin

Step by step

  1. Mark outMark the base 100mm longer and wider than the shed, plus room for the formwork, with string and pegs. Check the corners are square.
  2. Dig and compactDig out for the hardcore and concrete, then compact about 100mm of MOT Type 1.
  3. Build the formworkFix 25mm boards to pegs every metre and set them level.
  4. Membrane and mixBlind the hardcore with sand, lay the membrane, then mix 1 part cement to 5 parts ballast.
  5. Pour and levelFill from the far corner, tamp with a straight timber and leave the formwork for at least five days.

Tips from the manufacturers

  • Heidelberg Materials advises adding 100mm to the length and width of the shed, plus room for the formwork, and at least 75mm of concrete on a similar depth of sub-base.
  • A 1:5 mix of cement to all-in ballast is usually fine for a shed base (Heidelberg Materials). That works out at about 12 bags of cement per m³.
  • Getting the formwork level matters: Heidelberg notes that being 1cm out over 25 m² adds a quarter of a cubic metre of concrete.
  • If it will be 20 degrees or warmer, cover the slab with plastic for at least 24 hours so it does not dry too fast.

Mistakes to avoid

  • Mixing by "a bag of cement to four bags of ballast": the units are not comparable. Mix by volume, 1 part to 5.
  • Leaving out the hardcore, so the slab cracks or sinks.
  • Pouring without a damp-proof membrane.
  • Taking the formwork off too soon. Leave it for at least five days.

Frequently asked questions

How thick should a concrete shed base be?

Usually 100mm. Heidelberg Materials gives 75mm as the minimum, on a similar depth of compacted sub-base.

How many bags of cement for a shed base?

For a 1:5 mix, about 12 x 25kg bags per m³. An 8 x 6ft shed on a 100mm base needs about 7 bags.

What concrete mix for a shed base?

1 part cement to 5 parts all-in ballast is usually fine. Heidelberg suggests adding reinforcing fibres if the shed will hold something particularly heavy.

How big should a shed base be?

About 100mm longer and 100mm wider than the shed itself.

How long before I can put a shed on a new base?

You can walk on it after about 48 hours. Heidelberg advises leaving the formwork for at least five days before building the shed.

Sources

Quantities are estimates for typical jobs, using the coverage figures above. Products vary, so check the data sheet for the product you buy, and talk to a professional for structural, electrical or gas work. SmartBuild is independent and not affiliated with any of the manufacturers listed.

How much sand and cement do I need for bricks, blocks or slabs?Work out the volume of mortar first, then split it into cement and sand by the mix. Here are the standard UK rates for brickwork, blockwork and laying paving slabs. How much decking do I need?Boards are the easy part. The frame underneath is where most DIY lists go wrong, so this guide follows the Timber Decking & Cladding Association (TDCA) handbook. How many fence panels and posts do I need?Divide the length of the run by the panel width and round up. You need one more post than panels, plus one for each corner or gate.

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