How Much Concrete for Shed Base?
A solid concrete base is the foundation of any garden shed or outbuilding. Without a level, well-constructed base, your shed may settle unevenly, doors could jam, and damp may creep in from below. For a 2.44m x 1.83m shed base at 100mm depth, you may need approximately 0.447m³ of concrete, which equates to around 55 x 20kg bags of ready-mix concrete. This guide walks you through preparing the ground, building the formwork, and pouring a base that should keep your shed level and dry for years to come.
Planning and Preparation
Start by choosing a site that is as level as possible, away from overhanging trees and not in a low spot where water collects. Clear the area of turf and topsoil to a depth of at least 150mm, then compact the subsoil with a hand tamper or plate compactor. Build formwork from 25mm x 100mm timber boards, securing them with wooden pegs driven into the ground every 600mm. Check the formwork is level in all directions using a long spirit level or a laser level. The finished base should be at least 100mm thick and extend 50-100mm beyond the shed footprint on all sides to ensure the bearers have full support and rainwater drains away from the structure.
Mixing Concrete
For this project our estimate suggests around 0.447m³ of concrete. You have two options: use approximately 55 x 20kg bags of ready-mix concrete (such as Blue Circle, Postcrete, or Hanson Multicem), or mix your own using a C20 mix ratio of 1:2:3 by volume. To mix your own, you'll likely need around 122.8kg of Portland cement (5 x 25kg bags), 262kg of sharp sand, and 442.1kg of 20mm gravel aggregate — a total of roughly 827kg of materials. For small batches, mix by hand on a clean board or in a wheelbarrow: combine the dry materials first until the colour is uniform, then gradually add water until the mix holds its shape when squeezed but is still workable. For larger volumes, hire a cement mixer — a half-bag (electric) mixer handles most DIY jobs and costs around £25-35 per day from tool hire shops. Avoid adding too much water, as this weakens the finished concrete significantly.
Pouring Technique
Pour the concrete into the formwork in one go if possible, starting at the far end and working back towards your access point. Spread it roughly level with a shovel, then use a straight-edge timber (screed board) resting on the formwork to level it off by sawing the board back and forth across the surface. Tamp the surface with the screed board to bring the wet cement paste to the top and push down any aggregate. For a smooth finish, follow up with a steel float once the surface water has disappeared. Ensure the base is level in all directions — any dips will collect water under the shed and promote damp. If the base is large, consider placing a sheet of DPM (damp-proof membrane) under the concrete to prevent moisture rising through the slab.
Weather Considerations
Concrete should not be poured when the temperature is below 3°C or when frost is expected within 24 hours, as freezing conditions prevent the cement from hydrating properly and produce a weak, crumbly result. In autumn and winter, check the forecast carefully and plan your pour for a mild spell. If temperatures are above 25°C, dampen the sub-base and formwork before pouring to prevent the ground from drawing moisture out of the mix too quickly. Cover the finished slab with damp hessian or polythene and keep it moist for at least three days to ensure full hydration. Rain during pouring is problematic for a large flat slab — have polythene sheeting ready to cover the work at short notice. A few spots of drizzle will not cause harm, but a heavy downpour will pit the surface and weaken the top layer.
Curing Times
A concrete shed base achieves its initial set within 4-6 hours, after which you can walk on it carefully. However, you should wait at least 48-72 hours before placing the shed on the base, and ideally a full week for a heavy structure. The concrete reaches approximately 65% of its full strength after 7 days and its design strength at 28 days. For a shed base, 7 days is more than adequate before loading the slab with the weight of the shed and its contents. During the first 3-7 days, keep the slab damp by covering it with polythene or spraying it with water periodically. This is especially important in warm, dry, or windy weather. Proper curing prevents surface cracking and ensures the slab reaches its full potential strength.
Estimated Cost
Prices for concrete materials in the UK vary by region and supplier. Using ready-mix bags, approximately 55 x 20kg bags may cost around £220-£330 at typical retail prices of £4-6 per bag. If you mix your own, the individual materials may cost roughly £100-£155 (cement £25-£40 for 5 x 25kg bags, sharp sand £35-£55 for 1 bulk bag, aggregate £40-£60 for 1 bulk bag). For a job this size, mixing your own concrete from bulk materials may be significantly cheaper than buying individual ready-mix bags. Consider ordering a bulk bag (dumpy bag) of sand and aggregate from a local builders’ merchant for the best price. If you need to hire a cement mixer, expect to pay £25-35 per day from most tool hire companies. A wheelbarrow, shovel, and spirit level are also essential tools you may need to buy or borrow.