A complete step-by-step guide for laying a solid concrete base for your garden shed. Covers ground preparation, formwork, mixing, pouring, and curing — everything you need to know before you start.
Ordinary Portland Cement (OPC). Standard 25kg bags from any builder's merchant.
Mixed aggregate (sharp sand + gravel). Sold in 25kg bags or bulk bags (850kg).
Crushed stone for the sub-base layer. Compacts to a solid foundation.
50×100mm (2×4) treated timber for the frame. Pegs to hold it in place.
Spade, wheelbarrow, spirit level, string line, rake, float or screed board, tape measure.
Weed membrane, reinforcing mesh (A142), polythene sheet, cement mixer.
Enter your shed dimensions into our calculator and get exact quantities — in bags, bulk bags, or tonnes.
Open Calculator →Your concrete base should be at least 50mm larger than the shed on all sides — ideally 100-150mm. A 6×4 foot shed needs a base roughly 6'6" × 4'6". Mark the corners with pegs and run a string line to define the edges. Check the corners are square by measuring diagonally — both diagonals should be equal.
Dig out the marked area to a depth of 175-200mm (75mm sub-base + 100mm concrete). Remove all topsoil, roots, and soft spots. The bottom should be firm, level ground. If the soil is soft clay, dig deeper and add more hardcore.
Fill the excavation with 75mm of MOT Type 1 hardcore or crushed stone. Rake it level and compact it firmly — use a hand tamper (a heavy flat plate on a post) or a vibrating plate compactor if available. The sub-base prevents the slab from cracking due to ground movement and improves drainage.
Construct a timber frame (50×100mm treated timber) around the perimeter, matching the exact finished size of your base. The top of the formwork should be at the level where you want the finished concrete surface. Check it's level with a spirit level — this is your guide for screeding. Secure with wooden pegs driven into the ground on the outside.
Use a 1:4 mix — one part cement to four parts ballast. Mix dry first until the colour is uniform, then add water gradually. The consistency should be like thick porridge — holds its shape but isn't dry or crumbly. Mix in a wheelbarrow or cement mixer. For a typical 8×6 foot shed base you'll need roughly 0.5 cubic metres of concrete (about 20 bags of ballast and 5 bags of cement).
Tip the mixed concrete into the formwork and spread it with a rake or shovel, working it into the corners. Fill slightly above the formwork. Use a long straight piece of timber (screed board) resting on the formwork edges and drag it across the surface in a sawing motion to level the concrete flush with the top of the frame.
After screeding, use a wooden float to smooth the surface. Don't over-work it — a slightly textured surface is fine for a shed base and provides grip. Tap the sides of the formwork gently to settle the concrete against the edges and remove air pockets.
Cover the slab with polythene sheet or damp hessian sacking to prevent it drying too quickly — this is called curing and is critical for strength. Keep it covered and damp for at least 3 days. In hot weather, mist the surface with water before re-covering. The concrete will be hard enough to walk on after 24-48 hours but won't reach full strength for 28 days.
Too much water. The most common DIY mistake. Sloppy, runny concrete is easier to pour but dramatically weaker than a drier mix. If you can pour it like a liquid, it's too wet. Aim for thick porridge consistency.
No sub-base. Pouring concrete directly onto soil invites cracking. The hardcore sub-base distributes weight evenly and prevents the slab from moving when the ground expands and contracts with moisture.
Not levelling the formwork. If the formwork isn't level, your slab won't be level. Check with a spirit level on all four sides before you start mixing. It's much easier to fix the frame than to fix set concrete.
Pouring in bad weather. Rain washes the cement out of fresh concrete, leaving a weak, sandy surface. Frost destroys uncured concrete. Check the forecast and aim for a dry spell of at least 48 hours.
Base too small. Always make the base bigger than the shed — at least 50mm on each side, ideally 100-150mm. This gives you room for error when positioning the shed and prevents the edges of the shed from sitting right on the edge of the concrete where it's weakest.