A shed is only as good as the surface it stands on. A base that is out of level, soft in one corner or constantly damp will rack the frame, jam the door and rot the floor within a few seasons. A concrete slab is the most durable answer for a medium or large shed, and it is well within reach of a competent DIYer, because it is little more than a shallow box of hardcore and concrete. This guide walks through the decisions in the order you will meet them: whether you need permission, how big and how thick the slab should be, how to prepare the ground, how much concrete to order, and how to finish it so the shed stays dry.

Check the rules before you dig

The base itself rarely needs permission, but the building that sits on it can. In England, most garden sheds are outbuildings covered by permitted development, which means no planning application if you stay inside a set of limits. The Planning Portal lists the main ones: the outbuilding must be single storey, must not sit forward of the wall forming the principal elevation of the house, can have a maximum eaves height of 2.5 metres, and a maximum overall height of 4 metres with a dual-pitched roof or 3 metres for any other roof type. Within 2 metres of a boundary, the building can be no more than 2.5 metres high overall. No more than half the land around the original house can be covered by additions and other buildings. Raised platforms must not exceed 0.3 metres in height, which is worth remembering if your plot slopes and you are tempted to build the base up. Different, tighter rules apply in National Parks, the Broads, Areas of Outstanding Natural Beauty and World Heritage Sites, and any listed building or its curtilage needs permission for an outbuilding.

Building regulations are a separate test. The Planning Portal states that building regulations will not normally apply to a detached outbuilding with a floor area under 15 square metres that contains no sleeping accommodation. Between 15 and 30 square metres, approval is usually not needed if there is no sleeping accommodation and the building is either at least 1 metre from any boundary or built substantially of non-combustible materials. Above 30 square metres, or if anyone will sleep in it, expect building control to be involved. These figures are for England; Wales follows its own regulations, and Scotland and Northern Ireland have separate building standards systems with different thresholds, so check with your local authority if you are outside England. A dwelling-style garden room with insulation, power and plumbing raises more questions than a bare shed, and any new electrical circuit to the building is a separate matter covered by Part P.

Finally, check where services run. Drains, water mains and cables should not lie under a slab. Your water company can tell you if a public sewer crosses your garden; building over or close to one can require its consent.

Is concrete the right base for your shed?

Concrete is not the only option, and for a small lightweight shed it can be overkill. Compare the choices honestly:

  • Concrete slab. The most stable and long-lasting. Suits sheds from about 2.4 x 1.8 metres upwards, workshops with heavy benches, and anywhere you want a hard-wearing floor. It is permanent and hard to remove, and wet concrete is heavy work to mix and place.
  • Paving slabs on a compacted bed. Quicker for a small shed. It needs the same careful levelling, and individual slabs can settle unevenly over time.
  • Timber frame on concrete pads or piers. Lifts the floor above the ground for ventilation. Works well on gentle slopes. Each pad must be level with the others.
  • Plastic ground-grid systems. Interlocking panels filled with gravel. Easy to install and permeable, best for lighter sheds, but they rely on a well-compacted sub-base too.

Whichever you choose, the same principle applies: a firm, level, well-drained foundation that is slightly larger than the shed floor.

Sizing the slab

Make the slab the same size as the shed floor, or marginally smaller by 25 to 50 millimetres all round, so rain running off the walls does not land on the slab edge and wick under the floor. If the base is bigger than the shed, water pools on the exposed concrete and splashes onto the cladding. Check the manufacturer's installation sheet: many timber sheds specify the exact footprint of the floor frame and a maximum out-of-level tolerance.

For thickness, 100 millimetres of concrete is the usual figure for a domestic shed, over a layer of compacted hardcore or MOT Type 1 sub-base of about 100 millimetres. Increase to 125 or 150 millimetres of concrete for a workshop that will carry heavy machinery, a vehicle or a large storage load, and consider adding steel mesh. Where the ground is soft, made-up or clay that moves seasonally, or close to large trees, ask a builder or a structural engineer, because a thin slab on poor ground will crack whatever its mix.

Marking out and excavating

  1. Set the footprint. Mark the corners with pegs and string lines, allowing an extra 100 to 150 millimetres all round for formwork. Check the diagonals are equal. If they differ, the rectangle is not square.
  2. Choose a finished level. Aim for the top of the slab to sit 50 to 75 millimetres above the surrounding ground, so water drains away from it. On a sloping plot, decide whether to cut into the high side or build up the low side with formwork and fill. Remember the 0.3 metre limit on raised platforms if you are relying on permitted development.
  3. Dig out. Remove the turf and topsoil, which is organic and compresses over time, and keep digging until you reach firm subsoil. The depth you need is the slab thickness plus the sub-base thickness minus the height above ground. For a 100 mm slab on 100 mm of sub-base standing 50 mm proud, that is 150 millimetres below ground level.
  4. Compact the subgrade. Tamp or use a plate compactor so the bottom of the excavation is firm and level.

Sub-base and formwork

Lay the hardcore or MOT Type 1 in layers of no more than about 50 to 75 millimetres and compact each layer thoroughly. Blind the surface with a thin layer of sharp sand or fine crushed material so sharp stones cannot puncture a damp-proof membrane. A sheet of 1000-gauge polythene (250 micron) lapped at the joints stops the ground sucking water out of the concrete and reduces moisture rising into the slab. Lay it with a little slack so it is not stretched when the concrete goes in.

Build the formwork from straight timber boards, typically 100 to 150 millimetres deep, held by pegs knocked in at the outside and screwed to the boards. Pack the boards level using a long spirit level or a laser level. Brush a little oil on the inside faces so the boards release cleanly. Some builders give the slab a slight fall of around 1 in 100 to shed water, but if the shed manual requires a dead-level base, level it and rely on the surrounding ground falling away instead. Boards must be rigid enough that they do not bow under the weight of the wet mix.

How much concrete?

Concrete volume is length x width x thickness, all in metres. Always add an allowance for uneven ground, spillage and over-dig. Ten percent is a common figure.

Worked example (illustrative numbers): a 2.4 m x 1.8 m shed with a base 100 mm thick.

  • Volume = 2.4 x 1.8 x 0.1 = 0.432 cubic metres.
  • Add 10 percent: 0.432 x 1.10 = 0.475 cubic metres.
  • Round up to 0.5 cubic metres.

Now a larger 3.0 m x 2.4 m workshop base at 125 mm: 3.0 x 2.4 x 0.125 = 0.90 cubic metres; with 10 percent, 0.99, so order 1.0 cubic metre. The concrete slab volume and mix calculator does these sums and also converts the volume into cement, sand and aggregate quantities.

Roughly, one cubic metre of fresh concrete weighs around 2.4 tonnes, so a half cubic metre is more than a tonne. That is a lot to mix by hand. For anything over about 0.3 to 0.5 cubic metres, many people order ready-mixed concrete delivered by truck, or hire a drum mixer and organise a team. Ready-mix suppliers sell small loads in smaller volumes, often with a minimum charge; ask for a mix suitable for general domestic foundations and slabs, and tell them what it is for. A common DIY site mix is around one part cement to five parts all-in ballast by volume, but follow the guidance printed on the cement bag or the supplier's advice rather than relying on a rule of thumb.

For the sub-base, calculate the same way: 2.4 x 1.8 x 0.1 = 0.432 cubic metres, and compacted material needs about 30 percent more loose volume than the finished thickness, since it is squeezed down. Aggregate is sold by weight, and merchants will convert for you when you give them the area and depth.

Reinforcement: do you need it?

A 100 millimetre slab under a domestic shed on firm ground generally performs without steel, but a layer of A142 mesh (6 mm bars at 200 mm centres) held on spacers about 40 to 50 millimetres off the base helps to control cracking, and it is cheap relative to the cost of the job. If you use mesh, it must be positioned in the upper third to middle of the slab, not lying on the membrane, where it does nothing. Keep it at least 40 millimetres clear of the edges. For heavy loads or poor ground, seek design advice rather than guessing.

Placing and finishing

  1. Pick a dry day above 5 degrees Celsius with no frost forecast overnight. Cold slows strength gain and a frozen surface can scale off.
  2. Wet the sub-base lightly if it is very dry and hot.
  3. Tip the concrete in, working from one end, and rake it level with a shovel. Overfill slightly.
  4. Screed it off with a straight timber board resting on the formwork, using a side-to-side sawing action. Fill hollows and screed again.
  5. Tamp or compact to remove air pockets and bring a bit of paste to the surface. Tapping the outside of the formwork with a hammer helps settle the mix around the edges.
  6. Wait until the bleed water sheen has gone, then float the surface with a wooden float for a flat but slightly textured finish. A steel float gives a smoother, denser finish, but a very smooth surface can be slippery.

The surface will begin to stiffen within hours. Avoid overworking it, and do not sprinkle dry cement on the top to dry it; that weakens the finish.

Curing

Concrete gains strength by hydration, not by drying out. If it dries too fast, it cracks and dusts. Cover the slab with polythene or damp hessian once the surface is hard enough not to mark, and keep it covered for at least three days, ideally up to a week in hot or windy weather. In frosty weather, cover it with insulating blankets as well. Leave the formwork in place for at least a day or two, then strip it carefully. Concrete reaches the bulk of its design strength at around 28 days, but you can usually build a lightweight timber shed on it after a few days. Check the shed manufacturer's guidance for any minimum curing period, and wait longer before putting heavy equipment on the slab.

Keeping the shed dry

Standing a timber floor straight onto concrete invites rot, since concrete can stay damp. Place a damp-proof course strip, or a few plastic or treated timber battens, between the concrete and the shed floor frame. Ensure that the shed's bearers are not sitting in a puddle line, and that the ground around the slab falls away. A gap for air under the floor frame helps. Treat any cut ends of timber and check that the roof overhang throws rain clear of the base. Keep guttering in mind if you fit it; a small shed butt can overflow onto the slab.

Common mistakes

  • Skipping the compaction. Loose hardcore settles and leaves the slab unsupported, so it cracks.
  • Leaving the topsoil in. It rots and compresses, leaving voids.
  • Formwork that is not level, producing a slab that slopes towards one corner.
  • Ordering exactly the volume calculated. Ground is never perfectly flat, and running out half way through produces a cold joint.
  • Mixing too wet. Extra water makes pouring easy but weakens the concrete and increases shrinkage cracking.
  • Not allowing for a barrow route. Wet concrete in a wheelbarrow is heavy, so plan a smooth path and lay boards across soft ground.

Safety

Wet cement is alkaline and can cause serious skin burns, so wear gloves, long sleeves and wellingtons, and wash splashes off straight away. Wear eye protection and a dust mask when handling dry cement. Lift with your legs and get help to move bags; a 25 kg bag is easy to underestimate. If you cut mesh or concrete with a grinder, wear appropriate eye, ear and dust protection.

When to get help

Call a professional if the garden slopes steeply, if there are large trees nearby on clay soil, if you suspect services or a drain run beneath the site, or if the shed will be large, heavy or used as a habitable room. This guide offers general information, not engineering advice, and your local authority building control can tell you whether your specific project falls under the regulations.

Sources