How To Lay A Concrete Base For A Garden Shed | No-Sag Slab Plan

A shed only stays square if the base stays flat. A good slab does two jobs: it spreads weight and it keeps ground moisture from creeping into the floor frame. Get the prep right and the pour day feels calm.

Concrete Base Specs At A Glance

Decision	Common Pick	Why It Works
Pad size	Shed footprint + 100–150 mm each side	Room for cladding and a tougher edge
Slab thickness	100 mm for light storage sheds	Handles garden loads on firm ground
Perimeter	150–200 mm thickened edge	Stiffens the slab where cracks start
Sub-base	100–150 mm compacted crushed stone	Stops settling and drains water
Membrane	Polythene DPM under slab	Slows damp rise and holds mix water
Reinforcement	A142 mesh mid-depth	Helps control cracking
Finish	Float, then light broom	Flat for a shed, grippy for boots
Cure	Keep damp for 7 days	Better strength and less dusting

Tools And Materials You’ll Want Ready

Set everything out before you dig. A slab is a one-go job once concrete is on site.

Tools

• Measure, pegs, string line, square, marker paint
• Spade, shovel, rake, wheelbarrow
• Plate compactor (hire) or heavy tamper
• Level and a straight screed board
• Float, trowel, edging tool, broom

Materials

• Crushed stone, straight form boards, stakes, screws
• Polythene DPM and tape
• Steel mesh and small chairs/spacers
• Concrete (ready-mix or bag mix)

How To Lay A Concrete Base For A Garden Shed Step By Step

Pick The Spot And Check Rules

Choose a spot that sheds water after rain. If the ground stays soggy, raise the pad with extra stone or pick a drier area.

In the UK, skim the Planning Portal outbuilding rules so your base size and location match what’s allowed.

Set Out The Pad Square

Mark corners with pegs and run string lines. Measure both diagonals. When they match, the pad is square. Set string height to finished slab height, then measure down to plan your dig.

A gentle fall helps water run off. Aim for about 5 mm per metre away from the door side. Keep it mild so the shed still sits true.

Dig To Firm Ground

Strip turf and topsoil until you hit firm ground. Organic soil moves as it breaks down, and that movement shows up as cracks and rocking.

Depth target is simple: sub-base thickness plus slab thickness, with a little room to level stone.

Build And Compact The Sub-Base

Spread stone in 50–75 mm lifts and compact each lift. Two packed lifts beat one thick lift that never fully locks. If the stone is bone-dry, mist it so fines bind.

Check level with your straight board. If you want a fall, shape it now. Concrete follows what’s under it.

Fit Forms That Stay Put

Use straight boards for shuttering. Stake the outside and screw boards to stakes so you can tweak alignment. Re-check diagonals and check the board tops for level and fall.

Brace corners. Wet concrete pushes hard and a bowed form leaves a wavy edge.

Lay The Membrane

Lay polythene over the stone with 150–200 mm overlaps and tape the joins. Turn it up the inside face of the forms. This limits water loss into the ground and slows damp rise toward the shed floor.

If the new slab meets an old patio or wall, add an expansion strip so the slab can move a touch without cracking against the old concrete.

Place Mesh At Mid-Depth

Overlap mesh sheets by one full square and tie them. Lift mesh on small chairs so it sits near mid-depth, not flat on the membrane. Keep it back from edges by 40–50 mm to protect corners.

Choose Concrete And Keep Water In Check

Ready-mix gives steady quality and saves time. Site-mix can work too, yet it takes pace and planning. Either way, keep the mix stiff enough to hold shape. Extra water makes screeding feel easy and leaves weaker concrete once it cures.

Pour And Screed

Tip the concrete in, starting at the far end and working back. Push it into corners with a shovel. Tap the forms to help trapped air rise.

Screed with a straight board, sawing side to side as you pull. Fill low spots and re-screed until the surface sits on the form tops.

Float, Edge, Then Finish

Wait until bleed water fades. Float to flatten ridges and close the surface. Run an edging tool around the pad to round the edge and cut chipping.

A light broom finish gives grip. Don’t broom too early or you’ll drag stones and tear the skin.

Cure The Slab For Real Strength

Concrete gains strength by holding water while cement hydrates. Keep the slab damp and covered for seven days. Plastic sheeting weighed down at edges works well; lift it once a day to mist the surface.

The Federal Highway Administration sets out the same cure window in its curing guidance for Portland cement concrete, along with why that week matters.

When You Can Use The Base

Light foot traffic is often fine after 24–48 hours. Keep loads gentle early on. A small shed can usually go on after 3–7 days if you’re careful with anchors and you don’t drag walls across the surface. Full strength is often referenced at 28 days.

Work Out Concrete Volume Before You Order

Ordering short is a bad day. Ordering too much costs money, yet a small overage beats a half-finished pad. Measure length and width in metres, multiply by slab thickness in metres, then add a little for spillage and levelling.

Sample calc: a 2.4 m × 3.0 m pad at 0.10 m thickness is 2.4 × 3.0 × 0.10 = 0.72 m³. Add 10% and you order about 0.80 m³.

If you thicken the perimeter, add that volume too. A simple way is to treat the thickened edge as a ring: perimeter length × edge width × extra depth. Add it to the main slab number.

Decide If You Need A Thicker Slab Or A Thickened Edge

A thin slab works when the shed is light and the ground stays firm. Go thicker if you plan to store dense kit like a full log rack, a mower fleet, or a workbench with cast-iron tools. A thickened edge often gives you most of the benefit without pouring the whole pad deeper.

If you’re pouring on clay that cracks in summer, keep the stone layer generous, keep water routed away, and lean toward a thicker edge. Clay moves with moisture swings, and the slab needs stiffness to ride that movement.

Plan Your Pour Day So You Don’t Get Rushed

Concrete waits for no one. Clear the access route, stage tools at the slab edge, and line up one helper. One person spreads and one person screeds. That rhythm keeps the surface flat and stops frantic patching.

If you’re pouring in cold weather, protect the slab from frost for the first night with insulating blankets or straw over the plastic. In hot sun, start early, shade the pad, and sheet it once finishing is done.

Drainage Moves That Save You Headaches

Most slab trouble starts with water moving under the pad. Give water an easy route past the base.

• Keep soil around the pad slightly lower than the slab top so puddles don’t sit at the edge.
• Add a gravel strip on the downhill side to carry runoff away.
• Run gutter downpipes away from the shed base, not onto it.

Common Mistakes That Cause Cracks

Loose Ground Under The Slab

If the sub-base isn’t packed tight, it settles later and the slab bridges a void. That’s a classic crack line. Compact in layers and don’t rush this part.

Too-Wet Concrete

A sloppy mix bleeds more water, shrinks more, and ends up weaker. If you need flow, use a plasticiser and follow its label.

No Cure Plan

Hot sun and wind pull water from the surface fast. Sheet the slab early and keep it damp. Skipping cure is one of the fastest ways to get dusting and hairline cracks.

Mesh Left On The Bottom

Steel on the membrane won’t help much. Lift it on chairs, or pull it up gently as you place concrete so it ends up mid-depth.

Pour-Day Timing And Checks

Stage	When	Check
Final compaction	Before forms	Stone feels hard under heel
Form setout	Before pour	Diagonals match; boards braced
Membrane and mesh	Before concrete	Joins taped; mesh lifted; edge clearance kept
Screeding	Right after placing	No low ponds; surface on form tops
Floating	After bleed water	Ridges flattened; surface tightened
Finishing	When thumb marks	Edge rounded; broom lines light
Curing sheet	Same day	Plastic sealed at edges; slab kept damp

Final Checks Before You Build The Shed

Strip the forms once the edge is firm. Backfill with compacted soil and a narrow gravel strip so the edge stays clean and easy to mow against.

Lay a straight board across the slab in a few directions. If you spot a small high ridge, rub it down with a rubbing stone. If you spot a hollow, pack under the shed bearers with treated shims instead of trying to skim the slab top with thin mortar.

If you landed here for how to lay a concrete base for a garden shed, stick to the order: pack the ground, lock the forms, pour to height, then cure the slab like it counts.