How To Lay A Concrete Slab For A Garden Shed | Dry Base

A shed looks simple until it sits on a base that’s out of level. Doors stick. Panels rack. Rain sneaks under the floor. A well-made slab fixes those headaches in one go.

This walkthrough is for a plain slab-on-ground base for a metal shed. You’ll get sizes to aim for and a clean step order.

How To Lay A Concrete Slab For A Garden Shed In One Weekend

Plan ahead and you can prep one day and pour the next. Drying takes longer than the weekend, yet the heavy work can be done fast.

Stage	What You Do	What To Check
Measure and mark out	Set the slab size, add a border, square the corners	Diagonals match; pegs are firm
Dig to depth	Remove turf and soft soil, flatten the bottom	Base is even; no spongy spots
Add sub-base	Lay compactable gravel in layers and compact	No footprints; level is close
Build formwork	Fix timber boards to the slab outline and brace	Top edge is level; corners stay square
Lay barrier	Place a damp-proof sheet and tape overlaps	Sheet is unpunctured; edges turn up
Place steel	Set mesh or rebar on chairs so it sits mid-slab	Steel has cover all round
Pour and level	Fill the form, rake, tamp, screed to the board tops	No low pockets; edges are packed
Finish and cure	Float, edge, then keep the surface damp	Surface stays moist for days
Set anchor points	Press in bolts or mark drill points once set	Fixings align with shed base

Pick the slab size that fits your shed

Start with the shed’s base dimensions from the manual. Add a small margin so drips don’t fall onto bare soil and splash back onto timber.

• Typical overhang: 25–50 mm past the shed on each side.
• Height: set the slab a little above nearby ground so water runs away.

If you’re swapping an old shed, measure the footprint on the ground too. Sheds settle.

Plan thickness and materials before you dig

For most garden sheds, a slab in the 75–100 mm range works well on firm ground with a compacted sub-base. Go thicker if you’ll store a ride-on mower or a packed stack of logs.

Concrete and wet cement can burn skin without much warning. Gloves, long sleeves, and eye protection are smart habits when you mix, barrow, and clean up. OSHA’s PDF on Portland cement skin safety is a solid read if you’re new to it.

Materials for a small shed slab:

• Compacting gravel
• Timber boards, stakes, screws
• Plastic damp-proof sheet and tape
• Steel mesh or rebar with chairs
• Concrete (ready-mix or bagged)
• Tools: shovel, rake, tamper, level, straight board, float

Set out a square, level footprint

Mark one straight edge first. Use string lines and pegs to set the outline, then square it with the diagonal method.

1. Drive four corner pegs outside the slab area.
2. Run string lines between pegs to show the slab edges.
3. Measure both diagonals. Adjust until the diagonal lengths match.
4. Check the slab location against fences, tree roots, and door swing.

Set a finished height so rain can’t pool against the shed base.

Excavate and build a sub-base that won’t sink

Dig out turf and soft topsoil until you hit firm ground. The total dig depth needs to fit the sub-base plus the concrete thickness, plus room for the damp-proof sheet.

A simple target is 75 mm of compacted gravel under 75–100 mm of concrete. On wet clay, add gravel depth and shape the ground to drain away.

1. Scrape the bottom level with a shovel and rake.
2. Add gravel in 40–50 mm layers.
3. Compact each layer until it feels hard underfoot.
4. Check level across the area with a long straightedge and spirit level.

Loose fill is where slabs crack later.

Build formwork that stays put during the pour

Formwork is the temporary wooden frame that shapes the slab. For a shed base, simple straight boards work fine if they’re braced well.

1. Cut boards to the slab perimeter and fix them to stakes.
2. Set the board tops to your finished slab height.
3. Brace corners so the frame can’t spread when concrete pushes on it.
4. Re-check squareness by measuring diagonals again.

Push the boards. If they flex, add bracing.

Lay a damp-proof sheet and reinforcement steel

A plastic sheet under the slab slows moisture wicking up through the concrete. It also helps curing by keeping water in the mix longer. Overlap joints by at least 150 mm and tape them so slurry can’t seep through.

Steel helps the slab handle small ground movement. For light shed loads, welded mesh is common. Lift it onto chairs so it sits nearer the middle of the slab, not flat on the plastic.

• Keep steel back from the form boards so it won’t show at the edge.
• Tie laps so sheets act as one mat.

Mix, place, and screed the concrete without panic

Set up the pour

Ready-mix suits tight timelines. If you’re mixing bags, stage water, wheelbarrow route, and tools first.

Screed pass

Work in this order:

1. Wet the sub-base lightly so it’s damp, not muddy.
2. Place concrete into the form, starting at the far end.
3. Rake it into corners and along edges, then tamp with a straight board to pack it.
4. Screed: pull a straight board along the form tops in a sawing motion.
5. Fill low spots and screed again until the surface is flat.

If the mix feels soupy, resist adding extra water. Water makes placing easier for a minute, then costs strength and raises cracking odds.

Float and edge for a shed-friendly finish

Wait until surface water fades and the concrete starts to firm. Then use a float to smooth ridges from screeding. A slight broom finish gives grip and hides marks, which suits a shed floor area.

1. Run an edging tool around the form boards for a neat perimeter.
2. Float the surface with long, calm strokes.
3. For a broom finish, drag a soft broom across once the surface can take it.
4. Cut control joints on larger slabs to guide shrinkage cracks.

On a larger slab, cut shallow control joints to guide shrinkage cracks.

Cure the slab so it gains strength, not dust

Curing is the stretch after finishing where concrete needs moisture and mild temperatures. Fast drying can cause surface dusting and curl at the edges. A steady, damp cure gives a tougher top layer.

If you want a plain, official curing reference, the FHWA notes a common rule of curing for seven days on many mixes in its page on curing concrete guidance. Your local weather and mix choice still matter.

Time After Pour	What You Can Do	What To Do With Moisture
0–4 hours	Keep people and pets off the slab	None yet; let finishing set
4–12 hours	Start curing once the surface can take it	Cover with plastic or damp hessian
Day 1	Remove tools and tidy edges	Keep cover down, re-wet as needed
Day 2–3	Light foot traffic for checks and measuring	Lift cover, mist, then re-cover
Day 4–6	Drill pilot holes for anchors if you must	Keep curing plan steady
Day 7	Set the shed base and bolt down	Let the slab dry slowly after this
Day 28	Full strength period for many mixes	Normal use; avoid harsh chemicals early

Set the shed on the slab and deal with water at the edges

Before you carry panels onto the base, check the slab with a level in both directions. A tiny fall is fine if it drains away from the door side. A visible twist is not.

Seal gaps where rain can sneak under skirting. For timber sheds, keep any timber bearers off standing water. For metal sheds, bolt down through the base rails and use washers so the metal doesn’t deform.

Finish the ground around the slab so water runs away, then add gravel at the door side to stop mud.

Common slip-ups that cause cracks and rocking

Most shed slabs fail for plain reasons.

• Soft spots: if the shovel sinks, dig deeper and refill with compacted gravel.
• Weak formwork: bowed boards throw off the slab shape.
• Too much water: it weakens the surface and invites shrinkage cracks.
• Drying too fast: cover and keep it damp.

Quick slab checklist before you order concrete

Use this list right before you commit money to mix or delivery. It’s also handy if a friend is lending tools and you want to return them in one shot.

• Slab size set with 25–50 mm margin on all sides
• Diagonals match, strings are tight, finished height set
• Dig depth allows gravel plus 75–100 mm concrete
• Gravel compacted in layers until hard underfoot
• Forms braced; board tops level and straight
• Plastic sheet cut, overlaps taped, edges turned up
• Mesh or rebar ready with chairs to lift it
• Screed board, float, edging tool, broom within reach
• Plastic cover or damp hessian ready for curing

If you’re following how to lay a concrete slab for a garden shed step by step, the calm parts are set-out, compaction, and curing. Put care into those, and the pour itself tends to behave.

Once the slab has had a week of curing, you can set the shed and start using it. If you want to repeat the core process later, bookmark this: how to lay a concrete slab for a garden shed is mostly a tidy order of small checks, not brute force. No drama.