Pouring a slab is roughly 80% preparation and 20% concrete. The two hours the truck is on site get all the attention, but the outcome was decided days earlier by how well the ground was prepared and how accurately the formwork was set.
This walks through the whole sequence for a typical domestic slab — a shed base, garden building or small garage floor.
Planning the pour
Start with volume. Length × width × thickness, all in the same units, plus 5 to 10% for wastage. The slab calculator handles the conversions and gives you bag counts if you’re not ordering ready-mix.
Then answer four questions honestly:
- Can a truck reach the pour? If not, how far is the barrow run, and how many people do you have?
- How long will placing take? Concrete has about 90 minutes from batching.
- What’s the weather doing? Not just on the day — overnight frost or heavy rain in the following 48 hours matters too.
- Who’s helping? Anything over about a cubic metre needs at least two people, and three is better.
What you need on site
| Item | Why | Hire or buy |
|---|---|---|
| Plate compactor | Sub-base compaction | Hire, £45–£70/day |
| Formwork timber and stakes | Shape and level | Buy |
| Screed board | Longer than the widest span | Buy or make |
| Bull float | Initial surface levelling | Hire or buy |
| Hand float and edger | Finishing and edge detail | Buy |
| Jointing tool or disc cutter | Control joints | Buy / hire |
| Wheelbarrows | More than you think | Hire |
| Plastic sheeting | Curing and rain cover | Buy |
Marking out and excavating
Set out with pegs and string lines, then check the diagonals. For a rectangle they must be equal — if they aren’t, the shape isn’t square. The 3-4-5 method works for setting a right angle: measure 3 units along one line, 4 along the other, and the diagonal between should be exactly 5.
Dig to the total depth of slab plus sub-base. For a 100mm slab on 100mm of hardcore, that’s 200mm below finished level. Excavate 150mm wider than the slab on each side to give room for formwork and haunching.
Remove all topsoil and vegetation. Organic material rots and settles, and anything left under a slab will produce a soft spot later. Dig down to firm subsoil even if that means going deeper than planned.
Sub-base
This is the layer that decides whether the slab survives, and it’s the one most often skimped.
Use MOT Type 1 in the UK or crushed stone in the US, laid in layers no thicker than 75mm and compacted with a plate compactor between each. Two 75mm layers properly compacted are far better than one 150mm layer whacked on top.
The finished sub-base should feel solid underfoot with no give at all. Walk over it — if you can feel any movement, it needs more compaction. There’s more detail in our guide to sub-base depth and materials.
Formwork
Timber formwork sets the shape, the level and the falls. Use straight, sound timber — 25mm × 150mm is typical for a 100mm slab — held with stakes at no more than 1m centres.
Set the top edge of the formwork at exactly finished slab level, because you’ll screed off it. Use a level and a straight edge to get the top consistent, and if the slab needs a fall for drainage, build it into the formwork now rather than trying to introduce it while screeding.
Brace the outside of the forms well. Wet concrete pushes outward with real force, and a form that bows leaves a slab with a bulging edge you can’t fix afterwards.
Membrane and reinforcement
A damp proof membrane goes on top of the compacted sub-base for anything that will be enclosed or habitable. Lap sheets by at least 150mm, tape the joints, and turn it up at the edges. Protect it while you work — a punctured DPM is no DPM.
If the slab needs mesh, it must sit in the middle third of the slab depth, not on the ground. Plastic chairs or concrete spacers hold it up. Mesh lying on the sub-base does nothing at all, which is one of the most common and expensive mistakes on DIY slabs.
Placing the concrete
Start at the far corner and work back towards your access point, so you’re never walking through fresh concrete. Discharge or barrow in, then spread with a rake to roughly 20mm above final level.
Compact as you go. Tamp with a straight-edged board across the formwork, or use a poker vibrator on thicker sections. Compaction drives out trapped air, and air pockets are voids where strength should be. Work the board along the forms in a chopping motion, then a sawing motion to level.
Keep the pour continuous. If you have to stop for more than 30 to 45 minutes, form a proper joint rather than letting the second batch butt up against a partly-set face.
Screeding and finishing
Screed with a straight board resting on both forms, using a sawing action while pulling it back. Two people make this much easier on anything over a metre wide. Fill low spots as you find them and screed again.
Then wait. Bull-float once the surface water sheen has gone but before the concrete stiffens — floating too early works water back to the surface and weakens it.
Choose the finish to suit the job:
- Broom finish — drag a stiff broom across the surface for slip resistance. Standard for driveways, paths and any exterior slab.
- Float finish — slightly textured, good general purpose.
- Steel trowel — smooth and dense, for interior floors. Not for outdoors, it becomes lethal when wet.
Edge the perimeter with an edging tool to round the corner slightly. Sharp arrises chip; rounded ones don’t.
Timeline for a typical slab
| When | What happens |
|---|---|
| Day 1–2 | Mark out, excavate, remove spoil |
| Day 2–3 | Sub-base in layers, compact each one |
| Day 3 | Formwork, membrane, mesh on chairs |
| Day 4, morning | Pour, compact, screed |
| Day 4, +2–6 hours | Float and finish once the sheen has gone |
| Day 4–5 | Cut control joints, cover for curing |
| Day 5–6 | Strike formwork |
| Day 11 | Light vehicles |
| Day 32 | Full design load |
Joints and curing
Cut control joints within 12 to 24 hours, to a quarter of the slab depth, spaced at roughly 24 to 30 times the thickness. For a 100mm slab that’s every 2.4 to 3m in both directions. Concrete will crack as it shrinks; joints decide where.
Then cure it. Cover with plastic sheeting, damp hessian, or spray a curing compound, and keep it protected for at least seven days. This single step can add 30% to the finished strength and costs almost nothing.
Strike the formwork after 24 to 48 hours. Keep foot traffic off for two days, vehicles for at least seven.
Common questions
How thick should a concrete slab be?
100mm for sheds, paths and patios. 125 to 150mm for driveways and garages. Always on a compacted sub-base of at least the same depth.
Can I pour a slab in one day?
The concrete, yes. The preparation — excavation, sub-base, compaction, formwork, membrane, mesh — is typically two to three days before that. Rushing the preparation to pour sooner is the classic mistake.
What happens if it rains after pouring?
Light rain a few hours after finishing is survivable if the surface has set enough. Rain on fresh concrete washes cement from the surface and leaves it weak and dusty. Have plastic sheeting on site whatever the forecast says.
Do I need reinforcement in a slab?
Not for a 100mm shed base on good ground. Yes for driveways, garages, anything over about 10m², or where the ground is variable. Mesh must sit in the middle of the slab depth on chairs, never on the sub-base.
How long before I can build on a new slab?
A light shed after 3 to 4 days. A garage or masonry structure after 7 days minimum, ideally 14. Full design strength arrives at 28 days.