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How to pour a concrete slab: step-by-step guide
A concrete slab looks simple, but its strength and lifespan are decided before and after the pour: by the sub-base, the joints and, above all, the curing. This guide covers a small outdoor slab such as a patio or shed base.
1. Plan the size and thickness
For a patio, path or shed base, 10 cm of concrete is typical. Use 12–15 cm where cars will drive or heavy loads will stand, and follow a structural design for anything that supports a building. Plan a slight fall of 1–2% away from the house so water runs off.
Enter the length, width and thickness into the Concrete calculator to get the volume with a waste margin, the number of premix bags and the approximate weight. For more than about 0.5 m³, ready-mixed concrete from a truck is usually easier and more consistent than mixing by hand.
Safety: wet cement is strongly alkaline and can burn skin. Wear waterproof gloves, long sleeves, boots and eye protection, and wash splashes off immediately.
2. Dig and build the sub-base
Remove turf, roots and soft topsoil. Dig deep enough for the sub-base plus the slab. Lay 10–15 cm of crushed stone or gravel and compact it in layers with a plate compactor; a soft or uneven base is the main cause of cracked slabs. The Gravel calculator works out the quantity, including an allowance for compaction.
A polyethylene sheet over the gravel stops the ground from drawing water out of the fresh concrete and reduces rising damp under sheds.
3. Set up the formwork
Build a frame from straight timber boards, held by stakes driven into the ground on the outside every 60–90 cm. Set the top edge of the boards to the finished slab height, including the fall, using a string line and a level. Check that the frame is square by measuring both diagonals; they should be equal.
Brush or spray the inside faces with release oil so the boards come away cleanly.
4. Add reinforcement
Steel mesh or fibers help control cracking. Mesh must sit in the middle of the slab, not on the ground, so support it on plastic spacers (chairs). Overlap mesh sheets by at least one square and keep the steel about 5 cm away from the edges.
5. Mix and pour
Check the weather: avoid frost, heavy rain and very hot, windy days. Wet the sub-base lightly if it is dry and there is no sheet. If you mix by hand or in a mixer, add only the water on the bag; extra water makes concrete easier to place but noticeably weaker and more prone to cracking.
Start in the corner farthest from the access point and place the concrete in the forms, slightly above the top edge. Push it into the corners with a shovel and tap the outside of the forms to release trapped air.
6. Screed and float
Pull a straight board across the top of the forms with a sawing motion to strike off the excess (screeding). Fill low spots and repeat. Then smooth the surface with a float to push the stones down and bring a thin layer of paste to the top.
Wait until the water sheen (bleed water) has disappeared before the final finish. Working water back into the surface leaves a weak, dusty top layer.
7. Finish the edges, joints and surface
Round the edges with an edging tool. Cut control joints with a grooving tool, or with a saw within a day, about a quarter of the slab thickness deep. Space them roughly every 2.5–3 m for a 10 cm slab so that cracks form in the joints rather than across the slab.
For outdoor slabs, a broom finish gives a non-slip surface. A steel trowel gives a smooth surface for indoor use.
8. Cure the concrete
Curing means keeping the concrete moist and protected from extreme temperatures while it gains strength. Start as soon as the surface can be covered without marking it, and keep it up for at least 7 days. Cover the slab with plastic sheeting, keep it wet with damp burlap or a sprinkler, or apply a curing compound. Do not let it dry out and re-wet repeatedly, and protect it from frost.
Remove the forms after 1–2 days. Light foot traffic is usually fine after 24–48 hours and cars after about 7 days. Concrete reaches its design strength in about 28 days.
Common mistakes
- Adding extra water to make the mix easier to place, which weakens the concrete.
- Building on a soft or poorly compacted base, which leads to cracks and sinking.
- Leaving out control joints, so the slab cracks randomly.
- Finishing while bleed water is still on the surface, which leaves a dusty top layer.
- Skipping curing on a hot or windy day, which causes surface cracks and lower strength.
Frequently asked questions
How thick should a concrete slab be?
About 10 cm for patios, paths and shed bases, and 12–15 cm where cars drive. Structural slabs need an engineer’s design.
How long should concrete cure?
Keep it moist for at least 7 days. It can take light foot traffic after 1–2 days and reaches design strength in about 28 days.
Do I need reinforcement in a small slab?
Mesh or fibers are recommended because they hold cracks tightly closed. They do not replace a good base and control joints.
Can I pour concrete in cold weather?
Only with protection. Fresh concrete must not freeze; use insulating blankets and avoid pouring when frost is expected.
How much concrete do I need?
Multiply length × width × thickness and add 5–10% for uneven ground. The concrete calculator does this and also shows the number of bags.