A ground slab looks like the simplest thing on site — one flat grey rectangle. It isn't. A floor that stays flat, doesn't crack in a map of hairlines and doesn't sweat damp for the next twenty years is the sum of a dozen small decisions made before the truck arrives. Concrete is unforgiving in one specific way: once it's down, it's down. So you do the thinking up front.
Start below the slab
A floor is only ever as good as what it sits on. The prep is the job; the pour is the easy part.
- Compact the sub-base. Well-graded hardcore, blinded and rolled in layers. Soft spots become cracks — you're building the floor on this, not just tidying the ground.
- Lay a damp-proof membrane (DPM). Continuous polythene, laps taped, dressed up the edges and into the DPC. A missed lap is a wet floor you can't fix later.
- Insulate where the spec says. Rigid boards for a heated or habitable floor — get the build-up order right so the slab lands where the levels expect it.
Steel, mix and levels
- Position the reinforcement — don't just throw mesh in. Mesh or fibres control shrinkage cracking. Mesh belongs where the spec says (often the upper third), sat on chairs — not kicked to the bottom by boots as you pour.
- Order the right mix, and don't water it on site. Specify strength and slump for the job. Adding water at the truck to make it "flow nicer" quietly throws away strength and invites cracking.
- Set your datum and screed to it. Level pegs, laser or screed rails. Falls to drains where you need them. "Roughly flat" reads as a pond the first time it rains or you wash the floor.
- Don't over-work the surface or pour onto standing water. Bringing too much fines/water to the top makes a weak, dusty skin that fails early.
Finish, joints and cure
- Respect the finishing windows. Float once the bleed water has gone, then power-float / trowel as it stiffens. Too early traps water; too late and you're fighting concrete that's already set.
- Cut control joints — and cut them in time. Concrete will crack; joints decide where. Saw-cut early (typically within ~6–12 hours), roughly 24–36× the slab thickness apart, in tidy panels.
- Then actually cure it. This is the step everyone skips. Keep it damp/covered or use a curing compound for around 7 days. Concrete gains strength by hydrating, not by drying — walk away too soon and you've thrown away strength you already paid for.
Treat those as starting points, not gospel — thickness, mix, joint layout and reinforcement all come off the structural design and the loading. The floor in a bedroom and the floor in a warehouse are not the same conversation.
Concrete doesn't dry, it cures. Walk away too early and it'll tell everyone.