How To Lay Concrete
A straight length of timber and some careful prep are all it takes to lay a flat and solid concrete floor.
The short answer
To lay concrete, clear the ground, lay a polythene damp-proof membrane and fix timber edging where you want straight sides. Pour the freshly mixed concrete evenly, spread it with a shovel or trowel, tamp it with a straight-edged length of timber to drive out air, then draw the timber across to level it. Leave it 24 to 48 hours to set before removing the edging.
Laying concrete doesn't need specialist tools. What makes the difference is proper ground prep and a quick pace once the mix is ready, as fresh concrete only gives you a short window before it starts to stiffen.
The photos in this guide come from a floor we laid during one of our renovations, where a section of the old tiled floor needed filling with new concrete over a damp-proof membrane. If you haven't mixed yours yet, start with our guide on making concrete in a bucket with the right ratio, then come back here to lay it.
Planning The Pour Before You Mix
Have everything in place before the first batch goes in the bucket. That means the ground cleared, the membrane down, the edging timbers fixed and your tools within reach. Mixing in smaller batches helps if you're working alone, and a second pair of hands to mix while you lay makes the whole job far less of a rush.
What You'll Need
- Freshly mixed concrete
- Shovel and trowel
- Heavy-duty gloves
- Damp-proof membrane (for floors and bases)
- Straight-edged lengths of timber for edging and levelling
How To Lay Concrete Step By Step
1. Prepare The Ground
Clear out any loose debris, rubble and soft spots so the concrete sits on a firm base. For a floor, lay a damp-proof membrane over the ground to stop moisture rising up through the concrete. Polythene membranes come in different gauges, and a heavier gauge stands up better to being walked on and shovelled over.
Think about the edges at this stage too. Lengths of timber fixed along the sides give you crisp, straight edges and something to level against. They come out once the concrete has set.
2. Pour The Concrete
Use the concrete as soon as it's mixed, as it becomes unworkable once it starts to set in the bucket. Tip it along the area as you pour rather than dumping it in one heap, and you'll save yourself a lot of pushing around afterwards.
Any high spots can be moved with a trowel or a length of timber on a small area, or a shovel on a bigger one. Keep the shovel shallow so you don't catch and tear the damp-proof membrane underneath.
3. Tamp Out The Air Pockets
Trapped air leaves voids that weaken the concrete once it dries. Work along the surface with the edge of a straight length of timber, bringing it down in short, firm chops. As the mix compacts, a little moisture rises to the top, and that's the sign you're doing it right.
4. Level The Surface
Once the concrete is compacted, draw the same timber across the surface in a side-to-side sawing motion, resting it on the edging so it scrapes off the high points and drags concrete into any dips. The two photos below show the timber being pulled across the floor.
On this job we laid self-levelling compound over the new concrete afterwards, so the concrete didn't need a perfect finish. We still got it as flat as we could with the timber, as a flatter base means less compound and a smoother result.
5. Leave The Concrete To Cure
Concrete sets within 24 to 48 hours, at which point you can take out the edging timbers. It keeps gaining strength for weeks after that, so hold off any heavy work on top for at least a week, and longer for anything carrying serious weight. In hot weather, cover it with polythene or mist it with water so the surface doesn't dry out too fast and crack.
When You Can Walk And Build On New Concrete
Light foot traffic is fine once the concrete has set, but floor finishes, heavy loads and vehicles need it to be properly cured and dry. Our guide on how long concrete takes to dry covers the timings for different jobs and what slows drying down.
Ready-To-Use Bags Or Mixing Your Own
Mixing the concrete is the hardest part of the whole job. Ready-to-use bags that only need water skip the measuring, which makes them handy for small jobs like setting a post. For anything bigger, mixing your own from cement, sand and aggregate costs less, and a good paddle mixer takes most of the effort out of it.
Laying Concrete Without The Rush
Laying concrete well is mostly down to what you do before the first bucket is mixed, with the ground cleared, the membrane down and the edging fixed. After that, a straight length of timber handles the tamping and levelling. If you can rope in a helper, do, as someone mixing while you lay keeps you ahead of the concrete as it stiffens. If there's an old slab in the way first, our guide on breaking up old concrete covers taking it out.
Your Questions Answered
Do you need a damp-proof membrane under concrete?
A damp-proof membrane is needed under a concrete floor inside a house, as it stops ground moisture rising through the slab into the floor finishes. Use a polythene membrane and overlap and tape the joints. Outdoor bases for sheds and paths don't always need one, though it still helps keep the concrete drier.
How thick should a concrete floor be?
The thickness of a concrete floor depends on what it has to carry and what's underneath it. A patch infill matches the depth of the surrounding floor, while new floors, bases for outbuildings and anything taking vehicles need more depth. For new floors and structural work, check Building Regulations or ask building control before you start.
Can you lay concrete in the rain?
Avoid laying concrete in heavy rain, as the water washes cement from the surface and leaves it weak and dusty. Light drizzle is manageable if you cover the concrete with polythene as soon as it's levelled. Frost is the bigger risk, so don't lay concrete when temperatures are close to freezing.
How do you get a smooth finish on concrete?
Get a smooth finish on concrete by levelling it with a straight edge, letting the surface water disappear, then working over it with a steel float in sweeping arcs. Floating too early brings water and fine material to the surface and weakens it. For a perfectly flat floor ready for tiles or vinyl, finish with self-levelling compound.
Is a helper really necessary when laying concrete?
Not on a small patch, where one person working in smaller batches can keep up. On anything bigger, a helper makes a real difference. While one of you mixes, the other lays and levels, so each batch goes down while it's still workable instead of stiffening in the bucket.
What's the biggest mistake people make when laying concrete?
Starting to mix before the ground is ready. Once the concrete is in the bucket there's no time to go back for loose rubble, a soft spot or an edging board that isn't fixed. Close behind is skipping the tamping, which leaves air pockets that weaken the slab once it dries.
Does the concrete need to be perfectly flat if I'm covering it?
No. Our concrete went under self-levelling compound, so it didn't need a perfect finish. Getting it as flat as you can with the timber still pays off, because every dip you leave is more compound to buy and pour. Let the concrete dry before the compound goes on, and check the compound's instructions for how dry the base needs to be.
How do I stop the damp-proof membrane tearing while I pour?
Start with a heavier gauge of polythene, which copes better with boots and shovels. When you move the concrete, keep the shovel blade shallow so it skims the mix instead of digging down into the membrane. The less shifting you need to do, the safer the membrane is, so pour along the area instead of into one pile.
Dave, or Demolition Dave as the team calls him, worked as a builder for over 10 years before joining DIY Works. He now concentrates on his own properties and writes most of our construction content, along with tools, carpentry and plenty more.