Laying a concrete slab

The steps, in order

1. Set out and dig

   Mark the footprint with a string line and pegs, leaving room around it for the formwork. Strip the topsoil down to firm ground: the ground carries the slab, not the concrete.

   Tools: string line, pegs, laser level, spade, a mini digger beyond a few square metres

2. Compact the sub-base

   Spread the hardcore or crushed stone in thin layers and compact each one. A thick layer compacted in a single pass stays soft at the bottom, and the slab eventually shows it.

   Tools: wheelbarrow, rake, plate compactor

3. Form up and set levels

   Fix the formwork boards to pegs, set to the finished height of the slab. Give a patio, or any floor that will be washed down, a slight fall towards the outside so water does not sit on it.

   Tools: saw, drill driver, laser level, straight edge

4. Membrane and mesh

   Under an indoor slab, a polythene membrane limits rising damp. Steel mesh sits on spacers, never on the ground: it only works when concrete surrounds it. Sheets overlap where they meet.

   Tools: angle grinder or bolt cutters, mesh spacers

5. Pour in one go

   Work out the volume before you order or mix, with the concrete calculator. With a mixer, keep every batch to the same mix. Beyond a few cubic metres, ready-mix delivered by truck saves a day of mixing and the cold joints between batches.

   Tools: concrete mixer, wheelbarrows, shovel, a generator if the plot has no supply yet

6. Level and finish

   Tamp and level the concrete with a straight edge riding on the formwork, in a sawing motion. Float it once the surface water has gone, then trowel it smooth, or brush it for an outdoor surface with more grip.

   Tools: aluminium straight edge, float, finishing trowel, stiff broom

7. Joints and curing

   A large slab cracks where it pleases unless you tell it where. Control joints, sawn or formed, divide it into bays. Then keep the concrete damp and sheltered from sun, wind and frost for the first days: concrete that dries too fast loses part of its strength.

   Tools: floor saw or diamond-bladed grinder, tarpaulin

Consumables

• Concrete: ready-to-use bags for a small area, cement, sand and aggregate at your chosen mix, or ready-mix.
• Hardcore or crushed stone for the sub-base.
• Polythene membrane under an indoor slab.
• Steel mesh and spacers.
• Formwork boards, pegs and release oil.
• Tarpaulin or curing compound to protect the fresh concrete.

Safety on this job

• Wet concrete burns skin. It is strongly alkaline: gloves, long sleeves and boots, and rinse at once with clean water if any reaches your eyes.
• Cutting dust contains silica. Cut wet where you can, otherwise wear a suitable respirator.
• Plate compactor and floor saw: hearing protection and safety boots.