How to lay a slab

Written by anthony smith | 13/05/2017
How to lay a slab
Lay your slab correctly and it will last for years. (StockSolutions/iStock/Getty Images)

Many of the most common home improvement projects involve laying concrete slabs. Use stone or concrete slabs for patios, driveways and as foundations for sheds. A properly laid concrete slab can last a lifetime, whereas one that is poured incorrectly can crack and crumble in a relatively short period of time.

Prepare a base for the slab by excavating it to a depth of 10 cm, clearing and levelling the ground on which the slab will be poured. Use a heavy duty plate compactor to compact the soil firmly, then water down the area, allow it dry, and compact it again. Take note of any high or low spots in the ground during this process and add or remove soil as necessary.

Add a 10 cm layer of loose gravel to the area and level it. Lay down the special barrier membrane material, such as polythene, so that moisture cannot seep up from the ground to the concrete.

Build a form for the slab by nailing together timber along the perimeter of the area for the slab, on top of the membrane. For driveways or other slabs that will have vehicle traffic, add steel mesh in the form to serve as reinforcement for the slab before pouring concrete.

Add timber boards laid on edge on opposite ends of the perimeter to serve as screed rails. Set and secure these screed rails with stakes so that the tops are at the exact planned height of your concrete slab.

Pour the concrete, levelling the slab as you go. Level the slab by laying one timber plank so that it lies on, and spans, the two screed rails. Drag the piece of timber using the screed rails as a guide so that the excess concrete is removed.

Finish the concrete surface with a hand trowel, or power trowel if the size of the slab dictates it, and allow the slab to cure.

Things you need

  • Plate compactor
  • Gravel
  • Barrier membrane
  • 60 cm by 1.2 m timber
  • Stakes
  • Steel mesh
  • Concrete
  • Trowel

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