Building a shed base or foundation

LAST UPDATED: 4th January 2023 by The Editorial Team

Building a shed base or foundation

Summary: Step by step guide and video showing how to build a shed foundation and a shed base with concrete or slabs.

Building a concrete shed base

When constructing a shed, you need a firm and solid base on which to build on top of. The concrete will need to be made waterproof underneath to stop moisture from rising up. There are two main ways of laying the concrete base. You can have a base that is set into the ground or one that is raised above ground level.

Concrete entrenched base

  • For an entrenched base, the procedure is quite simple. Start by measuring the area 300mm wider and longer than the floor of your shed and excavate 100mm.
  • Once you have made the trench, simply lay the waterproof membrane and pour the concrete into the space.
  • See project on how to mix concrete.

Concrete raised base

  • You need to make sure the ground is firm enough for a concrete base before you start work. Do this by hammering a 50 x 50mm post into the ground. You will know the ground is hard enough if you are struggling after about 150mm into the earth.
  • If the ground is not hard enough you will need to lay a hardcore sub-base. To do this measuring an area 500mm wider and longer than the floor of your shed and excavate 4in.
  • Fill with scalpings or aggregate into the excavation and compact using a vibrating plate.
  • Once completed, you need to create a timber frame for the edges within which the concrete will be poured to form the base. The waterproof membrane is laid inside this framework.
  • The timber should be 150mm x 25mm and the timber frame should be 300mm longer and 300mm wider than the floor of your shed.
  • The timber should also be treated to protect it from weathering.
  • Peg the timber into position so it does not move under the weight of the concrete. Use a spirit level to check that the timber is level. Once you are sure it is level, nail the timber boards to the pegs using nails.
  • Spread a layer of sand into the square frame. This will prevent the membrane from being punctured. Lay the membrane on top of this sand and tuck into the corners as well as you can.
  • Mix the concrete to the correct ratio and wetness and pour into the square. Using a length of timber, you need to tamp the concrete so it is spread evenly and smoothly. You will need the help of another person to hold the other end as you both drag and pat the surface of the concrete.
  • The next stage is to construct a wall to run around the perimeter of the concrete base. This helps prevent damp from seeping between the concrete base and the membrane. To do this, excavate a trench around the perimeter of the concrete base 50mm deep and 125mm wide and fill with concrete.
  • Remove the timber frame once the concrete has dried. This should be after a day or so.
  • Lay two courses of bricks on the concrete foundation you have made surrounding the shed base. Make sure the membrane is tucked up between the brickwork and the concrete base. Cut away any surplus membrane.

How to make a concrete shed base video

How to lay a shed base with slabs video

Author

The Editorial Team

The Editorial Team at lets-do-diy.com are a team of experts led by Mark Wilson. We have been creating DIY projects, articles, costs guides and product reviews since 2008. For more info see our about page.