Building a Shed Base

Shed plans often tend to be a little vague about the type of base required., and readymade sheds usually come with wooden floors that seem to need no base. Some shed designs make no mention of a base.

In our experience, any shed will require a firm, level foundation which could be made from compacted gravel or paving slabs, but we always recommend building a shed base in concrete if possible.

This will ensure that, if you build your own shed, it will not shift over time (avoiding stuck doors and windows, for instance), will last for many years and will help protect the shed timbers from rot and rising damp.

If you decide to keep the wooden floor supplied with all ready-made sheds, prepare a firm substrate first. This can be gravel, hardcore or even rubble but it must be fully compacted and leveled off. The shed’s floor must be kept clear of this substrate by using wooden formers or bearers spaced close enough together to prevent sagging of the floor.

If you place paving stones on top of the substrate you will still need the wooden formers to keep the shed floor clear of moisture and give it adequate ventilation.

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Concrete makes the best base or foundation when building a shed. Level the site roughly, then mark out the exact location for the base from the shed plan.  Use marker pegs at each corner linked by string (a nail on the top of each peg makes a good anchor). The dimensions of the slab should be those of the shed floor with ¾ inch deducted from the length and breadth measurements (to allow a small overhang by the shed walls).

Dig out an area approximately 6 inches wider than the marked-out area, and to a depth of about 2 inches. If the ground is soft, you should dig deeper, about 6 inches, and lay a 4 inch substrate of compacted gravel or hardcore under the concrete foundation.

Now you need to make a mould for the concrete shed base. Use 1 inch timber planks with a width equal to the chosen depth of concrete – we recommend 4 inches, but 3 inches would be satisfactory for a small shed. Cut 2 planks to a length equal to the shorter sides of the shed less ¾ inch, and cut 2 planks equal in length to the longer sides plus 2 inches.

Using the marker string as a guide, screw these planks together to form a box in the shape of the concrete floor – it is a good idea to attach the corner pegs to this box, making sure that their tops do not protrude above the box timbers, to help with the leveling. Use a tape measure and long spirit level to ensure that the moulding box is squared and level, and check that the diagonals are equal.

Mixing your own concrete is simple if you use a motorized cement-mixer and a dry concrete mix from your local hardware store to which you add water. Or you can do it the hard way, using buckets and a wheelbarrow! Do not plan to lay concrete in frosty conditions.

To calculate the amount of concrete you will need for building a foundation, use this formula (all measurements in inches):

• [Depth x Short Side Length x Long Side Length] x 4/3 ÷ 1728 cubic feet, or
• [Depth x Short Side Length x Long Side Length] x 4/3 ÷ 15,552 cubic yards

Note that the 4/3 multiple allows a 1/3 excess for wastage – you always need more concrete than planned!

Mix the concrete with water until you have achieved a uniform color and the mix is workable but not runny (an over-wet mix will produce weak concrete). Pour the mix into the wooden mould, pushing it into the edges and working it to-and-fro with a shovel to eliminate as much air as possible.

As you get close to the top of the mould timbers start to smooth the concrete surface with a long wooden batten or other straight edged implement so that, eventually, the surface is level with the timber mould top edges.

Allow the concrete to dry slowly over several days, if necessary spraying it lightly from time to time with water to keep it moist. Cover the shed base with a plastic sheet until it is really hard.

Finally, to remove the moulding timbers, tap each plank lightly and unscrew. Your concrete base is now ready for erecting your do it yourself shed! If you would like more information about building a shed base, we recommend visiting MyShedPlans –  it  includes a good guide to preparing foundations and mixing concrete in its huge shed building plans kit.  You will find it very helpful.

Click here for more information about building a shed base

To prevent damp penetration when shed building you should place a membrane of plastic sheeting or roofing felt between the concrete and the shed wall foot plates, to act as a damp-proof course, and you should omit the wooden floor.

Also, painting the concrete floor with a good-quality flooring paint will greatly reduce concrete dust in your shed over the years – and it looks good too.