How to Build a Concrete Shed Base

Building a good base for your garden shed is very important. We provide a step by step guide on the best practices for building a concrete base area for your new shed to be sited on.

A vital part of the preparation for installing any shed is to create a strong and level base for it to sit on top of, if the base is not level you may find that vital screw holes or interlocking parts will not line up correctly making the final assembly difficult and unstable, especially in high winds.

When it comes to shed bases, there are several options available to you, these include:

  • Paving Slabs
  • Pre-Made Timber Frame
  • Pre-Made Plastic Frame
  • Concrete Base

Creating a base out of paving slabs can be pretty simple option, but it can be hard to achieve a flat surface if you're trying to create one on top of grass or soil. Pre-made timber and plastic bases are another possible solution, although for larger shed sizes can end up being more costlier. The later option being a concrete base, which can be a cheaper alternative, especially on larger size buildings and are almost always the most stongest, durable and long lasting.

If creating a concrete base sounds like the best solution for you, this is our 'how to' guide...

Building a Concrete Base

Step 1:

Before you build your shed base you will need to choose a suitable location that gives you enough distance from any fences or hedges to provide easy access to all sides of your shed. Try to choose a site where the land slopes away for better drainage and don't forget that, in addition to everyday tools such as spade, mallet and tape measure, you'll need some durable plastic sheeting to provide a moisture barrier, wooden pegs or stakes and timber to create a frame.

Step 2:

Level the area to be used with a rake and spade, removing all vegetation and roots. When measuring out the area for a shed base we recommend adding a 5cm (2ins) lip around the outside of the building, then use pegs and string to mark out the area. As the base will require a 7.5cm (3ins) layer of compacted hardcore then a 7.5cm (3ins) layer of concrete, the area within the pegs now needs to be excavated to a depth of least 6 inches. Hammer small stakes into the ground so that they are just above the top of the excavation. Nail strips of plywood or timber flush with the top of the stakes along the edge of the excavation, this forms what is called 'shuttering', a sort of mould for the concrete. There should also be extra stakes or pegs at a regular distance of 8 – 10 inches to keep the plywood in place when the shuttering is filled with gravel and concrete. Make sure that the distance between the two diagonals is the same and use a spirit level to ensure a 100% level base. 

Step 3:

Add the first layer which will be the hardcore and thoroughly compact it. You will want to spread a thin layer of sand over the top, and a damp-proof membrane (sized to the entire surface area the shed will occupy) should be placed 3ins on top of the sand. Metal or single walled vinyl clad sheds will suffer from condensation if any dampness is present and that's why plastic sheeting is so important. If you don't have any damp proofing, any moisture in the foundations, will enter the shed and react with the warmer air to cause condensation. The condensation cycle is difficult to stop, therefore, it is vital to prevent it as far as possible in the first place. For more information read our guide on How To Prevent Condensation In A Shed.

Step 4:

You will then be able to mix the concrete using one part cement to five parts ballast and only a small amount of water as you want the cement to be on the dry side. Dry-mixed concrete can be purchased which you just have to add water to, making sure you follow the instructions on the bag. Once mixed you will want to spread the concrete evenly into framework, for best results allow the concrete to go slightly higher than the framework as this will make levelling the concrete a lot easier as you can get rid of any excess. Using a straight edge of timber resting on the framework, you then use a sawing motion over the entire surface of the concrete to create a smooth level surface.

Step 5:

Once you have finished levelling the concrete it would be a good idea to check the weather forecast for any bad weather. If rain has been forecast you will want to cover the concrete with polythene for 24 hours, if warm weather is forecast then cover the entire surface with wet sacks and keep damp for 24 hours. Using these methods ensures that the concrete does not shrink or crack.