How to build a concrete shed base

A concrete base is by far the sturdiest, most permanent foundation for your shed, keeping it level and clear of groundwater seepage which causes rot. 

Follow our step-by-step guide to build a rock-solid concrete base for your shed, workshop, log cabin or large garden building. If you're reasonably fit and DIY-savvy, you can lay a concrete base in a day, and it will help keep your shed in excellent condition for years to come.

At a glance:

1. Choose the right location
2. Level the site
3. Build a timber frame to contain the concrete
4. Mix the concrete and pour it into the frame
5. Install your shed on top of the base.

You will need:

• Sand, cement and water (or ready-mixed concrete)
• Tape measure, pegs and string
• Set square and spirit level
• Spade and rake
• Finely-broken hardcore and earth rammer
• Timber (around 25mm thick)
• Cement mixer or mixing board
• Wheelbarrow
• Tamp board
• Floor bearers

How much concrete will I need?

To work out how much concrete you will need for your new base, use the following equation:

slab depth x slab width x slab length

Concrete is usually measured in cubic metres. So, for a 100mm deep concrete base that’s 3m x 2m, you’ll need 0.10m x 3m x 2m = 0.60 cubic metres. Add 10% contingency and round up small decimals (so, in this example round up to 0.70 cubic metres). It’s better to have too much concrete than not enough!

Once you have your tools and materials assembled, follow these steps to build your concrete shed base.

Step 1: Choose the right location

What will you use your shed for? Try to visualise it in position, and make sure you have sufficient access and services for its intended use. You’re unlikely to need permission to put up a shed – but if you’re unsure, see our guide to planning permission.

Try to leave a one-metre gap between your shed and any surrounding walls or fencing. This will make construction easier, and you’ll have access for regular maintenance. You’ll probably want to clear away weeds, treat the timbers, or even paint your shed – so allow space for this. This also lets the building ‘breathe’, reducing damp and the potential for rot.

Avoid a shady spot if you need natural light (but be wary of overheating in direct sunlight!) If you need water and/or electricity, think about the location of the mains supplies when siting your shed. Or you could look into powering your shed off-grid.

Step 2: Level the site

Ensure the ground is clear and level. The flatter your site is to begin with, the easier the job will be. If your base isn’t level, it won't support your shed properly and may compromise the integrity of the structure over time. The timbers will flex with changes in temperature and humidity – and the doors and windows might not shut properly.

Use the tape measure, pegs and string to mark out the area for the base. It’s advisable to make your base slightly larger than your shed – so add 30-40mm to each side. Use a builder’s set square to check each corner is a 90-degree angle.

Remove all vegetation and turf inside the perimeter, and dig out the topsoil to a minimum depth of 150mm: 75mm for hardcore, plus 75mm of concrete. Remove the marking-out strings, but leave the pegs in place for later use. Add the hardcore, rake it level and compact it with the earth rammer. Finish it off with compacted sand if you wish.

Step 3: Build a timber frame to contain the concrete

Build a timber frame to contain the concrete while it sets. Make sure the inside measurements match the dimensions of your base. Measure the diagonals and make sure they’re equal to guarantee a square base. Use a spirit level to check the frame sits level on top of the hardcore, and make any necessary adjustments.

Use additional wooden pegs, approximately every 600mm, inside the frame, to hold it securely in place. Make sure your pegs sit below the top level of the frame, though, or it will be very difficult to level off the surface of the concrete.

Step 4: Mix the concrete and pour it into the frame

Either use ready-mixed concrete, or make your own by mixing sand, cement and water according to the manufacturer’s instructions (usually a cement to sand ratio of 1:5). Add water slowly until you have a workable consistency. If you mix a small amount at a time, use the same amount of water each time. For large quantities, hire a cement mixer.

Pour a layer of concrete into the frame, making sure you get it into the corners and edges. Spread it evenly and compact it with the rammer. Tap the sides of your frame with a hammer to knock out air bubbles and get a nice solid edge to the slab. Continue pouring and spreading layers of concrete until it slightly overfills the frame.

Use a length of timber as a ‘tamping beam’. Use sawing motion across the slab, working from one end to the other. This will level off the concrete, and leave it flush with the top of your frame. Use a spirit level to ensure you have a flat, even surface.

Step 5: Install your shed on top of the base

Leave your new concrete base to set for at least three days before removing the frame. If it rains, cover it with plastic sheeting. Don’t allow it to dry out too quickly: in warm weather, prevent cracking by spraying it with water and covering with some old damp sheets.

Before assembling your shed on top of your new base, add floor bearers. These raise your shed a few centimetres from the base, and help protect your shed from damp by allowing air to circulate.

Use 75mm x 50mm timber bearers, pressure-treated against insect attack and rot. Space them equally at intervals of approximately 400-600mm, and lay them perpendicular to any floor joists which already come built into your shed. Make sure they’re level with one another using a spirit level. You’re now ready to install your shed on top of your base!

Building a concrete shed base is an achievable task for any competent DIY-er. A well-maintained garden building on a solid, level base will give you years of service and enjoyment.