How to build a concrete shed base
By Tom Drake | 2nd November 2024 | 6 min read
Concrete is one of the best options for building a shed base. It’s strong, easy to level, and resistant to shifting or settling. In this guide, we outline how you can lay your concrete shed base.
Laying a concrete shed base steps:
1. Mark & clear the area
Before you can start building your shed base, you need to mark out the area where you’ll build the base and clear it.
Your shed base should be roughly the same size or slightly bigger than the size of your shed. Use a builders square, brick line and wood stakes to mark out the size and shape of your concrete base. You can also use a line marker spray.
Once you’ve marked out the dimensions of the base, remove any turf and dig to the correct depth. For most sheds, you’ll need to dig to a depth of 150mm. This allows for 75mm of concrete and a 75mm sub base.
2. Lay the sub base
Once the base is marked out and cleared, you can add the sub base. We recommend adding 75mm of MOT type 1 for your shed base.
You can use wood stakes to ensure the base is the correct height and level. Hammer the stakes so the gap from the ground to the top of the stake is 75mm, then lay a spirit level between each of the stakes to check they're level.
You can then add your sub base and compact it using a wacker plate. Remember to wear ear defenders, safety boots, and safety goggles when using a wacker plate.
Run the wacker plate over the base until it’s level with your stakes. You may need to add more as you go.
3. Build the formwork
With your base laid, you can create the formwork that will support the concrete until it's set. Your formwork should be constructed from 25mm thick timber that is as wide as the depth of your concrete base.
Cut a piece of timber to the length of one end of your shed base and place it in the correct position. Hammer in two stakes at both ends of the piece of timber, use a spirit level to check its level, then secure it to the stakes.
With the first piece of timber in place, you can continue building the rest of the frame. Position stakes at 1 metre intervals around the edge of the formwork to ensure it stays solid as you pour the concrete. Use a spirit level and builders square to ensure the formwork stays square and level.
Concrete will harden two hours after it’s been mixed, so it needs to be laid and tamped within that time. If you don’t think you’ll be able to lay an entire base in this time, divide your base into sections that you can complete one at a time.
4. Add damp proof membrane
Before pouring concrete into the formwork, add a damp-proof membrane above the sub base. A damp-proof membrane will help prevent condensation issues inside your shed and will stop the concrete drying too quickly.
Cut the damp-proof membrane to size, tape any joints, and lay it inside your formwork like a tray.
5. Mix the concrete
With your formwork and damp-proof membrane in place, you’re now ready to mix up your concrete.
For a concrete shed base, a 1:2:3 concrete mix ratio of 1 part cement, 2 parts sand and 3 parts aggregates (or 1 part cement and 4 parts ballast) will usually be adequate. For more information on concrete mix ratios, take a look at our guide.
You can mix concrete either by hand or with a cement mixer. Unless you’re building a small shed base, using a cement mixer is recommended. We’ve created a comprehensive mixing concrete guide to help you achieve the perfect concrete consistency.
6. Pour the concrete
Use a wheelbarrow to pour the concrete into a corner of the formwork.
Spread the concrete with a rake or a shovel, leaving it around 18mm higher than the top of the formwork. Work in manageable sections about 1 metre to 1.5 metres square. Push the blade of your shovel up and down the wet concrete to remove any air pockets.
7. Tamp the concrete
Once you’ve poured the concrete into your formwork, you need to level it and tamp it down.
With a helper, compact the concrete using a straight piece of timber that's longer than the width of the shed base. Start at one end and use steady blows of the plank across the base to tamp the concrete down. Move the plank slowly from one end of the shed base to the other.
To remove excess concrete and level the surface, go back to the end where you started and slide the tamping board backwards and forwards in a sawing motion across the base. Fill any depressions in the surface and repeat if necessary until you have an even surface flush with the top of your formwork.
Once you’ve finished tamping, run an edging trowel along the edge of the formwork to round off the exposed edges of the concrete and prevent chipping.
When can I build my shed?
You should wait at least 24 hours, 48 hours if possible, before removing the formwork and walking on your new shed base. However, it can take up to 28 days for concrete to reach full strength, so it’s best to wait until this time before building your new shed.
Can I pour concrete straight onto soil?
Pouring concrete straight onto soil is likely to lead to movement and cracking over time. There will also be a higher chance of moisture damage, so a sub base is recommended.
If you don't think concrete is the right choice for your shed base, you can also use paving slabs, timber, plastic and a range of other options. Take a look at our shed base range and our shed base guide to ensure you make the right choice for your shed.
