Building a shed base might seem simple, but using the wrong concrete mix is one of the most common mistakes DIYers make. Get it wrong and you could end up with a base that cracks under weight, shifts over time, or crumbles when the weather turns cold. Getting it right is not complicated once you understand what the mix actually does.
Why the Shed Base Matters
A shed base does more than just give your shed something to sit on. It spreads the weight of the structure evenly across the ground, stops moisture from working its way up into the floor, and keeps the whole thing level over time. Without a solid base, a shed can warp, the doors will stop fitting properly, and the floor joists can rot from underneath.
Most people underestimate how much weight a shed can carry. Even a modest garden shed with a lawnmower, tools, and some stored boxes can put significant pressure on the ground beneath it.
What Mix Should You Use?
For a standard shed base, a 1:2:3 mix is the most widely recommended starting point. That means one part cement, two parts sharp sand, and three parts aggregate (usually gravel or crushed stone). This produces a concrete that is strong enough for most domestic shed bases without being over-engineered or expensive.
Some builders opt for a slightly stronger mix if the shed will be used as a workshop or will hold heavier equipment. In those cases, a 1:1.5:3 mix or a C20 ready mix concrete (which delivers around 20 N/mm2 compressive strength) is worth considering.
If you are ordering ready mix rather than mixing by hand, tell the supplier what the base is for. They will advise on the right strength class for your project.
How Much Concrete Do You Need?
This is where many people run into trouble. Ordering too little means a second delivery and an awkward join in the slab. Ordering too much wastes money and leaves you with concrete to dispose of.
To work out the volume you need, multiply the length by the width by the depth of your base. A typical shed base is around 100-150 mm deep. So for a 3 m x 2 m base at 100 mm depth, you would need 0.6 cubic metres of concrete.
Rather than doing this by hand, a shed base concrete mix calculator takes the guesswork out of the process. You enter your dimensions and it tells you exactly how much material to order.
Preparing the Ground First
Concrete is only as good as the ground it sits on. Before you pour, you need to:
- Remove all topsoil and vegetation from the area
- Dig down to a firm, stable sub-base
- Lay a layer of compacted hardcore or MOT Type 1 (roughly 100 mm deep)
- Use a spirit level and timber formwork to make sure the pour will be level
Skipping the sub-base is a common shortcut that causes problems later. Even firm-looking soil can shift seasonally, especially in clay-heavy ground.
Pouring and Finishing
Once the formwork is in place and the sub-base is compacted, you are ready to pour. Work the concrete into the corners and edges first, then use a straight timber to screed across the top. A steel float can be used to finish the surface, but a slightly textured finish from a wooden float is actually better for a shed base as it provides grip.
Leave the concrete to cure for at least 48 hours before placing anything on it, and ideally wait a full week before the shed itself goes up. Covering it with polythene sheeting helps during cold or wet weather, as concrete needs moisture to cure properly and should not dry out too fast.
Common Mistakes to Avoid
- Adding too much water to make mixing easier. This weakens the final mix significantly.
- Pouring in freezing conditions without protection. Concrete will not cure properly below around 5 degrees Celsius.
- Not checking levels before the pour. Once it sets, there is no adjustment.
If you are unsure how much to order, use a shed base concrete mix calculator before you book delivery. It takes seconds and can save a costly mistake.
A well-built shed base takes a few hours to lay and can last decades if done correctly. The concrete mix is a small but critical part of the process.
