How to Lay a Concrete Shed Base Guide

Thanks to its strength, versatility and remarkable durability, concrete is ideally suited to home construction tasks.

By using concrete, you can quickly, easily and affordably create a shed base, which is dependable and extremely hard-wearing.

Below, we have shared a thorough step-by-step guide covering the entire process of how to lay a concrete shed base.

And, if you’ve never worked with concrete before, don’t worry. So long as you prepare the area with precision, and work with accuracy, it is a brilliantly easy material to use.

7 step guide to laying concrete for a shed base

Here at iMix Concrete our team can supply the exact the amount of concrete you need, at the grade you require. Contact our team for a quote today. See below our 7 steps for laying your shed base.

1. Measure

First, you will need to mark out the area where you will lay the concrete. Decide where you want to position your shed, and then mark out the area where the slab will be.

Remember that, when measuring the area, you will need to make it wider and longer than the shed itself will be. You also need to allow extra space for the wooden frame (also known as the formwork, which you will prepare in step 4).

We recommend adding an extra 100mm to both the width and length of the base, and adding a further 75mm to all dimensions, for the timber formwork.

At this stage, it’s critical that the area you mark out is even and symmetrical. You can avoid any uneven angles by driving wooden pegs in the ground at each of the area’s corners. Then, run a string between the two pegs at opposite corners, and measure the diagonal length. Repeat for the other pegs, and check that they are the same. If the lengths differ, then adjust your area accordingly.

2. Dig out the base area

Once the area is measured, you need to dig it out, to a depth of at least 150mm.

The concrete base itself will need to be a minimum of 75mm thick, and the sub base will need to achieve a similar depth.

As you dig, make sure that any plants, grass, stones and other debris are removed. Then, you need to make sure the bottom of the pit is level. So, once you’ve finished digging, compact the ground, either with a roller or a tamper.

3. Lay a sub-base

The sub-base acts as a barrier between the concrete and the ground, which is essential, because you cannot lay the concrete base onto soft ground directly.

So, prepare and lay the sub-base onto the ground. For this, we recommend using MOT Type 1 hardcore.

Then, once poured, compact it thoroughly and spread a layer of sand on top, to ensure the sub-base stones don’t puncture the damp-proof membrane.

4. Frame the slab area

The wooden frame (formwork) will go around the edge of the slab area. This is designed to keep the wet concrete in place while it dries.

Begin by constructing the frame, to the same size of your slab. We suggest you use treated timber (at lengths of 150mm x 25mm) and drill the pieces together.

Then, position the frame into the base, and drive wooden posts into the ground around the formwork to keep it in place. We recommend doing this at one-metre intervals, across the entire formwork.

It’s essential that the formwork is completely level, across all areas. You can do this by using a spirit level, and checking the position of all sides of the formwork and posts.

5. Add a membrane

A damp-proof membrane (DPM) is critical to ensuring the durability of your shed. Not only does it minimise the risk of condensation issues occurring, but it also helps to prevent the concrete from cracking when it dries.

For your DPM, choose some heavy-duty plastic sheets. Then, lay the sheets over the entire sub-base, and turn up the edges, so that it sits in the formwork like a tray.

If there are any joints, overlap the sheeting in these areas, and tape them in place.

6. Spread your concrete

Now, you are ready to pour the concrete into the base. As you pour it into the base, use a shovel or a rake to spread it across the entire area.

Then, once all the concrete is poured, tamp it down. This will ensure any air bubbles are removed, and that you achieve a neat, level finish.

7. Wait for it to cure

After you have tamped the concrete, lay a raised polythene sheet over the top (this will further reduce the risk of cracking).

Then, you will need to allow the concrete to cure. After 48 hours, you will be able to walk on it and you will be able to remove the framework. But, it will take a minimum of 28 days before the concrete is fully cured and ready for the shed.

What concrete is best for shed bases?

For shed bases, we suggest that you use one of the following concrete types:

• C20 / Gen 3 / ST1
• C30 / ST3

These will provide you with the required strength and durability for the base, while still being easy to work with.

At iMix Concrete, we are trusted by homes, businesses and construction sites across the country, to supply and prepare their concrete.

Our team of concrete specialists are efficient, highly skilled and work to an exceptional degree of accuracy. So, with their specialisms at your disposal, you can be certain that your DIY project will achieve a flawless finish, and remain in perfect condition for many years to come.

To find out more about any of our concrete supply and delivery services, contact our team today.