How to lay a shed base
Concrete is perfect for creating a solid, hard-wearing base for your garden shed. The first step is mark out the area where the concrete will be laid, adding an extra 100mm to the width and length of the finished shed to allow a small edge around the base. Also add in a further 75mm all-round to accommodate the timber formwork which will hold the wet concrete in place while it is drying. The easiest way to mark the area is with a string line and stakes, making sure the corners are right angles.
The concrete base should be at least 75mm thick, with a similar layer of sub base, so you will need to excavate to the required depth and ensure that all plants, stones and debris are removed before compacting the ground with a roller or tamper.
Building formwork
The formwork is constructed using 25mm timber planks and it is vital that this is level as it will determine whether your finished concrete slab is level. Place additional pegs at 1 metre intervals around the outside of the formwork to provide extra support when the concrete is being poured.
Then add 75-100mm of hardcore and compact before adding a layer of building sand to protect the damp-proof membrane. If there are any joints in the membrane, these should be overlapped and taped and the edges should be turned up against the formwork to form a tray.
Concrete needs to be poured and levelled quite quickly as it begins to ‘go off’ within two hours of being mixed, so you will need to make sure everything is ready before the concrete delivery truck arrives. Calculate the volume of concrete you need (length of concrete slab x width x depth or use our concrete volume calculator) and make sure you order a concrete pump if your site is hard to reach or the volume of concrete if too large to place by wheelbarrow.
Concrete mix for shed base
A standard concrete mix ratio of 1 part cement to 5 parts ballast (aggregate and sand) is usually perfectly adequate for shed bases but you can add reinforcing fibres if you are housing anything particularly heavy. if you are unsure ask for advice from your ready-mixed concrete supplier.
It’s extremely important that you wear the correct safety clothing (long sleeves and trousers, gloves, rubber boots and safety goggles) as concrete is caustic and can cause serious injury if it comes in contact with skin or eyes. Learn more about safety precautions you should take here.
Laying a concrete shed base
Begin to pour the concrete into the furthest corner of the formwork and use a rake or shovel to spread it evenly, ensuring it is around 10mm higher than the formwork to allow for it to be compacted to remove any air pockets. This is a two-person job and requires the use of a straight piece of timber, longer than the width of your formwork. With one person at either end, the timber can be used to compact the concrete and then level it in line with the top of the formwork by performing a sawing motion. Ensure there are no low spots and the corners are filled before running a trowel along the sides of the formwork to smooth the edges and reduce the chances of it chipping.
It is important that the concrete doesn’t dry out too quickly so, if the temperature is likely to reach 20°C or above, use plastic sheeting to cover the slab for at least 24 hours. The concrete can be walked on within 48-hours but ideally the formwork should remain in place for at least five days before removing and building the new shed.
