Shed Base Calculator

We put the same 5 x 4m base through the shed base calculators that come in the top 10 on Google, and the MOT Type 1 answers ran from 2.2 tonnes to 5.04 tonnes for the same 100mm layer. Outdoor Aggregates' works from what a bag of our stone actually weighs.

Which Shed Base Do You Need?

The right shed base depends on the size of the shed, what goes in it, and how much digging you want to do. I always go for concrete for a big or heavy shed and slabs for a normal garden shed. A plastic grid is also effective for a small one on good ground, and you can use a timber frame where you want to stay away from concrete altogether.

I like paving slabs for most sheds: quick, nothing to wait for, and they lift again if the shed ever moves. For a workshop with heavy kit inside, I would always pour concrete. Whichever you pick, the turf and topsoil come off first and the MOT Type 1 goes down compacted. Our guide to laying a sub-base covers the dig and the compacting step by step.

How Much Concrete Do I Need for a Shed Base?

A concrete shed base needs its length times its width times its thickness, plus about 10% for spills and an uneven dig: an 8 x 6ft base at 100mm is 0.49m³. Here are the common shed sizes at 100mm thick on 100mm of MOT Type 1, mixed with 10mm Gravel:

The mix is 1 part cement, 2 parts sharp sand and 4 parts gravel, by volume. Our ballast and cement calculator goes into the mix in more detail. Above about 1m³, ready-mixed concrete delivered to the site is usually quicker and often cheaper than mixing it yourself.

Paving Slab, Plastic Grid and Timber Frame Shed Bases

How Many Paving Slabs for a Shed Base?

A paving slab shed base needs the base area divided by the area of one slab, plus about 10% for cuts and breakages: 14 slabs of 600 x 600mm for an 8 x 6ft shed. The 10% covers the slabs you cut to fit along the edges and the odd one that cracks. The slabs stop at the edge of the shed floor rather than running out past it: here is why a shed base has no overhang. Lay them so every timber under the shed floor sits on a slab, never on a joint. Our guide to laying paving slabs on a dry mix covers the bed, and the patio calculator handles bigger slabbed areas.

What Goes Under a Plastic Grid Shed Base?

A plastic grid shed base goes on weed membrane over level, firm ground, with the cells filled with gravel at about 10kg per 500 x 500mm panel. An 8 x 6ft base of 20 panels takes about 188kg of 10mm or 20mm Gravel. Grids are only as good as the ground under them, so pick "Needs levelling" in the calculator on a lawn that dips, and it adds 50mm of MOT Type 1. I would not put a heavy shed on a grid at all. Our page on what to put under gravel explains the membrane.

What Timber Do You Need for a Shed Base Frame?

A timber frame shed base uses 4 x 2 (about 47 x 100mm) treated bearers running the length of the shed at 400mm centres, with a rail across each end. For an 8 x 6ft shed that is 6 bearers of 2.44m and 2 end rails of 1.83m, about 18.3m in all. The frame sits on weed membrane over 100mm of compacted MOT Type 1. Level it with packers rather than by digging under one corner, and use timber treated for ground contact.

How Much Bigger Should a Shed Base Be Than the Shed?

A shed base should be the same size as the shed floor, not bigger, so rain running down the walls drips off onto the ground instead of onto the base. Some guides say to make the base 50 to 75mm bigger on every side. I disagree. That lip of concrete or slab sticks out past the cladding, catches every drop that runs off the walls, and holds it in a puddle against the bottom rail. The timber sits wet for days after rain, and that is where sheds rot first. Many shed makers say the same: the base should be the exact same size as the shed floor.

Check the real floor size with the shed maker before you dig, because an "8 x 6" shed is rarely exactly 8ft by 6ft. Type that figure into the calculator rather than using the size buttons.

Next Steps

We stock MOT Type 1, 10mm Gravel, 20mm Gravel, grano dust and the heavy duty weed membrane with its pegs. A bulk bag of MOT Type 1 is £114 delivered to your kerb. If your shed base is bigger than the calculator is happy with, or the ground is something odd, call us on 0330 043 7585 and we will work it out with you.

Have you ever laid a base for a shed: which kind did you go for, and how deep did you dig? Tell us how it has held up in the comments below.

FAQs

How long before a shed can go on a new concrete base?

A new concrete shed base can take foot traffic after a day or two, but leave it about a week before the shed goes on. Concrete keeps gaining strength for about 28 days, so keep heavy kit off it until then.

Is 3 inches of concrete enough for a shed base?

A 75mm (3 inch) concrete shed base is enough for a small, light shed on a well compacted MOT Type 1 sub-base. For an average garden shed, 100mm is the standard, and 150mm suits a workshop or anything heavy.

Can you put a shed straight onto gravel?

A shed should not go straight onto loose gravel, because the bearers rock and sink into it. Gravel works when it is locked into a plastic grid on membrane, or used as the stone in concrete.