Shed Bases

Why every shed needs a proper base

A shed is only as good as the ground under it. If one corner drops by a centimetre the whole frame racks out of square. That's when doors start to stick, windows crack and the roof lets water in. A good shed base stops that happening.

Whatever it's made of, a garden shed base has four jobs.

• Stay level across the full footprint, so the shed floor and walls sit true.
• Carry the weight of the building and everything you store in it without sinking.
• Drain, so rain runs away rather than pooling under the floor.
• Lift the floor off the soil, so airflow underneath keeps damp and rot away.

Grass on its own does none of this. It's soft after rain, it moves as it dries out in summer and it holds moisture against the floor. So whichever shed you buy, plan the foundation at the same time.

Match the foundation to your ground

Every UK garden behaves differently after a week of rain, so take a look at yours before you choose.

• Flat, firm lawn. The easy one. Lift the turf and an interlocking grid kit goes straight down.
• Heavy clay. Clay holds water in winter, can heave in a hard frost and shrinks in a dry summer. A compacted layer of sharp sand or hardcore under the grids keeps the shed foundation stable through both.
• A gentle slope. Dig the high side down rather than piling loose soil on the low side, or lift the building on adjustable legs.
• Puddles or a flood risk. Free drainage matters most here. An open grid lets water run straight through instead of sitting under the floor.
• An old patio. If it's level and sound it can carry a small shed as it is. If it isn't, ProBASE makes QuickJACK kits that level a shed on hard surfaces.

How plastic shed base kits work

A plastic shed base is a set of square grids that clip together into one flat platform. Every kit here is made by ProBASE. Each grid measures 500 x 500mm and is 40mm deep, moulded from 100% recycled plastic in an open pattern that water and air pass straight through.

You lay a weed membrane on the prepared ground, click the grids together on top, then fill every cell with pea gravel. The grid locks the stones in place so they can't spread or rut the way a loose gravel bed does. The gravel gives the base its weight and strength. The grid keeps it flat.

That mix is why plastic grids have become the most popular shed foundation for DIY. The grids are lightweight enough to carry round the side of the house, yet strong and stable once they're full. They're durable too, because recycled plastic won't rot in wet soil the way timber does. And reusing waste plastic makes them an eco-friendly choice.

Here's what you get in each kit.

• The interlocking recycled plastic grids, from 9 to 120 depending on the kit.
• A heavy-duty, commercial grade weed membrane to go underneath.
• Instructions, plus a 2 year guarantee from ProBASE.

ProBASE advises about 15kg of 10mm pea gravel per grid, which isn't in the box, so book a delivery from a local builders' merchant for the same week as the kit. There's no concrete to mix, no cutting and no special tools. And because nothing is fixed down, you can lift the grids and relay them if you ever move the shed. You can read more about the system on the ProBASE website.

Shed bases compared

Here's every ProBASE kit Shedstore sells, smallest first. The platform size is the real size of the finished base in metres, which is the figure to check against your shed. Gravel is the total at 15kg per grid. Prices move every day and the grid above always shows Shedstore's current price, so here they're grouped into bands.

The kits are named after the shed they're meant for, not the base itself, so the platform is often a little bigger or smaller than the name suggests. The 11x11 kit, for example, builds a 4 x 4m square.

What size shed base do you need?

Measure the shed, not the name on the box. A shed sold as 8x6 is rarely exactly 8ft by 6ft, so take the external floor size in metres from the product listing. Then pick a kit whose platform is the same size or slightly larger.

• Keter Factor 8x6 measures 2.57 x 1.82m. That's a few centimetres wider than the 2.5m platform of the 8x7 kit, so the 9x9 kit (3 x 3m) is the one that covers it fully.
• Keter Newton Plus 7.5x9 is 2.30 x 2.87m and sits neatly on the same 9x9 kit.
• Keter Stronghold 10x8 is 3.05 x 2.43m, so look at the 11x9 kit, which builds 3.5 x 3m.

A slightly bigger base is no bad thing. A margin of gravel around the walls gives you somewhere dry to step, keeps grass and soil off the panels and stops rain splashing mud up the sides. You'll find the exact sizes for each model on our Keter sheds and Palram Canopia sheds pages.

How to lay a plastic shed base kit

Most people can install a plastic grid base in a morning. Getting the ground ready takes the longest. Shedstore doesn't offer installation for these kits. If you've booked its team to build your shed, the base has to be finished before the fitters arrive.

1. Mark out the area a little larger than the platform with pegs and string.
2. Strip the turf and any soft topsoil, then rake the ground flat.
3. Compact the ground with a tamper or by treading it down firmly. On soft soil, tamp down a thin layer of hardcore first so the grids can't settle.
4. Check it's level with a long spirit level on a straight board, in both directions.
5. Roll out the membrane over the whole area.
6. Join the grids on top of it, working out from one corner.
7. Fill every cell with gravel, brush it across and check the level once more.

Then build your shed straight on top. Don't underestimate the gravel, though. The 8x7 kit takes about 375kg and the 20x12 kit over 1.4 tonnes. Ask for it to be dropped as close to the spot as you can, then share the barrow work.

Plastic grids vs concrete, slabs and timber

A plastic grid kit isn't the only way to build a shed base. Here's how the main options compare for a typical garden shed.

Concrete shed base

A poured concrete slab is the strongest and most permanent foundation. It means digging out around 150mm, laying compacted hardcore and a damp-proof membrane, then building a timber frame to hold the edges. After the pour it needs at least 48 hours to cure. A concrete base suits a big workshop or log cabin that will never move, especially one carrying a heavy load of machinery. For a storage shed it's a lot of work and cost. It's also very hard to take up later.

Paving slabs

Slabs bedded on sharp sand give a firm, flat surface for a smaller shed. You may even have some spare. Each slab has to be levelled on its own, which takes patience. Any gaps or rocking slabs will show up in the shed floor.

Timber frame and gravel

A pressure-treated timber frame filled with compacted gravel is cheap and drains well. Some people go simpler still with timber skids, a few pressure-treated bearers laid on the ground. The weak spot is the timber itself. Where it touches wet soil it will rot in the end, often long before the shed does, which makes it a budget fix rather than a lasting one.

Ground screws and adjustable bases

On a slope, levelling the ground by digging can be a big job. Adjustable systems lift the shed on height-adjustable feet instead. ProBASE's QuickJACK kits come in versions for hard surfaces such as a patio and for soft ground, where the larger sizes use screw piles. Compare them on our adjustable shed bases page.

For most garden sheds a gravel-filled plastic shed base is the best balance. It drains better than concrete and won't rot like timber. It goes down in a few hours without a mixer. Once you add up the materials and labour, it also costs far less than a slab.

Do plastic sheds need a base?

Yes, even more than a timber one does. A plastic shed is light and flexible, so a flat foundation matters more, not less. Resin panels are made to fit each other exactly. A floor that dips or twists pulls the doors out of line and stops the panels locking together during the build.

Many resin sheds include a floor. You'll find them in our plastic sheds with floors collection. That floor spreads the load but it isn't a foundation. On soft lawn it will flex and sink unevenly. Some models, like the Keter Darwin potting shed, come with no floor at all, so they need a solid, level platform to stand on.

The best base for a plastic shed is a gravel-filled grid. It's flat, drains freely and won't rot under the shed. Planning something bigger? Our large plastic sheds need the same firm start.

Planning permission and your shed base

A garden shed base on its own rarely needs permission. In England most sheds count as permitted development, as long as they meet a short set of rules. Among them, the building can't cover more than half the garden or sit in front of the house. Within 2m of a boundary it can't stand taller than 2.5m.

The base matters in one way. A platform raised more than 30cm above the ground is treated as a raised platform, which needs planning permission. A ProBASE grid is only 40mm deep, so it stays well within that. The Planning Portal guide to outbuildings has the full rules. The government's permitted development technical guidance explains each one with diagrams. Scotland, Wales and Northern Ireland have their own versions. Listed buildings and conservation areas may differ too.

Shed base FAQs

How much gravel do I need for a plastic shed base?

Allow roughly 15kg for each grid, then multiply by the grids in your kit. An 8x7 kit with 25 grids needs about 375kg. A 12x10 kit with 48 needs around 720kg.

Can I put a shed base straight onto grass?

Not on its own. Lift the turf first, then level and firm the soil underneath. Grass left under the membrane rots down. As it does, the base sinks unevenly and moisture collects under the floor.

Are plastic shed bases better than concrete?

For the average garden shed, yes. They're quicker and cheaper, they drain better and you can lift them if you move. Concrete is still the choice for a heavy workshop or a building that needs a permanent slab.

How long does a plastic shed base last?

The recycled plastic grids won't rot or rust the way timber and steel can. The maker backs each kit for 2 years. With the gravel in place they're built to sit under a shed for many years.

Can I use a plastic grid base for a summerhouse or log cabin?

Yes, the larger kits are sized for them. The 16x10, 18x11, 20x12 and 5 x 6m kits suit summerhouses, garden offices and log cabins. For any heavy garden building, follow the maker's own base advice too.

Do I need a shed base kit for a small storage box?

A small bin store or garden storage box still lasts longer on a level, dry surface. The 5x5 kit is the smallest here, or a few paving slabs work well. Browse our plastic garden storage boxes for the units themselves.