Concrete vs Paving Slabs vs Plastic Grid: Which Shed Base Should You Buy? (UK Guide)

Every garden shed needs a proper shed base underneath it, but that doesn't mean every shed needs a poured concrete slab. The right choice comes down to how big and heavy the shed is, how flat your garden already is, and how much time you want to spend. A plastic grid base goes down in an afternoon and suits most small to medium sheds. Paving slabs cope with a bit more unevenness for a bit more effort. Poured concrete is really only worth it under a large workshop-style shed or anything holding serious weight. Skip a base altogether and even a well-built shed will lean, sink or rot within a couple of seasons.

In this article

• Do you actually need a shed base?
• Concrete vs paving slabs vs plastic grid: at a glance
• Concrete shed bases: cost, pros and cons
• Paving slab shed bases: cost, pros and cons
• Plastic grid shed bases: cost, pros and cons
• How big should the base be?
• Common shed base mistakes to avoid
• FAQ

Do you actually need a shed base?

Yes, in effectively every case. A shed put straight onto grass or bare soil sinks and twists as the ground shifts, and the timber floor bearers sit in permanently damp ground and rot from below. It's a common story: sheds installed straight onto a lawn start sinking within a season, grass grows up through gaps in the floor, and in some gardens foxes dig underneath an unsupported shed and cause it to subside further still.

Even an existing patio isn't automatically safe to build on. If it isn't level, rainwater pools under one side and the timber floor rots the same way it would on grass. A shed floor also needs an air gap underneath it, not just something solid to sit on, or damp gets trapped even on a good concrete base. See our guide to shed size and planning permission if you haven't settled on a shed size yet.

Concrete vs paving slabs vs plastic grid: at a glance

Here's the short version before the detail, based on a typical small to medium shed in the 6x4ft to 8x6ft range:

Concrete shed bases: cost, pros and cons

Poured concrete is the strongest and most permanent option, and the only sensible choice under a large workshop-style shed, anything holding heavy machinery, or a building you want to stay flat and solid for decades.

Expect to pay roughly £200 to £350 in materials for a DIY base under a typical medium shed, or from around £850 to £1,000 for a professionally installed base nearer 3m x 4m, based on UK tradesperson cost-guide data. A poured slab needs the ground excavated to around 150 to 200mm, a compacted hardcore sub-base, and then time to cure: at least 48 hours before you can build on it, and closer to 28 days before it reaches full strength.

The trade-off is effort and permanence. Concrete is hard to remove or relocate later, needs real DIY confidence (or a tradesperson) to get right, and copes badly with being rushed. Get the levelling wrong while it's wet and you're stuck with it.

Paving slab shed bases: cost, pros and cons

Paving slabs sit in the middle: sturdier than a plastic grid, less permanent and easier to DIY than poured concrete.

Materials typically cost £100 to £200, cheaper still with secondhand slabs. Because water drains through the gaps rather than pooling on a solid surface, paving generally handles rain better than a concrete pad. The catch is that each slab has to be individually bedded and levelled on compacted sand or hardcore, which is fiddly across a whole shed footprint, and any slab that's out of level tends to rock or lets one corner of the shed sit low.

Paving slabs suit small to medium sheds on ground that's already fairly flat. On a sloped or uneven garden, expect considerably more prep work than the other two options.

Plastic grid shed bases: cost, pros and cons

A plastic grid base is the fastest and cheapest option to install, and it's what Opal Retail sells directly. Interlocking panels click together on firm, level ground: no cement, no curing time, and no damp-proof membrane needed underneath a shed that already has its own floor. Opal's ECO Plastic Grid range is independently tested to support more than 150 tonnes per square metre, is made from recycled plastic, and comes with a 25-year manufacturer's warranty.

Real feedback on plastic grid bases is genuinely mixed, worth knowing before you buy. Some DIYers find the panels fiddly to get flat on uneven ground and would rather have laid slabs; others get on fine, especially once the panels sit on a shallow bed of fine (roughly 10mm) gravel rather than directly on soil. It's the quickest and least permanent option, and suits a small or medium shed on ground that's already reasonably flat far better than a large, heavy structure or a sloped garden.

SHED BASE, 6X4FT

ECO Plastic Grid Shed Base, 6x4ft

Interlocking recycled-plastic grids sized for a typical small to medium shed. Click-fit, no cement needed.

From £89.99 Shop This Size →

SHED BASE, 9X6FT

ECO Plastic Grid Shed Base, 9x6ft

The larger option for bigger sheds, independently tested to support over 150 tonnes per square metre.

From £139.99 Shop This Size →

How big should the base be?

Whichever base you choose, get the size right. For a concrete or paving base, extend it beyond the shed's actual footprint: add roughly 25mm on each side for a shed under 10m² floor area, and 50mm on each side for anything bigger. That overhang gives rainwater running off the roof somewhere to land other than bare soil at the shed's edge. Plastic grid and timber-frame bases are the exception: size these to match the shed's footprint exactly, rather than oversizing.

Before laying any base, mark out the area with pegs and string and check the diagonals match, so the base is genuinely square rather than just roughly the right size. Clear turf, roots and topsoil from the area, and compact the ground properly before adding your chosen base material. This groundwork step is where most DIY bases actually fail, not in the base material itself.

Common shed base mistakes to avoid

The same mistakes come up again and again with DIY shed bases:

• Building straight onto grass or soil with no base at all
• Skipping ground compaction, which lets the whole base settle and twist later
• Ignoring drainage, so water pools under the shed instead of running away from it
• Not checking the base is level with a spirit level before building on it
• Undersizing the base, leaving part of the shed floor over unsupported ground
• Building on fresh concrete before it's had time to cure properly
• Leaving out a damp-proof membrane under a paving or gravel base

Any one of these on its own can be enough to make a shed lean, let a floor rot, or force a rebuild within a year or two. See our guide to choosing between metal, wooden and plastic sheds if you haven't settled on the shed itself yet.

Frequently asked questions

Do I need a base for a garden shed?

Yes. Every shed needs something firm, level and dry underneath it. A shed set straight onto grass or bare soil will sink, twist out of shape and start rotting from the floor up, usually within a season or two.

What's the cheapest shed base?

A plastic grid base is typically the cheapest and quickest option for a small or medium shed on flat ground, from around £89.99 for a 6x4ft kit. Paving slabs come in at a similar or slightly higher materials cost, and poured concrete is the most expensive option by some margin.

Can I put a shed straight onto an existing patio or concrete slab?

Sometimes, but only if the existing surface is genuinely level and well-drained. An uneven or sloped patio will still let water pool under one side of the shed, and the floor needs to overlap any damp-proof layer underneath it, or the exposed edge will still get wet and rot the bearers.

Are plastic shed bases actually any good?

They do the job well for most small to medium sheds, but they're not universally loved. Some people find the panels fiddly to level on uneven ground; others get on well with them, particularly laid over a shallow bed of fine gravel rather than directly on soil. They're not designed for large, heavy structures like garden offices or workshops.

Does a shed base need planning permission?

Not on its own, in most cases. Garden sheds and other outbuildings are usually permitted development, provided they meet standard conditions: single storey, under 2.5m eaves height if within 2m of a boundary, no more than half the garden covered, and not built forward of the house. Check the Planning Portal's outbuildings guidance or your local planning authority if you're unsure.

How big should my shed base be?

For concrete or paving, size the base slightly bigger than the shed itself: roughly 25mm larger on each side for a shed under 10m² floor area, and 50mm each side for anything larger. Plastic grid and timber bases should match the shed's footprint exactly.