Plastic Shed Base Kits: How to Fit One, and Are They Any Good?

Plastic grid base kits are the biggest change in shed foundations in decades: a base you can lay in an afternoon with a spade, a rake and no cement mixer. They are genuinely good, within limits that the marketing rarely mentions. This guide gives you the honest verdict and then the exact installation method, because nearly every plastic base failure is an installation failure.

What They Are and Why People Use Them

A plastic shed base is a set of interlocking honeycomb panels, usually recycled polypropylene, that clip together into a rigid raft. Laid on prepared ground over a weed membrane and filled with pea gravel, the cells confine the gravel so it cannot spread, and the filled raft spreads the shed's weight across the whole footprint.

The attractions are real:

• Speed: a 8x6 base is a two to three hour job, and you can build on it the same day, no curing time
• No concrete: no mixer, no barrowing, no waste, and the ground underneath is recoverable if you ever move the shed
• Drainage: rain drains straight through, so the shed floor sits on a dry, ventilated base rather than a puddle
• Weight: the kit for a typical shed arrives in two manageable boxes rather than a tonne of aggregate

The Honest Verdict: When They Work and When They Don't

Use a plastic grid base when:

• The shed is a typical garden shed, up to around 10x8, holding normal garden contents
• Your ground is reasonably firm and can be levelled, lawns, compacted soil, existing gravel
• You want the job done this weekend without hiring a mixer or paying for a groundworker

Stick with concrete or slabs when:

• The building is heavy: log cabins, workshops with machinery, anything with masonry. Grid ratings look enormous on paper but the limit is really the ground beneath, and heavy point loads find soft spots
• The site slopes more than a few centimetres across the footprint; grids follow the ground, they do not fix a slope. Level the ground first or build up properly
• The ground is soft, boggy or made-up fill; confined gravel on mud is still mud underneath
• You need to bolt down a metal or plastic shed, or a security shed with a ground anchor, those want concrete

Cost-wise, a quality 8x6 kit plus gravel lands around £100–£180, comparable to paving slabs, far less than ready-mix concrete once you price delivery, and far less effort than either. Our full foundation guide covers the traditional methods if your site rules the grid out.

Installation Step-by-Step

1. Mark out and excavate

Mark an area 50–100mm larger than the shed on every side with string lines, checking the diagonals match so it is square. Strip the turf and dig down 50–70mm, deep enough that the filled grid finishes at or just above the surrounding ground.

2. Level and compact

Rake the exposed ground level, then check it properly with a straight edge and spirit level, working both directions and the diagonals. Fill hollows with sharp sand rather than loose soil, then compact the whole area firm with a tamper or plate compactor. This step decides whether your shed sits level for twenty years; do not rush it.

3. Membrane and grids

Roll the weed membrane across the whole excavation with generous overlaps. Lay the grid panels on top and clip them together, staggering joints like brickwork if your kit allows. Trim edge panels with a handsaw if the footprint demands it.

4. Fill, check, build

Fill every cell with pea gravel (10mm is ideal), brushing it level with the cell tops. Run the level over the finished raft one last time, then place your bearers and build. No curing, no waiting.

Common Mistakes

Grid vs Concrete vs Slabs at a Glance