Do plastic shed base grids actually work?

The marketing implies these replace a base. They do not — they replace the *surface* of a base. The grid spreads point loads, holds gravel in place so it cannot migrate, and lets water through instead of standing on it. Everything below it is still the job you were trying to avoid.

Step by step

1. Understand what the grid actually does.It is an interlocking plastic honeycomb, typically 30 to 50mm deep, laid over compacted sub-base and filled with gravel. It spreads the shed's bearer loads over a wider area, stops the gravel spreading sideways, and drains freely so nothing stands under the shed.
2. Accept that the excavation is unchanged.Strip the topsoil and all vegetation, dig to firm subsoil, and lay and compact at least 75 to 100mm of MOT type 1 in thin layers. Grid laid on topsoil sinks with the topsoil, exactly as slabs would.
3. Check the load rating against your shed.Manufacturers quote a load figure. A light timber or plastic shed with garden tools is well within it. A large workshop with machinery, a motorbike or heavy timber stock is not what these are designed for, and neither is a garden building someone will spend time in.
4. Get the surface flat, not just level.Grids follow the surface underneath. A hump or a hollow in the sub-base becomes a hump or a hollow in the shed floor, and metal and plastic sheds will not assemble squarely over one.
5. Fill the cells properly.Pea shingle or 10mm clean gravel, brushed in until every cell is full and the surface is flush. Half-filled cells give the bearers nothing to bear on and the grid flexes.
6. Keep the shed bearers clear of the gravel.The grid drains, but a timber bearer bedded down into damp gravel is still in permanent contact with moisture. Set the shed on the grid surface, not into it, and use bearer pads or a DPC strip under the timber where you can.
7. Extend the grid beyond the shed footprint.150 to 300mm all round gives a free-draining margin, catches roof run-off if the shed is ungutted, and keeps vegetation away from the cladding.

Common mistakes

• Laying grid straight onto grass or topsoil — This is the failure mode the product is blamed for. Topsoil is compressible and full of organic matter that decomposes; whatever sits on it settles unevenly within a year or two.
• Using it under a heavy building — The load rating is real. A workshop, a garden office or a shed storing building materials exceeds what a light plastic grid is designed to spread.
• Skipping the gravel fill — An empty grid is a plastic honeycomb with no compressive support inside it. The bearers crush the cells and the shed drops.

Alternatives

• Paving slabs on compacted sub-base: The traditional answer. More work, more weight to handle, and a solid, unambiguous surface that any shed sits on happily. Better for heavy sheds and for anything you will roll a mower or a bike onto.
• Concrete pad: Most work, most cost, most permanent. The right answer for a large workshop, a garden office or anything carrying real weight, and the only one that gives a genuinely flat floor across a big footprint.
• Treated timber frame on gravel: A frame of treated bearers on compacted gravel and pads. Quick, cheap, drains well and lifts the shed clear of the ground. Needs the timber treated for ground contact and the pads properly bedded.

Questions people ask