Roadbase for a Shed Base

Preparing a Foundation for Your Shed

A solid foundation is essential for any shed, whether it will sit directly on compacted roadbase or receive a concrete slab. The prepared base prevents the structure from settling unevenly, keeps moisture away from the building, and provides a level surface for construction. For a typical garden shed measuring 6 metres by 3 metres, you'll need approximately 2.7 cubic metres of DGB20 roadbase at 150mm depth. This weighs around 4.9 tonnes, a standard bulk delivery for most landscape suppliers.

The 150mm depth works well for most garden sheds and light storage buildings. If you're planning to store heavy equipment, vehicles, or anticipate significant point loads from machinery or racking, consider increasing to 200mm. The additional base material provides extra load distribution and long-term stability.

Preparing the Site

Before placing roadbase, the area needs proper excavation. Remove all topsoil and organic material to reach stable subsoil. Any grass, roots, or soft material left beneath the base will eventually decompose and cause settling. Excavate slightly larger than the shed footprint to allow room for drainage and edge compaction. On sloping sites, cut into the high side rather than filling the low side, as fill material is always less stable than undisturbed ground.

Concrete Slab Over Roadbase

If pouring a concrete slab for your shed, the compacted roadbase layer serves as the sub-base. After compacting the roadbase to level, lay a vapour barrier (builders plastic) before placing concrete. This prevents moisture wicking up through the slab. For shed slabs, 100mm of concrete over 150mm of compacted roadbase provides adequate support for most garden storage buildings. Larger sheds or workshops may require engineering specifications for slab thickness and reinforcement.

For a similar sized carport foundation, see our carport calculator. Use our main roadbase calculator for custom dimensions.