A well-prepared base is the single most important factor in ensuring your garden building lasts for years. An uneven or poorly supported base will cause doors to misalign, walls to twist, and can lead to premature timber decay. This guide covers the four most common base types and how to prepare each one.
Why the Base Matters
Your garden building needs a flat, level and stable surface to sit on. Without it:
- Doors and windows won't open properly
- The building may twist or warp over time
- Gaps can appear, allowing water ingress
- The timber may rot prematurely where it contacts damp ground
- The building warranty may be voided
The base should be the exact footprint of the shed or slightly larger — not smaller. It must be perfectly level in all directions.
Option 1: Concrete Base
A concrete base is the most durable and permanent option, ideal for larger buildings and workshops.
What You'll Need
- Wooden pegs and string
- Spade
- Hardcore/rubble
- Sand
- Cement and ballast (or ready-mix concrete)
- Timber for formwork
- Tamping board
- Spirit level
Steps
- Mark out the area using pegs and string — make it 50–100mm larger than the shed footprint on each side
- Excavate to a depth of 150mm (6 inches)
- Lay a sub-base of 75mm of hardcore, compacted with a plate compactor
- Add a layer of 50mm of sand on top of the hardcore and level it
- Build formwork — timber boards around the perimeter, set to the correct level
- Pour the concrete to a depth of 75mm (or 100mm for larger buildings)
- Tamp and level the surface using a straight timber board across the formwork
- Cure — cover with plastic sheeting and leave for at least 48 hours (7 days for full strength)
Option 2: Paving Slab Base
Paving slabs are quicker to install than concrete and provide an excellent, level surface.
Steps
- Mark out the area, allowing a small margin around the shed footprint
- Excavate to a depth of 75mm
- Lay a sub-base of 40mm of MOT Type 1 or hardcore, compacted
- Add a layer of 30mm of damp sharp sand and level with a screeding board
- Lay the slabs starting from one corner, tapping each one into the sand with a rubber mallet
- Check levels frequently with a spirit level — the surface must be perfectly flat
- Leave to settle for 24 hours before installing the shed
Option 3: Timber Bearers
Timber bearers are a cost-effective option for smaller sheds on already-level ground. They keep the shed floor off the ground and allow air circulation.
Steps
- Level the ground where the shed will sit
- Lay treated timber bearers (100mm × 50mm pressure-treated) at 400–600mm intervals, running perpendicular to the shed floor joists
- Check levels — use paving slabs or bricks under the bearers to level them if the ground isn't perfectly flat
- Ensure all bearers are in contact with the ground and level with each other
Option 4: Plastic Grid Base
Plastic grid systems are a modern, eco-friendly alternative that allows water to drain through while providing a solid surface.
Steps
- Level the ground and remove any turf
- Lay a weed membrane over the area
- Clip together the plastic grids to cover the shed footprint
- Fill the grids with angular gravel or pea shingle
- Level the surface with a screeding board
General Tips for All Base Types
- Check the level multiple times during construction — a 5mm slope over a 2m span is noticeable
- Allow access on all sides — leave at least 30cm clearance for installation and future maintenance
- Consider drainage — ensure the base is slightly above surrounding ground level to prevent water pooling
- Don't forget the approach — make sure there's a clear path for our installation team to access the site with materials
We're Here to Help
If you're unsure about base preparation, our team is happy to advise. All our garden buildings include free professional installation, so once your base is ready, we take care of the rest.
Browse our garden sheds, summerhouses and garden rooms — or contact us if you need a building tailored to your space.
