A shed is only as good as what it sits on. Put a shed directly on grass or bare soil and within a year or two the base will rot, the floor will sag, the doors will jam, and damp will ruin everything inside. A proper foundation takes half a day to build, costs relatively little, and gives your shed a solid, level, long-lasting base.
Types of Shed Foundation
There are four main approaches, each suited to different situations:
| Foundation Type | Best For | Difficulty | Cost (6×4 shed) | Durability |
|---|---|---|---|---|
| Paving slabs | Small to medium sheds on flat ground | Easy | £40–£80 | Very good |
| Concrete slab | Heavy sheds, workshops, permanent buildings | Moderate-hard | £100–£200 | Excellent |
| Timber frame (bearers) | Sloping ground, quick installation | Easy | £30–£60 | Good (with treatment) |
| Plastic grid system | Eco-friendly, flat ground, quick install | Very easy | £60–£120 | Very good |
Method 1: Paving Slab Foundation (Most Popular)
This is the most common DIY shed base and works well for the majority of garden sheds. You’re creating a flat, level platform of paving slabs that the shed sits on top of.
What You’ll Need
- 600×600mm concrete paving slabs (calculate quantity based on shed size)
- Sharp sand
- Cement (optional — for a mortared base)
- Spirit level (long — 1200mm or longer)
- String line and pegs
- Spade
- Rubber mallet
- Tape measure
Step-by-Step
- Mark out the area — use string lines and pegs to mark a rectangle 50–100mm larger than the shed footprint on each side. Check the diagonals are equal to confirm it’s square
- Excavate — dig out the marked area to a depth of about 75mm. Remove any turf, roots, and organic matter. The base of the excavation should be roughly level
- Lay a sand bed — spread a 25–30mm layer of sharp sand across the excavated area and level it roughly with a rake
- Position the first slab — place it in one corner, check it’s level in both directions with a spirit level, and tap it into position with a rubber mallet. This is your reference point for all other slabs
- Continue laying slabs — work outward from your reference corner, butting slabs together and checking each one is level with its neighbours. Use the spirit level constantly — a base that’s out of level means a shed with doors that don’t close
- Check the entire base — lay the spirit level diagonally across the finished base and along each edge. Adjust any high or low spots by adding or removing sand underneath
- Fill the joints — brush kiln-dried sand into the joints between slabs
Method 2: Concrete Slab Foundation
A concrete slab is the strongest and most permanent option. It’s essential for heavy workshops, large sheds, and any building where you’ll be doing work that creates vibration (machinery, heavy bench work). However, it’s more work and more expensive than slabs.
Brief Overview
- Excavate to 150mm depth, 100mm larger than the shed on each side
- Build a timber formwork frame to contain the concrete
- Lay a 50mm compacted sub-base of MOT Type 1 or crushed hardcore
- Lay a damp-proof membrane over the sub-base
- Pour 100mm of C20 concrete (1 part cement, 2 parts sharp sand, 4 parts gravel)
- Level with a straight edge (screed board), working the concrete between the formwork rails
- Allow 48–72 hours to cure before placing the shed
For a typical 6×4 shed, you’ll need about 0.4 cubic metres of concrete. That’s roughly 10 bags of premix or the equivalent in separate materials. The Paving Expert concrete guide has comprehensive mix ratio tables.
Method 3: Timber Frame (Bearer Base)
A timber frame base uses pressure-treated bearers (typically 75×50mm or 100×50mm) laid directly on the ground. The shed sits on top of the bearers, raising it off the soil and allowing air to circulate underneath — which helps prevent rot.
Step-by-Step
- Level the ground and lay a weed membrane
- Cut pressure-treated bearers to the width of the shed
- Space them 400–600mm apart, running front to back
- Level them using spirit level and packing material (slate shims or concrete blocks) where needed
- The shed floor sits across the bearers at right angles
This method is especially useful on sloping ground — you can pack one end higher than the other to create a level base without extensive excavation. Use tanalised (pressure-treated) timber rated UC4 (ground contact use) for maximum longevity.
Method 4: Plastic Grid System
Plastic grid foundations (like the Hawklok or similar brands) are a relatively recent alternative. They’re interlocking plastic grids that you fill with gravel, creating a permeable, level base. No excavation, no concrete, no mixing — lay the grids, fill with gravel, level, and you’re done.
They work well on flat ground and are particularly popular with eco-conscious homeowners because they’re fully permeable — rainwater drains straight through rather than running off, which helps with surface water drainage. Wickes sells several grid base systems if you want to see the options.
Drainage Considerations
- Never position a shed at the lowest point of your garden where water collects
- If the area is prone to standing water, consider a French drain around the base
- Slope the ground very slightly away from the shed on all sides so rainwater flows away from the base
- Leave a gap between the shed walls and any fences or walls — airflow underneath and around the shed prevents damp
Common Mistakes
- Not checking level — a base that’s out by even 10mm over a metre causes problems. Check and recheck
- Making the base too small — always make it at least 50mm larger than the shed on each side for drainage and access
- Skipping the membrane — a weed membrane underneath prevents weeds growing up around and through the base
- Using untreated timber bearers — they’ll rot within 2–3 years. Use UC4 pressure-treated timber only
- Building on topsoil — topsoil settles and is uneven. Excavate down to firm subsoil or use a sub-base material
