The shed base is the least exciting part of getting a new garden building. It doesn't look impressive, nobody photographs it for Instagram, and it's tempting to rush through it to get to the part where an actual structure appears. This is a mistake that almost always reveals itself within two or three years: doors that no longer close, floors that flex or rot, walls that pull out of square as the base settles unevenly. A shed that sits on a well-built, level, drained base will outlast the building by decades. A shed on a poorly prepared base won't last as long as the building deserves.
Choosing the Right Base Type
There are four main shed base options, each suitable for different situations:
Concrete slab. The strongest and most permanent option. Ideal for heavier buildings (log cabins, timber-frame workshops, metal sheds used as garages), sites with soft or variable ground, and anywhere you want a base that definitively won't move or settle. Most durable option but requires the most effort to install and is permanent once laid.
Paving slabs. A practical compromise — stable, relatively easy to install over compacted hardcore, adjustable during laying to achieve level, and reversible if you want to repurpose the space later. Suitable for most standard garden sheds. Not ideal for very heavy buildings or soft ground where individual slabs may sink.
Plastic grid system. Interlocking grids filled with gravel provide a permeable, level base that drains well and requires minimal excavation. Suitable for smaller, lighter sheds. Easier to install than concrete or slabs but less rigid for larger or heavier buildings.
Compacted gravel. The simplest option, suitable only for very small lightweight structures. Not recommended for buildings with timber floor joists, as ongoing moisture contact from the gravel will accelerate decay even with treated timber.
Planning the Base Size
The base should be the same size as the shed's floor footprint — neither significantly larger nor smaller. A base that extends beyond the building perimeter collects rainwater that sits against the shed's lower frame. A base smaller than the shed leaves floor joists unsupported at the edges, causing flex and ultimately structural failure.
Use our square footage calculator to confirm your exact base area from the shed's floor dimensions. For a 3m × 4m shed: base area = 12m². For an irregular footprint, measure each section separately and total the areas.
Calculating Concrete Quantities
A concrete slab for a shed should be at least 100mm thick for standard shed use, or 150mm for heavier vehicles, machinery, or log cabin buildings. The concrete volume is: area × depth.
For a 3m × 4m slab at 100mm (0.1m) depth: volume = 3 × 4 × 0.1 = 1.2 cubic metres. Use our concrete calculator to confirm volumes for any slab dimensions and to calculate the quantities of cement, sand, and aggregate needed for a site-mixed batch, or to determine how many ready-mix lorry loads are required.
For a 1.2m³ slab, ready-mix concrete is the practical choice — hand-mixing that volume would require around 24 bags of 25kg pre-mixed concrete and several hours of physical effort. A ready-mix minimum delivery is typically 0.5–1.0m³ depending on supplier, making it viable even for relatively small slabs. Request C20 or C25 concrete for a standard shed base.
Ground Preparation: The Most Important Step
No base material will perform well on poorly prepared ground. The preparation sequence for a concrete slab:
- Mark out the area using pegs and string lines, extending 150mm beyond the final slab edge on all sides.
- Excavate to depth — for a 100mm slab, excavate 250mm (100mm concrete + 150mm compacted hardcore sub-base). On well-drained firm ground, 100mm sub-base may suffice. On clay or soft ground, use 200mm or more.
- Lay hardcore sub-base (crushed concrete or Type 1 MOT stone) and compact thoroughly with a plate compactor. Hiring a plate compactor for a day (around £60–£80) is genuinely worth it — hand-tamping produces insufficient compaction.
- Check for level using a long spirit level or a laser level. The sub-base should be level to within 10mm across the entire area. Low spots will transfer to the finished slab surface.
- Install formwork (timber shuttering) around the perimeter to contain the concrete during pouring. Ensure the top of the shuttering represents the desired finished slab level.
Reinforcement: Do You Need It?
For a standard garden shed base, a concrete slab without reinforcement is adequate. For log cabins, workshops with machinery, or any application where point loads will be significant, adding A142 steel mesh reinforcement (available in sheets from builders' merchants) increases crack resistance and load distribution. The mesh sits in the lower third of the slab thickness, supported on plastic spacers ("chairs") to keep it off the sub-base.
Pouring and Finishing the Slab
Work systematically from one end to the other. Pour concrete to slightly above the formwork level and use a straight-edged board (screed board) moved in a sawing motion across the top of the formwork to level the surface. Work any air pockets to the surface by tamping or using a vibrating screed. Finish with a float for a smooth surface or a stiff brush for a textured non-slip finish.
Protect freshly poured concrete from direct sun, frost, and rain. Cover with polythene sheeting for at least 72 hours. Do not load the slab for a minimum of 7 days (foot traffic only); avoid full loading for 28 days, which is when concrete reaches most of its design strength.
The Cement and Concrete Alliance provides detailed guidance on mixing ratios, curing times, and slab design for DIY applications, with free technical data sheets for standard residential concrete applications.