A garden shed needs a level, well-drained base, or the door will stick, the floor will rot and the walls will rack. This calculator compares three common foundations from the size of your shed: a gravel pad, a concrete slab and deck blocks or piers.
Gravel pad
A compacted gravel pad is the most common base for wood sheds with a built-in floor. It drains well, is cheap and is easy to build.
- Make it larger than the shed, usually 1 ft beyond each side, so rain dripping from the roof lands on gravel.
- Dig out the topsoil, add a weed barrier and fill with 4 to 6 in of crushed stone (¾ in minus or similar), compacted in layers.
- Surround it with treated timbers to hold the stone.
volume = (length + 2 × overhang) × (width + 2 × overhang) × depth
A 10 × 8 ft shed with a 1 ft border gives a 12 × 10 ft pad. At 4 in deep: 120 × 4 ÷ 12 = 40 cu ft, or 1.48 cubic yards, about 2.1 tons of gravel.
Concrete slab
A slab gives a solid floor for heavy sheds, workshops or sheds without a floor. A 4 in slab over a compacted gravel bed is typical:
concrete = length × width × thickness = 10 × 8 × 4 ÷ 12 ÷ 27 ≈ 0.99 cubic yard
Order about 10% extra for uneven ground and spillage. Check local rules: some areas require frost footings or permits for slabs.
Deck blocks or piers
Precast concrete deck blocks are quick and work on gently sloping ground. Place them under the floor beams about every 4 ft in both directions:
blocks = (floor(length ÷ spacing) + 1) × (floor(width ÷ spacing) + 1)
A 10 × 8 ft shed with blocks every 4 ft needs 3 × 3 = 9 blocks. Level each one carefully on compacted gravel.
Permits
Under the International Residential Code, one-story detached accessory buildings of up to 200 sq ft are often exempt from a building permit, but many towns set a lower limit, and zoning rules on setbacks, height and easements still apply. Homeowners associations may have their own rules. Call your local building department before you dig, and call 811 to locate buried utilities.