How do I build a shed base?

Almost every shed that rots early sits on a poor base. Timber in contact with damp ground wicks water upwards, doors that do not shut are a base out of level, and a base out of square shows up on the very first wall.

Safety

Step by step

1. Size the base to the shed, not the other way round.Measure the shed floor's actual outside dimensions from the supplier's specification, not the nominal size. The base should match or be very slightly smaller so rain drips clear rather than sitting on a ledge.
2. Choose a position that drains.Avoid the lowest corner of the garden and anywhere water collects after rain. Leave enough space around the shed to walk and to treat the timber — tight against a fence makes maintenance impossible.
3. Mark out and check the diagonals.Set out with pegs and string, then measure both diagonals. They must be equal — that is what makes it square. Adjust until they match before digging anything.
4. Excavate and lay a compacted sub-base.Dig down enough for around 100mm of compacted hardcore or MOT type 1 plus the slab or timber above it. Compact in layers with a whacker plate or a heavy tamper. Uncompacted sub-base settles unevenly later.
5. Lay slabs on a full mortar bed or sharp sand.Work from one corner outwards, checking level constantly in both directions and across the diagonals. A slight fall away from any building is fine; a twist is not.
6. Consider a timber frame on gravel as an alternative.Pressure-treated bearers on a compacted gravel bed, levelled with packers, are quicker and cheaper than slabs and drain very well. Suits small and medium sheds and is easier to correct later.
7. Use concrete only for large or heavy sheds.A concrete pad needs shuttering, reinforcement for anything substantial, and time to cure. It is the most work and the most permanent, and it is unnecessary for a typical garden shed.
8. Lift the shed floor clear of the base.Even on a perfect base, sitting timber directly on it holds water. Pressure-treated bearers underneath the floor create an air gap, which is what actually prevents rot.
9. Check level along, across and diagonally before building.A long spirit level or a straightedge with a level on it. Every door and window problem afterwards traces back to this step, and it is far easier to correct now.
10. Leave a clear margin around the finished base.Gravel or a mown strip around the shed keeps vegetation off the timber and lets you see problems early. Grass growing up against a shed wall keeps it permanently damp.

Tips

• Order about ten per cent more aggregate than the calculation suggests. Running out mid-pour or mid-layer is worse than a small surplus.
• Do the marking out and levelling on a dry day and do not rush it. It is the only part of the job that cannot be corrected afterwards.
• Check the shed supplier's own base requirements before starting — some warranties specify a base type.

Questions people ask