The base is smaller than the shed, and the hole is bigger
Both of those are true at once, and getting either one backwards is what puts shed floors in a skip after five years.
The base goes inside the footprint. A concrete or paved base that oversails the shed is a shelf. Rain runs off the roof, lands on the slab, and sits against the bottom rail where it wicks straight up into the floor. Hold the base 25mm inside the shed on every side and the drip line falls on soil instead. With a typical 75mm roof overhang that puts the water 100mm clear of anything structural.
Almost every calculator asks for the shed size and hands back a base the same size, because that is the obvious answer. It is also the one that keeps the timber wet.
The excavation goes outside it. The sub-base has to extend 100mm past the base on every side, because unconfined aggregate spreads sideways under load. Cut the sub-base flush with the slab and the corners settle first, and the corners are exactly where a shed puts its weight.
The size on the box is not the size of the shed
A "6x4" is a nominal description, not a measurement. The floor panel is usually a little under it. The roof overhangs by 50 to 100mm all round, and on most designs the eaves overhang further than the gable ends, so the drip line is not even the same distance out on all four sides.
So set a base out to the nominal size and you can end up with something simultaneously too small for the floor to sit on properly and far too small to keep the water off. Measure the actual shed, or the floor panel if it is still flat-packed, and put that in.
Sub-base, and why you order more than fits in the hole
MOT type 1 compacts by roughly a quarter as it is whacked. So the volume to order is the compacted depth you want plus that, and the tool does it that way round.
Whack it in layers of no more than 75mm. A single deep layer whacked from the top compacts the top and leaves the bottom loose, which produces a base that looks perfect on the day and settles over the first winter anyway. It is the most commonly skipped step on a small job because the difference is invisible until it is too late.
The membrane goes under the sub-base, and it is not there to stop water. It is there to stop the aggregate and the soil mixing. Without it the stone works its way down into the subgrade over a couple of winters and the base loses the depth you paid for, silently.
Which base for which plot
- Concrete slab. Permanent, dead level, and it does not drain. Fine on a dry plot, and the least forgiving option if the ground holds water.
- Paving slabs. Same sub-base as concrete, and liftable if you change your mind. Laid straight onto soil without a sub-base they will move, which is how most of the wonky ones happen.
- Plastic grid on aggregate. Drains straight through, so it is the most forgiving on a damp plot and the least likely to rot a floor. Fill the cells: an empty one deforms under a point load, and a shed concentrates its weight on a handful of bearer points rather than spreading it.
- Timber frame on pads. Air moves under the floor, which is the single best thing you can do for a timber shed, and it lets you level a slope without building a retaining structure. The pads must bear on something firm rather than on topsoil, and you want at least 50mm of gap underneath or the ventilation does nothing.
The costs people forget
The spoil. Excavated soil swells once it is out of the ground, so the pile is bigger than the hole. On a shed base the muck away is frequently a larger cost than the materials, and it is much easier to price before the digging than after it is sitting on a tarpaulin across the lawn.
Fall. A base that is dead level holds water on it. A millimetre or two of fall per metre, running away from the door, does not. And where the ground slopes, build up rather than cutting in if you can: a base cut into a bank collects everything running down the hill towards it.
Common questions
Should a shed base be bigger or smaller than the shed?
Smaller, and this is the one almost everybody gets wrong. A concrete or paved base that sticks out past the shed is a shelf: rain running off the roof lands on it, sits against the bottom rail and wicks up into the floor. Keeping the base 25mm inside the footprint on every side means the drip line falls on soil instead, and it is the single biggest thing you can do for how long the shed lasts.
Is a 6x4 shed actually 6 foot by 4 foot?
No, it is nominal. The floor is usually a little under, the roof overhangs by 50 to 100mm all round, and on most designs the eaves overhang further than the gable ends. Set a base out to the nominal size and it can be too small for the floor and much too small to keep the drip line off it. Measure the actual shed or its floor panel.
How deep does a shed base need to be?
For a concrete slab, 100mm of concrete on 100mm of compacted sub-base, so a 200mm dig. For paving, the same sub-base plus a 40mm sharp sand bed plus the slab, so around 240mm. Both need the sub-base to run past the edge of the base, because unconfined aggregate spreads sideways under load and the corners settle first.
Why order more sub-base than the hole holds?
Because MOT type 1 compacts by roughly a quarter as it is whacked. Order the loose volume for the compacted depth you want, and whack it in layers of no more than 75mm. One deep layer whacked from the top compacts the top and leaves the bottom loose, which gives you a base that looks right and settles anyway.
Do I need a membrane under the base?
Yes, and not for the reason people assume. It is not there to stop water, it is there to stop the aggregate and the soil mixing. Without it the stone works down into the subgrade over a couple of winters and the base quietly loses the depth you paid for.
What is the best base on a damp or sloping plot?
On damp ground, a plastic grid filled with aggregate, because it drains straight through rather than holding water under the timber. On a slope, bearers on pads, which keep air moving under the floor and let you level without building a retaining structure. A slab is the worst of both on a wet plot and the most permanent.