Advanced options
Make the base the shed's exact footprint or a touch smaller: an overhanging slab collects rain and splashes it at the timber. Concrete at 100mm suits any shed and workshops; slabs on a sharp-sand bed suit small-to-mid sheds; plastic grids filled with gravel are the light-duty quick option. All three want a firm, level, weed-membraned sub-base.
What The Shed Base Calculator Works Out
- Base area: shed length x width. Size the base to the shed's footprint or a touch smaller, because an overhang becomes a rain shelf splashing the cladding. Sheds die from below, so the base is the whole game.
- Material volume: area x depth: 100mm for a concrete slab (the permanent, workshop-grade answer), a 50mm sharp-sand bed under 600mm slabs, or 50mm of gravel in plastic grids. A 2.4 m x 1.8 m shed needs about 0.43 m3 of concrete, a wheelbarrow-and-mixer weekend.
- Shopping list: concrete at roughly 1:6 cement to ballast (1,800 kg of ballast and 12 bags of cement per m3), slabs at 0.36 m2 each plus 5% spare with sharp sand at 1,600 kg per m3, or 500mm grid panels with gravel at the same density, all in 25 kg bags.
- Depth override (advanced): 150mm of concrete for a heavy workshop or log store; 30mm of sand where slabs sit on an already firm base.
- Hardcore sub-base (advanced): compacted MOT Type 1 at about 2.1 tonnes per m3, in bulk bags of around 850 kg at GBP 50 to GBP 70 each. 100mm is the usual minimum under any base; the hired wacker plate is the least skippable tool on the job.
- Base oversize (advanced): make the base bigger or smaller than the shed by a few centimetres each side. Slightly smaller keeps the drip line off the slab; larger only where a step or path is wanted.
- Materials cost (advanced): bags, slabs or panels at your prices plus hardcore and a weed membrane at about GBP 1 per m2, excluding mixer and wacker hire. Typical UK prices are GBP 2.50 to GBP 4 per 25 kg bag of ballast or sand, GBP 6 to GBP 9 for cement and GBP 6 to GBP 12 per 600mm slab or grid panel.
- Ready-mixed concrete (advanced): the slab volume at a delivered price per m3, noting that most suppliers charge a 1 m3 minimum, so a small shed base usually costs less in bags. Volumetric mixer lorries that charge for what you use are the exception.
- Membrane and bearers (advanced): membrane for the whole base against weeds and rising damp, and pressure-treated bearers at 400mm centres so air moves under the floor; the ventilation gap is the difference between a 25-year floor and a 10-year one. The table compares all three base types for the same footprint.