Four bases, one job, very different commitments
The comparison people usually make is cost, and cost is the least interesting axis. A gravel pad and a slab under the same shed differ by a factor of five in effort and by infinity in reversibility, and both of those matter more than the difference in the material bill. What follows is what each one is actually for.
| Base | Bearing | Suits | The catch |
|---|---|---|---|
| Skids on stone | Narrow strips under the runners | Sheds you may move, sell or reposition | High pressure on a small area, so soft ground rutts under the runners |
| Blocks or piers | Point loads at each block | Uneven sites and maximum clearance under the floor | Every block is an independent settlement experiment |
| Gravel pad | The whole footprint | Most sheds on most ground, especially clay | Moving the most material by weight of the four |
| Concrete slab | The whole footprint, rigidly | Workshops, heavy contents, rolling equipment | Permanent, and the floor is now part of the building |
Read the bearing pressure column in the results before anything else. Three skids under a twelve foot shed give perhaps ten square feet of contact for the whole building, while the same shed on a gravel pad spreads over ninety-six. That is a ten to one difference in what the ground feels, and it is the entire reason skid sheds sink into lawns and pad sheds do not.
Clearance is half the point
Every one of these bases exists to keep wood out of standing water as much as to carry weight. A shed floor sitting on soil rots from underneath, and it does so invisibly, over years, until a foot goes through it. Skids lift the joists by the depth of the runner. Blocks lift them by the height of the block, which is usually the most generous of the four. A gravel pad lifts by however far it stands proud of the surrounding grade, which is zero if you dug the pad into a hollow, and that is a common and expensive mistake.
The corollary is that the ground around the shed matters as much as the ground under it. A base built correctly in a spot where water collects still ends up with a wet shed. Where the water goes when it comes off the roof and off the surrounding grade is worked out on the grading and swale calculator, and the fall across a hard surface on the hardscape slope calculator.
Clearance also decides the ramp
Whatever the base lifts the floor by is what a wheelbarrow, a mower or a bike has to climb every single time it goes in. Blocks give the most clearance and therefore the worst threshold. A four inch gravel pad plus a 2x6 joist and 3/4 inch decking puts the floor around eleven inches up, which needs a real ramp rather than a board. What slope actually works depends on what is being pushed: a self-propelled mower will climb something a loaded wheelbarrow will not, because the wheelbarrow has one wheel, all its weight ahead of you, and nothing but your arms holding it. Work the length out on the loading ramp calculator, and pick the gentler target if a wheelbarrow is in the picture.
The soil setting is doing real work
Clay is the case that catches people. It carries a shed perfectly well when dry, softens when wet, and changes volume with the seasons, so a base with small contact patches works fine for a year and then goes out of level. Free-draining sand and gravel is the opposite: it settles a small amount quickly and then stops. Soft, filled or recently disturbed ground is not a soil type so much as an unknown, and the honest response to it is to spread the load as widely as you can and accept that the surface tells you nothing about what is three feet down.
None of that is a bearing capacity. This page divides weight by contact area and prints the result. What that number is allowed to be where you are building comes from the adopted code, and on genuinely poor ground it comes from someone who has put a hole in it and looked.
Before you order stone
Whether this shed needs a permit, how far it has to sit from a boundary, how tall it may be and what your deed or association allows are all local determinations, and none of them can be answered from a page. A shed placed over a boundary, an easement or a buried service can be ordered moved at your cost after it is built. Settle the position with the building department and with a survey you trust before any material is ordered, and call the dig line before a shovel goes in the ground.
Frost depth, wind uplift and snow load are local figures and this page states none of them. A shed is a large light box with a big roof, which makes it an efficient sail, and unanchored buildings do move. What the anchoring has to resist where you are building comes from the adopted code or from an engineer.
Once the base is chosen, the quantities live on separate pages: concrete volume and bag count for a slab, crushed stone tonnage for a pad, excavation volume for whatever comes out first, and the shed material takeoff for everything that lands on top of it.
Questions people ask
Does a shed need a concrete slab?
Only if the use requires one. A slab is the right answer for a workshop with machines, for anything on wheels that gets rolled in and out, and for a shed you never intend to move. For a garden shed holding a mower and tools, a compacted gravel pad does the same two jobs — spreading the load and keeping the timber dry — for a fraction of the effort and with the option of changing your mind. What decides it is usually contents and permanence rather than size.
How thick should a gravel pad under a shed be?
The default here is four inches of compacted clean crushed stone, which is the figure most shed pads are built to, extending about a foot past the walls on every side so the drip line lands on stone. Deeper is normal on soft or wet ground, and the depth that is actually right depends on what is underneath, which is a site question. Two things matter more than the depth: use angular crushed stone rather than round pea gravel, because angular stone locks together and round stone does not, and compact it in layers rather than dumping it in one lift.
How many blocks go under a shed?
Enough that the beams or joists spanning between them are not overspanned, which makes it a span question rather than a block-counting question. The default of nine on a 12 by 8 is a common three-by-three arrangement, and the calculator will show you what each block is being asked to carry so you can see whether adding a row changes anything meaningful. What it will not do is tell you the spacing is adequate. Solid blocks are used rather than hollow ones, and they want to sit on compacted stone rather than on soil.
Can I put a shed straight on the grass?
It will stand up and it will rot. Timber in contact with soil stays damp, and a floor frame sitting on turf has no way to dry from underneath, so the underside goes first and you find out when something breaks through. Beyond the moisture, grass and topsoil are the least load-bearing material on any site and the shed settles into it unevenly, which racks the frame and stops the doors closing. Every option on this page exists to avoid those two outcomes.
Do I need to worry about frost under a shed?
Frost depth is local and this page states no figure for it. What is worth understanding is the mechanism: water in the ground freezes, expands and lifts whatever is above it, and it does not lift evenly. A shed on skids or a floating gravel pad is generally allowed to move as one and settle back, which is why those are common in cold climates. Anything rigid and anything bearing at points is the case where differential heave shows up as a cracked slab or a door that only closes in July. What applies to your site comes from the building department.