Build a Shed Base: Project Walkthrough

1. Lay out the pad and check what's actually underneath it

Mark the base's footprint with stakes and string, square it using the diagonal-measurement method, and dig a test hole or two to see what's actually under the topsoil — clay, sand, existing fill from a previous structure, or rock close to the surface all change the excavation plan before you've moved the bulk of the dirt. A base that's slightly oversized on all four sides, an inch or two past the shed's actual footprint, gives some working room for the gravel or forms without needing to re-dig an edge later.

2. Choose gravel pad or poured slab before ordering anything

A compacted gravel pad is faster, cheaper, and lets water drain through rather than around the shed, and it's the common choice for a smaller, lighter shed without plumbing or a permanent foundation requirement. A poured concrete slab costs more and takes longer (forming, reinforcing, curing) but gives a genuinely flat, permanent floor, handles a heavier shed or one with anchored walls better, and is often what's required once a shed crosses a size or use threshold that your local code treats as a permanent accessory structure rather than a portable one.

This decision changes every calculator that follows: a gravel pad skips the concrete slab calculator and rebar calculator entirely and only needs the gravel calculator, while a poured slab needs a compacted gravel sub-base plus the concrete and reinforcement on top of it, so effectively all four calculators.

3. Excavate to the depth the chosen base actually needs

A gravel-only pad typically needs 4-6 inches of compacted crushed stone; a slab needs that same compacted base underneath plus the slab's own thickness on top, commonly landing total excavation somewhere around 8 inches for a light-duty shed slab. Run the excavation spoil calculator with that depth, and pick the right entry on soil swell and compaction for your actual dirt rather than leaving the calculator on its default clay-adjacent assumption.

For most of the pad, none of the dug soil goes back into the hole — gravel or slab fills that space instead, all the way up to grade — so unlike a deeper excavation where some native soil gets backfilled around a buried structure, a shed base's spoil pile is close to the full swollen volume of the hole, not a fraction of it.

4. Build the base and let it settle before the shed goes up

Compact gravel in thin lifts with a plate compactor rather than one thick loose layer — a compactor only effectively densifies the top few inches per pass, so a 6-inch pad built and compacted as one pass ends up less dense than the same depth built and compacted in two lifts. For a slab, run the rebar calculator on the grid spacing your slab thickness calls for, place the concrete, and give it real cure time before loading it with the shed's weight — a few days for enough strength to build on, longer for full cure strength, longer still in cool weather.

Why crossing the frost-footing threshold changes the dig by a factor of two

A 10 by 12 foot pad excavated 8 inches deep for a slab-on-grade base works out to about 80 cubic feet, just under 3 cubic yards bank measure. At a 25 percent swell factor for ordinary earth, that comes out to a little over 3.5 cubic yards loose — a pile that fits in the back of a full-size pickup in two or three loads, or gets spread across a couple of garden beds without much trouble.

Many jurisdictions exempt a small detached shed under a stated size from needing footings that reach the local frost line, treating it as a portable accessory structure. Cross that size threshold, or add plumbing or a permanent power run that changes the shed's classification, and the base commonly needs a perimeter footing that actually reaches frost depth rather than a shallow slab-on-grade. In a climate with a frost line around 42 inches, a footing trench dug 12 inches wide around that same 10 by 12 pad's roughly 44-foot perimeter adds close to 154 cubic feet on its own — nearly 5.7 cubic yards bank, over 7 cubic yards once it swells. The shed didn't get bigger and the slab on top of it barely changed; crossing a size or use threshold most people don't think about until they call the building department roughly doubled the dig and the spoil that has to leave the site.

What goes wrong when the base gets built before the code question gets asked

Pour a slab-on-grade base first and find out afterward that the shed's size or intended use actually required frost-depth perimeter footings, and the fix isn't an add-on — it's breaking out what's already cured and starting the excavation over at a much greater depth, on a site that now also has a shed-shaped slab in the way. The code question belongs in step 2, before any gravel or concrete is ordered, specifically because it changes which of the four calculators in this project even apply and by how much.

The same reversal shows up on a smaller scale with the spoil pile itself: dig first and figure out where the dirt is going afterward, and it's common to end up with a pile sitting on the exact spot where gravel delivery needs to drop its load, or blocking the path a concrete truck's chute needs to reach. Deciding where excavated soil goes — spread on site, or hauled — before the hole is dug avoids the dirt itself becoming the next obstacle in the build.