A concrete slab is the one part of a shed you cannot redo. Pour it too thin, set it on loose fill, or lay it on bare dirt, and within a winter or two it cracks, a corner heaves, and the doors stop latching. There is no un-pouring it. Done right, a slab is the flattest, most permanent base a shed can sit on, and the easiest surface to anchor a shed to when the wind picks up. Here is how to build one that lasts.
TL;DR: Pour a concrete slab 4 inches thick from a 3,000 psi mix on 4 inches of compacted gravel, reinforced with welded wire mesh or a rebar grid and thickened at the edges. You can walk on it in 24 to 48 hours, but wait about 7 days before you build.
Four inches. A standard backyard shed sits on a 4-inch slab, which carries a wood or metal shed plus normal storage without trouble. Before you form anything, confirm a slab is the right base at all in our how to build a shed foundation guide, since it is only one option. Step up to 5 or 6 inches if you will roll a mower, ATV, or other heavy equipment across it. Use a 3,000 psi mix at minimum, or 3,500 to 4,000 psi if the slab is heavily loaded or faces freeze-thaw winters. Pour on 4 inches of compacted crushed stone, never on bare dirt: strip the sod and organic topsoil first, or the ground settles under the slab. Reinforce with 6x6 W1.4 welded wire mesh, or #3 (3/8 inch) rebar in a 12 to 16 inch grid for heavy loads, set on chairs in the lower-middle third, not laid on the gravel. Thicken the perimeter into a turned-down edge 12 to 18 inches deep so the edges carry the walls. Then cure it before you build: about 7 days.
Split the job into two stages: build the form and base, then pour the concrete.
Forms and base:
Concrete and finish:
This guide sticks to the build and skips dollar figures; for what a finished slab and shed actually cost, see our cost to build a storage shed guide. What you do need here is the volume. Multiply the slab area in square feet by the thickness in feet (4 inches is 0.33 feet) to get cubic feet, then divide by 27 for cubic yards. A 10x12 slab at 4 inches is 120 x 0.33 = 40 cubic feet, or about 1.48 cubic yards.
An 80 lb bag yields 0.60 cubic feet and a 60 lb bag 0.45, so a cubic yard takes about 45 of the 80 lb bags or 60 of the 60 lb bags. Hand-mixing tops out around 1 cubic yard, roughly 55 bags of the 80 lb size, or about a 10x10 slab at 4 inches. Past that, mixing by hand is a brutal all-day job and the pour stiffens unevenly, so order a ready-mix truck.
| Slab size (4 in) | Concrete | 60 lb bags | 80 lb bags | Bags or truck? |
|---|---|---|---|---|
| 8x8 | 0.79 cu yd | 48 | 36 | Bags OK |
| 8x10 | 0.99 cu yd | 60 | 45 | Bags (borderline) |
| 10x12 | 1.48 cu yd | 89 | 67 | Ready-mix |
| 12x16 | 2.37 cu yd | 142 | 107 | Ready-mix |
| 12x20 | 2.96 cu yd | 178 | 134 | Ready-mix |
A truck costs less per yard, goes faster, and gives one continuous pour with no cold joints. Whichever route you take, order about 10 percent extra so a miscount or a deeper spot does not leave you short mid-pour with the first batch already stiffening. To pin down your exact bag count or truck order, run your dimensions through the free QUIKRETE calculator.
Mark the slab with batter boards and string, then check for square by measuring both diagonals: equal diagonals mean square corners. Excavate deep enough for 4 inches of gravel plus the 4-inch slab, and dig the perimeter trench deeper for the turned-down edge.
Build the form from pressure-treated 2x lumber set on edge, screwed at the corners and braced every 2 to 3 feet with stakes driven flush to the top. Level the top edges carefully, because the form is your screed guide and any dip in it shows up in the finished slab.
Add the crushed stone in two lifts, compacting each with a plate compactor until it stops moving under the plate. That compacted base is what keeps the slab from cracking later. Finally, lay the wire mesh or rebar grid and lift it onto chairs so the steel sits in the lower-middle third of the slab, not down on the gravel where it does nothing.
Start the pour in the far corner and work back toward your exit so you never trap yourself against wet concrete. Fill to the top of the forms and rake it roughly level as you go, working it into the corners and the turned-down edge. Whatever you are setting on top, from a garden shed to one of our wood storage sheds, the surface under it has to be flat and true.
Screed right away: rest a straight 2x4 across the forms and pull it toward you in a side-to-side sawing motion, dragging the excess and leaving a level surface. Follow with a bull float to knock down ridges and bring up the paste. Once the surface sheen leaves, run an edger around the perimeter for clean, chip-resistant edges. Finish with a broom, dragged lightly across the fresh concrete, so the slab has grip and is not slick when it gets wet.
Concrete sets fast but gains strength slowly, and rushing it is how you get a weak, dusty slab. You can usually walk on it and strip the forms after 24 to 48 hours. Then keep it damp: mist it or cover it with plastic for about 7 days, because concrete cures through a chemical reaction that needs moisture, not dry air. By day 7 the slab reaches roughly 70 percent of its strength, which is normally enough to set a shed on. Full design strength takes 28 days, so if you are loading it with something heavy, give it the full month. For a deeper walkthrough of moist-curing, see this Concrete Network curing guide.
Get the base compacted, the steel set right, and the cure unhurried, and the slab will outlast the shed on top of it. Pour it wrong, and there is no fixing it after the fact.
Four inches is standard for a typical wood or metal shed. Step up to 5 or 6 inches if you will drive or park heavy equipment like a mower or ATV on it. Do not shave the thickness to save a few bags.
No. Strip the sod and organic topsoil first, then pour on 4 inches of compacted crushed stone. Bare soil holds water and settles unevenly, so a slab poured straight on dirt is the one most likely to crack and heave.
Use a 3,000 psi mix at minimum. Go to 3,500 or 4,000 psi if the slab carries heavy loads or lives through freeze-thaw winters. Bagged concrete mix works for small slabs, but order ready-mix once you pass about 1 cubic yard.
Yes, and it is one of the best bases you can build on. Anchor the shed with wedge or sleeve anchors driven into the slab so wind cannot lift it, and set the sill on a foam sill gasket to block moisture wicking from the concrete into the wood frame.
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