Quick answer
Most shed and patio slabs are 4 inches thick on 4 to 6 inches of compacted gravel, with welded wire mesh or #3 rebar set in the middle of the slab and control joints every 8 to 10 ft. Go to 5 or 6 inches for heavy loads like a hot tub, garden tractor or vehicle. Bagged concrete makes sense for about 1 cubic yard or less; beyond that, ready-mix delivery is usually easier and often cheaper.
How thick should the slab be?
| Use | Slab thickness | Gravel base |
|---|---|---|
| Patio, walkway, small shed | 4 in | 4 in |
| Shed with mower, workshop | 4 to 5 in | 4 to 6 in |
| Hot tub pad | about 4 to 6 in (check the maker's spec) | 4 to 6 in |
| Garage or vehicle parking | 5 to 6 in | 6 in |
Check local rules before building. Some towns require permits for sheds or slabs over a certain size, and some require thickened edges or footings to frost depth.
The base
- Call 811 and have utilities marked.
- Excavate sod and topsoil to firm soil. Total dig depth is the gravel plus the slab thickness, minus however much the slab will sit above grade.
- Compact the soil with a plate compactor.
- Add 4 to 6 inches of crushed stone such as 3/4 inch crushed gravel or crusher run, compacted in layers. The base gives uniform support and drains water from under the slab. The gravel calculator sizes it.
- Build forms from straight 2x4s (for a 4 inch slab, a 2x4 is actually 3 1/2 inches, so many people set it slightly above the gravel or use 2x6s). Stake every 3 to 4 ft and check the diagonals to confirm the corners are square.
- Set the slope. Patios should drain away from the house, commonly about 1/8 inch per foot. Shed slabs are usually built level and raised a few inches above surrounding ground.
Reinforcement: mesh or rebar?
Reinforcement does not prevent cracks; it holds cracked pieces together so they do not separate or shift in height.
- Welded wire mesh: 6 x 6 inch grid, often labeled 6x6 W1.4xW1.4 (formerly 10-gauge). Common for 4 inch patio and shed slabs. Buy flat sheets rather than rolls if you can; rolls are hard to keep flat.
- Rebar: #3 (3/8 inch) bars on about an 18 to 24 inch grid, or #4 (1/2 inch) bars for heavier slabs. Stiffer and easier to keep in position.
- Fiber-reinforced mix: some bagged and ready-mix concrete includes fibers that reduce surface shrinkage cracks. It is often used along with, not instead of, steel in slabs that carry load.
Either type of steel only works if it sits in the middle third of the slab. Set it on plastic or concrete "chairs" about 1 1/2 to 2 inches high, not on the gravel.
Control joints
Concrete shrinks as it cures and will crack. Control joints tell it where. A common rule is joints spaced no more than 2 to 3 times the slab thickness in inches, read as feet: about 8 to 12 ft for a 4 inch slab. Cut or tool them about one-quarter of the slab's depth (1 inch on a 4 inch slab) and keep panels close to square. A 10 x 12 ft shed slab can be split into two 10 x 6 ft panels, or left as one panel near the upper limit.
Bags or ready-mix?
Volume in cubic yards = length x width x thickness in inches / 12 / 27. Add about 5 to 10 percent for uneven base and spillage. The concrete calculator converts that into bags of each size.
| Slab (4 in thick) | Cubic yards | 80 lb bags (0.60 cu ft) | 60 lb bags (0.45 cu ft) |
|---|---|---|---|
| 4 x 4 ft | 0.20 | 9 | 12 |
| 8 x 8 ft | 0.79 | 36 | 48 |
| 10 x 12 ft | 1.48 | 67 | 89 |
| 12 x 16 ft | 2.37 | 107 | 143 |
(Bag counts are before the waste allowance.) One cubic yard takes 45 bags of 80 lb mix, which is 3,600 lb to lift and mix by hand. Ready-mix commonly runs about $130 to $190 per cubic yard depending on region and strength, and orders under about 4 to 5 yards usually carry a short-load fee, often in the range of $40 to $100 or more. At $5 to $8 per 80 lb bag, 1 yard in bags is about $225 to $360, so a delivered truck is often competitive from about 1 1/2 to 2 yards up, and far less work. Ask whether the truck can reach your site; if not, budget for a concrete pump or plenty of wheelbarrows and helpers.
Pour and cure
- Dampen the gravel so it does not pull water from the concrete.
- Place concrete starting in a far corner, working it around the steel and lifting mesh into position as you go.
- Screed with a straight 2x4 across the forms, then float with a bull float or hand float.
- Run an edger along the forms and cut control joints with a groover.
- Broom the surface for a non-slip finish once bleed water has gone.
- Keep the slab damp, or covered with plastic or a curing compound, for about 7 days. Avoid pouring when frost is expected.
- Light foot traffic is usually fine after about 24 to 48 hours, building a shed on it after about a week, and concrete reaches its design strength at about 28 days.
Setting fence posts or a gate at the same time? See the fence post concrete calculator.