How to Pour a Concrete Slab: Forms, Base, Pour and Finish

Pouring a 20×20 slab yourself: $2,344–$2,718 in concrete, forms and rentals, 24–40 hours with three helpers.

Before you start

Read the cost comparison first. A 10×10 or 12×12 at grade is a fair DIY job that saves about $100 and teaches you the trade; a 20×20 saves about $1,000 but needs a crew of three or four and a rented skid steer for the excavation. This guide is written for the small slab. The steps are the same for a large one — it’s the pace that changes.

Two conditions before you commit: nothing will be built on the slab that needs a permit and an inspection, and a mixer or a ready-mix truck can get to it. If either isn’t true, get quotes.

What you need

For a 10×10, 4 inches thick: about 60 bags of 80 lb mix or a 1.5-yard short load of ready-mix, 1.25 yards of 3/4-inch crushed stone, four straight 2×4s plus stakes and screws, a sheet of welded wire mesh with chairs, an expansion-joint strip if it meets a wall, and plastic sheeting or curing compound.

Tools: a rented mixer for bagged mix, a hand tamper, a bull float with handles, a hand float, an edger, a groover, a steel trowel, a straight 2×4 for screeding, a 4-foot level, mason’s line and a line level — plus rubber boots, gloves and glasses, because wet concrete burns skin. The priced list is on the cost page.

The two things that decide how it turns out

The base. A slab cracks and settles because of what’s under it, not what’s in it. Dig to firm ground, compact the gravel in thin lifts, and keep the slab a uniform 4 inches. Every shortcut here shows up as a crack in a year.

The clock. Once concrete is mixed you have about 90 minutes to place, screed and float it, then a waiting period, then a short window to edge, joint and broom before it’s too hard. Everything else in this job can be done slowly and checked twice. Do the slow parts perfectly so the fast part is just execution.

Step by step

1. Check rules and mark the layout

   Call the building department about permits and setbacks, and call 811 to have utilities marked (free, takes a few days). Drive stakes at the corners, run mason's line, and square it by checking that the diagonals match. Set the finished slab height so it sits 1–2 inches above grade and slopes away from any building at 1/8 inch per foot — mark that height on the stakes with a line level.

2. Excavate

   Dig the whole footprint plus a foot outside it to 8 inches below the finished height: 4 inches for the slab, 4 for the base. Remove roots, topsoil and anything soft; the slab needs undisturbed or well-compacted ground. A 10×10 is about 2.5 cubic yards of soil, most of which can be spread on site. Keep the bottom flat — high spots thin the slab, low spots waste concrete.

3. Build and set the forms

   Frame the perimeter with straight 2×4s, screwed at the corners, top edges at the finished-height line. Drive stakes every 2–3 feet on the outside and screw the forms to them, checking level along each side and the fall across the slab with a 4-foot level. Sight down each board for bows. Brush the inside faces with form-release oil or vegetable oil. Where a wall or existing slab meets the pour, nail an expansion-joint strip to it.

4. Lay and compact the base

   Spread 3/4-inch crushed stone in two 2-inch lifts, compacting each with a hand tamper (small pad) or plate compactor. The finished base should be 4 inches below the top of the forms everywhere — check with a straightedge across the forms and a tape. Dampen it before the pour so it doesn't suck water out of the concrete.

5. Add vapor barrier and reinforcement

   For a slab that will be enclosed, roll out 10-mil poly over the base, lapped 6 inches and taped. Lay welded wire mesh or tie a rebar grid, stopping 2 inches short of the forms, and set it on chairs so it sits in the middle of the slab. Steel lying on the base does nothing. Don't plan to hook it up during the pour — that's how it ends up on the bottom.

6. Order or mix the concrete

   Measure length × width × average depth in feet, divide by 27 for cubic yards, and add 10%. Ask for 3,500–4,000 psi, air-entrained where it freezes, and a 4–5 inch slump; tell the plant it's a homeowner pour so they can set a realistic unload time. For bagged mix, rent a mixer, stage the bags and a hose at the forms, and mix to a stiff, workable consistency — no extra water. Have everyone and every tool at the forms before the truck arrives.

7. Pour and screed

   Place the concrete starting at the far corner, raking it into the corners and slightly above the forms, and work toward the truck. Screed as you go: two people draw a straight 2×4 across the form tops with a sawing motion, filling low spots ahead of it, then pass again. Tap the forms with a hammer to release air along the edges.

8. Float, edge, joint and finish

   Run the bull float right after screeding to push down aggregate and bring up paste — three or four passes, each barely overlapping. Then wait: bleed water rises and must evaporate before you touch the surface again (20 minutes to 2 hours depending on weather). When a thumb leaves a 1/4-inch print, run the edger along the forms, cut control joints with a groover every 8–10 feet, hand-float any rough areas, and finish with a broom drawn in one direction for a non-slip texture.

9. Cure and strip

   Cover with plastic or spray curing compound as soon as the surface won't mar, and keep it damp for 7 days. Strip the forms after 24–48 hours and backfill the edges. Foot traffic at 2 days, a shed at 3, vehicles at 7, full strength at 28. If you missed the joints while finishing, saw-cut them within 18 hours.

What would this cost you? The concrete slab installation calculator prices the DIY version with the tools you already own and a value on your time, next to a pro quote.