How to Pour a Concrete Slab

Intermediate · 1–2 days

A concrete slab is one of the most useful things you can build: a base for a shed, a patio, an AC pad, or a garage floor. The pour itself happens fast, so the work is mostly in the prep — a level, compacted base and square, braced forms are what separate a slab that lasts decades from one that cracks and heaves the first winter. Grab a friend or two to help on pour day; fresh concrete waits for no one.

What you'll need

• Ready-mix concrete or 60/80 lb bags (use the calculator to size the order)
• 2x4 or 2x6 lumber for forms, plus stakes and screws
• Compactable gravel base (3–4 in) and a plate compactor or hand tamper
• Welded wire mesh or #3 rebar on chairs for reinforcement
• Screed board (a straight 2x4), bull float, edger, and a magnesium hand float
• Wheelbarrow, mixing hoe or mixer, gloves, and rubber boots

Steps

1. 1

   Lay out and excavate

   Mark the slab with stakes and string, checking for square by measuring the diagonals (they should match). Excavate to allow for 3–4 in of gravel plus your slab thickness, and slope the grade slightly away from any building to shed water.

2. 2

   Build and level the forms

   Set 2x4 or 2x6 forms to the slab outline, staking every 2–3 ft. Level the top edges (or build in a slight 1/8 in per foot slope for drainage) — the form tops are what you screed against, so getting them right is critical.

3. 3

   Add and compact the base

   Spread 3–4 in of compactable gravel and run a plate compactor over it until it stops shifting. A solid base prevents settling and cracking later.

4. 4

   Place reinforcement

   Lay welded wire mesh or a rebar grid, supported on chairs so the steel sits in the middle third of the slab thickness — not on the ground, where it does nothing.

5. 5

   Pour and screed

   Pour from the far corner toward the exit, filling to the top of the forms. Drag a straight 2x4 across the form tops in a sawing motion to strike off the excess and level the surface.

6. 6

   Float, edge, and finish

   Once the surface sheen disappears, bull-float to smooth it, run an edger along the forms, and finish with a hand float. For grip outdoors, drag a broom lightly across the surface for texture.

7. 7

   Cure and strip forms

   Keep the slab damp (cover with plastic or mist it) for at least 3–7 days — concrete gains strength by curing, not just drying. Strip the forms after 24–48 hours; wait a week before heavy loads.

Pro tips

• Order 5–10% more than the bare calculation — uneven subgrade and spillage always eat into the total.
• Past about one cubic yard, a ready-mix truck is usually cheaper and far less work than mixing bags by hand.
• Pour in mild weather. Above 90°F the mix sets too fast to finish; below 40°F it can freeze before it cures.
• Tool control joints (or saw-cut them within 24 hours) every 8–10 ft so the slab cracks where you want it to, not randomly.

FAQ

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