How to Pour a Concrete Slab: Foundation, Patio & Garage Floor

Before any concrete goes down, you need the right substrate. Concrete is strong in compression but brittle in tension. If the ground under it moves — from frost heave, poorly compacted fill, or expansive clay — the slab cracks. Every minute spent on subbase preparation pays dividends.

Lay out the slab using batter boards and string lines. Check square by measuring diagonals — they must match within 1/4 inch on any slab under 20 feet. Mark the perimeter, then excavate to a depth of your slab thickness plus base material thickness: typically 4-inch slab + 4-inch gravel base = 8 inches total below final grade.

Excavation for a 20x20 foot slab removes roughly 2.5 cubic yards of soil (about 5 tons). Have a plan for spoil removal before you start — a 10-wheel dump truck typically holds 10 to 14 cubic yards. For small slabs under 200 square feet, wheelbarrow removal is practical.

A 4-inch compacted gravel subbase is the minimum for any slab exposed to vehicle loads or freeze-thaw cycles. Use 3/4-inch crushed stone (ASTM #57 stone) or Class II base aggregate. Pour, rake level, and compact with a plate compactor in 2-inch lifts. A plate compactor rental runs $75 to $150 per day.

The ACI (American Concrete Institute) Standard 332 recommends a minimum 4-inch granular subbase under all residential slabs. Some local codes require vapor barrier over the subbase for garage floors and any slab at risk of moisture transmission. Use 6-mil polyethylene sheeting, lapping seams 12 inches and taping.

If you're pouring over clay soil — common throughout the Midwest, Southeast, and Pacific Northwest — compaction alone isn't enough. Clay expands when wet and contracts when dry, creating the heave cycles that crack plain slabs in 5 to 10 years. For slabs on clay, either over-excavate and replace with 8 to 12 inches of engineered fill, or design thicker slabs with more steel.

Forms define the slab edges and hold concrete in place during placement. For a 4-inch slab, use 2x4 lumber; for a 6-inch slab, use 2x6 dimensional lumber. Forms must be straight, plumb, and staked every 18 to 24 inches — concrete exerts significant hydrostatic pressure that will bow unstaked forms.

Set forms to your finished elevation. Use a builder's level or laser level to ensure the slab pitches correctly: a minimum 1/8 inch per foot fall away from structures for exterior slabs. Interior slabs on structures need control joints designed into the form layout.

Apply form release agent (motor oil, commercial form release, or diesel fuel) to the inside faces of forms before pouring. Without it, the concrete bonds to the wood and form stripping becomes destructive. Forms can be stripped when concrete reaches approximately 400 PSI — typically 24 to 48 hours after pour in normal temperatures.

IRC Section R506 requires reinforcement for slabs over fill material or where the subgrade has questionable bearing capacity. Beyond code minimums, reinforcement is good practice for any slab that will see vehicle loads, thermal cycling, or will be over expansive soils.

#3 rebar (3/8-inch diameter) at 18-inch centers on center in both directions is the standard residential specification. Use wire bar chairs or concrete block chairs (dobies) to hold rebar at mid-depth — approximately 2 inches from the bottom in a 4-inch slab. Rebar resting on the subbase adds no value; it needs to be in the concrete matrix. This is the single most common DIY error I see.

6x6 wire mesh (W1.4/W1.4) is an alternative to rebar for light-duty patios. Mesh provides temperature and shrinkage crack control but less load-carrying capacity than #3 rebar. For garage floors, driveways, and any slab carrying vehicles, use rebar. For decorative patios with no vehicle access, mesh is acceptable.

Fiber reinforcement (polypropylene or steel fibers added to the mix) supplements but does not replace structural rebar. Fibers reduce plastic shrinkage cracking in the first 24 hours post-pour — they're a useful addition to exterior slabs but not a structural substitute.

Order your concrete based on the intended use. Ready-mix concrete is priced per cubic yard and delivered by transit mixer. In 2026, ready-mix costs $125 to $160 per cubic yard nationally, with significant regional variation — Gulf Coast markets average $120 to $135/CY; Northeast urban markets run $145 to $175/CY per BuildEstimatory's 2026 pricing data.

Specify mix design, not just PSI. For exterior slabs in freeze-thaw climates, specify: 4,000 PSI, 0.45 water-cement ratio maximum, 5% ± 1.5% air entrainment, 3/4-inch maximum aggregate. The air entrainment creates microscopic bubbles that absorb expansion pressure during freezing, preventing surface scaling.

Adding water to concrete on-site to improve workability is the single most destructive thing a finisher can do. Every gallon of water added to a cubic yard of concrete drops compressive strength by approximately 200 PSI and dramatically increases shrinkage cracking. Specify a 4- to 5-inch slump for most pours; add a mid-range water reducer (plasticizer) if more workability is needed.

Over-ordering wastes money; under-ordering creates cold joints — structural weaknesses where fresh and partially set concrete meet. Order 8 to 10% more than calculated volume as a buffer. Use this formula: Length (ft) × Width (ft) × Thickness (in) ÷ 12 ÷ 27 = cubic yards.

Example: 20 × 20 foot slab, 4 inches thick: 20 × 20 × 4 ÷ 12 ÷ 27 = 4.94 cubic yards. Add 8%: order 5.3 cubic yards. Round up to 5.5 cubic yards (minimum practical delivery in many markets).

For small slabs under 1.5 cubic yards (about 10x10 at 4 inches), you may be better off using bagged concrete mix from a home center. Ready-mix trucks typically have minimum delivery charges of $150 to $400 regardless of volume. A 200 sq ft slab uses about 2.5 cubic yards — right on the borderline where ready-mix becomes economical.

Have everything staged before the truck arrives. Concrete begins setting the moment water contacts cement — you have a working window of approximately 90 minutes in 70°F weather. In hot weather (above 85°F), that window shrinks to 45 to 60 minutes. In cold weather (below 50°F), the window extends to 2 to 4 hours, but strength gain slows.

Direct the chute or pump to place concrete as close to final position as possible. Never use a vibrator to move concrete horizontally — this causes segregation, where heavy aggregate sinks and water bleeds to the surface, creating a weak top layer. Move concrete with rakes, come-alongs (square-head rakes), and shovels only.

Use an internal concrete vibrator to consolidate concrete around rebar and eliminate voids (honeycombs). Vibrate at 18-inch intervals, inserting the vibrator vertically to within 2 inches of the bottom. Each insertion should last 5 to 10 seconds until large air bubbles stop surfacing. Vibrating too long causes segregation; not enough leaves voids that weaken the slab.

Screed the surface with a 2x4 or magnesium screed bar to bring concrete flush with the forms. Use a sawing back-and-forth motion while moving forward. Fill low spots with shoveled concrete and re-screed. The goal is a flat surface — use a 10-foot straightedge to identify low and high spots.

Bull float with a magnesium or resin float to bring paste to the surface and fill screed marks. Two passes at 90 degrees to each other produces the best result. Wait for bleed water to evaporate before final finishing — finishing over standing bleed water is one of the most common causes of surface scaling and dusting.

The timing of final finishing is the highest-skill part of the operation. Too early and you entrap bleed water. Too late and you can't close the surface. The surface is ready for final troweling when a footprint compresses the concrete about 1/4 inch and the sheen of bleed water is gone — typically 1 to 4 hours after pour depending on temperature and mix water content.

For exterior slabs, finish with a broom texture (drag a stiff bristle broom across the surface in one direction) for traction. For interior garage floors, a steel trowel finish creates a denser, harder surface. Decorative options — stamping, exposed aggregate, colored concrete — must be specified at the mix design stage, not as afterthoughts.

Concrete shrinks approximately 1/16 inch per 10 feet as it cures. Without control joints, that shrinkage cracks randomly — ugly, hard to seal, and eventually a waterproofing problem. Control joints are tooled or saw-cut grooves that direct cracking to predictable locations where it can be sealed.

Space control joints at intervals equal to 2 to 3 times the slab thickness in feet — so a 4-inch slab gets joints every 8 to 12 feet. Maximum spacing is 15 feet in any direction per ACI 302.1R. Make joints 1/4 of the slab thickness deep — 1 inch deep on a 4-inch slab. Shallower joints don't control cracks; they just add a decorative groove.

Saw-cut control joints within 4 to 12 hours of finishing using a walk-behind concrete saw. Too early and the saw ravels the joint edges; too late and the random cracking has already started. If you're hand-tooling joints (using a grooving tool immediately after floating), keep them deep enough — the most common error is cutting joints too shallow.

Curing is the process of maintaining moisture and temperature conditions for concrete to achieve design strength. Per ACI 308 Standard Practice for Curing Concrete, exterior slabs need 7 days of proper curing for adequate strength and durability. Concrete that dries too fast in low-humidity or high-wind conditions loses moisture before hydration is complete, creating a weak, dusty surface layer.

The three main curing methods for slabs: wet curing (plastic sheeting, burlap, or wet blankets maintained for 7 days), curing compound (sprayed on immediately after final finishing — covers the surface and retains moisture), or water curing (continuous wetting). Curing compound is the most practical for residential projects. Apply it immediately after the final finish pass before the surface dries.

Do not use calcium chloride as an accelerator on any slab with steel reinforcement — it accelerates corrosion. In cold weather, use non-chloride accelerators or blanket insulation to maintain concrete temperature above 50°F for 7 days per ACI 306 cold-weather guidelines.