A concrete slab poured correctly lasts 30 years or more. One poured badly cracks within months and costs twice as much to tear out and redo. Learning how to pour a concrete slab saves you $3 to $6 per square foot in labor costs compared to hiring a contractor, and it gives you full control over thickness, finish, and timing. This guide covers everything from building forms to the final cure for slabs up to 200 square feet, the practical upper limit for most DIY pours.
Planning and Layout
Start by defining the slab’s purpose. A shed pad, hot tub base, or patio each have different thickness and reinforcement requirements. Standard residential slabs are 4 inches thick for foot traffic and light loads. Increase to 6 inches for vehicle parking or heavy equipment.
Mark the perimeter with stakes and string lines. Check for square by measuring diagonals. When both diagonals are equal, the rectangle is square. For a 10×10-foot slab, your diagonals should both measure 14 feet 1-7/16 inches. Add 2 inches beyond the finished slab size on each side to account for form board thickness.
Excavation and Base Preparation
Dig out the area to a depth of 8 to 10 inches to accommodate 4 inches of gravel base plus 4 inches of concrete. Remove all organic material, roots, and soft soil. Compact the subgrade with a plate compactor until no footprints remain when you walk across it.
Spread 4 inches of crushed gravel or road base (3/4-inch minus) over the compacted subgrade. Grade it smooth and compact again. The gravel provides drainage, prevents frost heave, and creates a stable base that keeps the slab from settling. Skip the gravel and you will likely see cracks within two winters in any climate that freezes.
Building the Forms
Use 2×4 lumber for 4-inch slabs or 2×6 lumber for 6-inch slabs. Straight boards are critical since any bow in the form transfers directly to the slab edge. Drive 18-inch wooden stakes every 3 to 4 feet along the outside of the form boards and screw them together from the stake side.
Set the forms to slope 1/8 inch per foot away from any adjacent structure for drainage. Verify with a 4-foot level and shims. Apply form release oil or spray cooking oil on the inside faces to make stripping easier after the concrete cures. For curves, use 1/4-inch Masonite or thin plywood bent to shape.
Reinforcement Options
Reinforcement holds cracks tight if they develop and adds tensile strength the concrete lacks on its own. Three common options:
- Welded wire mesh (6×6 W1.4/W1.4): The most common residential choice. Costs about $0.15 per square foot. Lay it on wire chairs or rebar chairs to position it in the middle third of the slab.
- Rebar (#3 or #4 bars on 24-inch centers): Stronger than mesh and better for slabs supporting heavy loads. Material cost runs $0.50 to $0.75 per square foot.
- Fiber mesh: Polypropylene fibers mixed directly into the concrete. Controls plastic shrinkage cracking but does not replace structural reinforcement for load-bearing slabs.
Mixing and Pouring
For slabs under 50 square feet, bagged concrete mix works fine. An 80-pound bag of standard mix yields 0.6 cubic feet. A 4-inch-thick, 10×10-foot slab requires about 1.24 cubic yards or roughly 56 bags. For anything larger, order ready-mix delivery. A typical residential load costs $150 to $200 per cubic yard with a minimum order of 1 yard and a short-load fee for orders under 5 yards.
Pour into one corner and work across the forms. Spread the concrete with a shovel or come-along, pushing it into corners and against forms. Do not throw or dump from a height, which causes aggregate separation. Tap the outside of the forms with a hammer to settle the mix against the edges and eliminate voids.
Screeding and Floating
Screed immediately after filling each section. Rest a straight 2×4 on top of the forms and pull it across the surface in a sawing motion while drawing it toward you. Two people make this much easier. Fill any low spots and screed again until the surface is flat and level with the form tops.
Wait until the bleed water rises to the surface and evaporates. This can take 20 minutes to 2 hours depending on temperature and humidity. Never work bleed water back into the surface. Once the sheen disappears, run a bull float or magnesium hand float across the surface using long, sweeping strokes to smooth out screed marks and push aggregate below the surface.
Finishing and Curing
For a broom finish, drag a concrete broom across the surface in straight, parallel lines after the final float pass. This creates a slip-resistant texture ideal for outdoor slabs. Cut control joints every 8 to 10 feet with a groover tool to a depth of at least 1 inch. These joints give cracks a predetermined path so they stay hidden.
Curing is where most DIY jobs go wrong. Concrete needs moisture to reach full strength. Cover the slab with plastic sheeting or apply a liquid curing compound within an hour of finishing. Keep the surface moist for at least 7 days. Concrete reaches about 70 percent of its rated strength at 7 days and full strength at 28 days. Avoid heavy loads for the first week and vehicle traffic for at least 14 days.
Strip the forms after 24 to 48 hours. Backfill against the slab edges with soil or gravel, sloping away for drainage. A properly poured and cured slab following this process resists cracking, settling, and surface deterioration for decades.
