How to Pour a Concrete Slab

Before you dig: plan the slab

Most slab failures start before any concrete arrives: soft ground, no base, water running toward the house, or a pour that is too big for the crew. Settle these first.

• Permits. Many towns let you pour a small patio without a permit, but driveways, garage floors, slabs under living space and anything near a property line often need one. Call your building department.
• Buried lines. Contact 811 a few business days before you dig. The service is free and marks gas, power and water lines.
• Drainage. Plan a slope away from the house. A common target is 1/8 to 1/4 inch per foot, which is 1.25 to 2.5 inches over a 10 ft patio.
• Weather. Pick a mild, dry, calm day. The concrete should stay above 50°F while it cures, and frozen ground is never a base.
• Crew. Concrete does not wait. For a 10 x 10 slab from bags, plan on two or three people. For a truck, have helpers with wheelbarrows ready before it arrives.

The International Residential Code (IRC) sets 3.5 inches as the minimum for a floor slab on ground, a 4-inch base course in most soils, and a 6-mil vapor retarder under slabs of conditioned space (section R506). Unheated sheds, garages and open patios are exempt from the vapor retarder. For porches, carport and garage slabs exposed to severe freeze-thaw weather, the IRC asks for 3,500 psi air-entrained concrete; many states amend these rules, so check yours.

Step 1: Lay out and dig

Mark the corners with stakes and run mason's string around the outside. Check that the layout is square by measuring the two diagonals: when they match, the corners are 90 degrees. Remove sod, roots and topsoil; organic soil rots and settles.

Dig to the depth of the slab plus the base. For a 4-inch slab on 4 inches of gravel that sits flush with the lawn, that is 8 inches. Dig about 6 inches wider than the slab on each side so you have room for the forms and stakes. Then compact the bottom of the hole with a plate compactor or hand tamper; any soft spot you leave becomes a crack later.

Step 2: Put down a compacted gravel base

Spread crushed stone with fines (often sold as crusher run, road base or 3/4-inch minus) in two layers of about 2 inches. Wet each layer lightly and run a plate compactor over it until it no longer moves underfoot. Rent the compactor; it costs far less than a cracked slab. Rake the top so it follows the same slope the slab will have, which keeps the concrete an even thickness.

Our gravel calculator converts the area and depth into tons. A 10 x 10 base 4 inches deep is about 1.5 tons. If the slab is the floor of a heated space, lay 6-mil polyethylene on top of the base with the seams lapped at least 6 inches.

Step 3: Build the forms

Most DIY slabs are formed with straight 2x lumber screwed to wood stakes driven every 3 to 4 feet on the outside of the boards. Keep the stakes below the top of the form so the screed board can ride over them. Remember a 2x4 is only 3 1/2 inches tall: it makes a 3 1/2-inch slab unless you set it on top of the base and the base is 1/2 inch low, or you use a 2x6.

Set the top of the forms to the finished height and slope with a string line and level. Use the forms to build in the slope: for a 10 ft run at 1/8 inch per foot, the far form sits 1 1/4 inches lower. Brush form oil or vegetable oil on the inside faces so they release cleanly. Where the slab meets the house foundation, steps or an existing walk, attach a strip of isolation joint material (asphalt-impregnated fiber or foam) so the two can move separately.

Step 4: Add rebar or mesh

Reinforcement does not stop concrete from cracking. It holds cracks tight so the two sides stay level. A common grid for a patio or shed is #4 (1/2-inch) bar 18 inches on center each way, tied at the crossings and kept 2 to 3 inches from the forms. Welded wire mesh is a lighter option for walks and patios.

The steel only works if it sits inside the concrete, not on the ground. Set it on plastic chairs or concrete blocks so it stays between the middle and the upper third of the slab, which is what the IRC asks for in floor slabs. "Pulling the mesh up" with a rake during the pour rarely works.

Step 5: Bags or a truck, and how to mix

For the 10 x 10 example, our concrete calculator puts bags with a day of mixer rental at about $467 and a 1.5-yard truck load at about $423, including a short-load fee and delivery. Under about 1 cubic yard, bags usually win. Above 2 to 3 yards, a truck is cheaper and spares you hours of mixing.

Mixing bags. Quikrete's data sheet for its standard concrete mix calls for about 6 pints of water per 80 lb bag to start and no more than about 9 pints, aiming for a 2 to 3 inch slump: stiff enough to hold a shape, with no dry pockets and no puddles. Allow about 1 cubic foot of mixer drum for each 80 lb bag. Extra water makes concrete easier to place and weaker, more prone to cracking and dusting. If it is too stiff, add water a little at a time.

Ordering a truck. Tell the plant the job (exterior slab), the volume and the access. For outdoor slabs in freeze-thaw climates, ask for air-entrained concrete of at least 3,500 psi, or what your code requires. Check where the truck can park; a loaded truck can crack a driveway, and chutes only reach so far. Have wheelbarrows, a hose and every tool laid out before it arrives.

Step 6: Place, screed and float

Dampen the base so it does not suck water out of the concrete, but leave no puddles. Start at the far end and fill the forms to full depth, working continuously. Do not fill halfway and come back later: the line between the two batches becomes a weak "cold joint."

• Screed. Drag a straight 2x4 across the top of the forms with a sawing motion to strike off the excess. A helper shovels concrete into low spots ahead of the board.
• Bull float. Right after screeding, push a bull float (or a darby on small slabs) across the surface to flatten ridges and push the stones down slightly.
• Edge. Run an edging tool between the concrete and the forms to round and compact the edges, so they do not chip.

Step 7: Cut the control joints

Concrete shrinks as it dries and it will crack. Joints decide where. The National Ready Mixed Concrete Association (NRMCA) recommends a maximum joint spacing of 24 to 36 times the slab thickness, which is about 10 feet for a 4-inch slab, and no more than 15 feet. Panels should be square or nearly so, with the long side no more than 1.5 times the short side. Avoid L-shaped panels.

A 10 x 10 patio sits at the edge of that range as one panel. Splitting it with one joint down the middle makes two 5 x 10 panels, which breaks the 1.5:1 rule; two joints that make four 5 x 5 squares are better. Quikrete's instructions for its bag mix are more conservative: 1-inch-deep joints every 6 to 8 feet. When in doubt, go closer.

You can tool the joints with a groover while the concrete is soft, using a straight board as a guide, and rerun them during finishing. Or saw them the next day: NRMCA puts conventional saw cuts at 4 to 12 hours after finishing, as soon as the edges do not ravel. Wait too long and the slab cracks on its own first. Saw wet, or wear a respirator, because cutting concrete makes silica dust.

Step 8: Finish the surface

After floating, water rises to the surface ("bleed water"). Wait until that sheen is gone before you touch the slab again. Working the surface while it is wet pushes water and fine material to the top and leaves a weak skin that dusts or flakes.

When you can press a thumb in about 1/4 inch, go over it with a magnesium hand float, then rerun the edger and groover. For outdoor slabs, finish with a broom pulled across the slope: it leaves a slip-resistant texture. Steel troweling to a smooth shine is for indoor floors, and outdoors it makes the slab slick when wet. Do not sprinkle dry cement or water on the surface to make finishing easier, and do not overwork it.

Step 9: Cure it, and keep it from freezing

Concrete hardens by a chemical reaction with water, not by drying. If it dries out early it never reaches its strength: NRMCA notes that concrete left in a dry environment can lose as much as 50% of its potential strength compared with concrete that is moist-cured.

• How long. Quikrete says to start curing as soon as possible and continue for 5 days in warm weather (70°F and up) or 7 days at 50°F to 70°F.
• How. Spray or roll on a curing compound once the surface is hard, or cover with plastic sheeting sealed at the seams and weighted at the edges, or keep it wet with burlap or a sprinkler that runs without gaps. Plastic can blotch colored concrete.
• Cold. Protect the slab from freezing for the first 48 hours with plastic and insulated blankets if temperatures will drop below 32°F. Do not apply curing compound if rain or temperatures below 50°F are expected within 24 hours.
• Heat and wind. Pour early in the day, keep the base damp, and start curing right after finishing. Fast drying causes surface cracks.

Bagged 4,000 psi mix reaches about 2,500 psi at 7 days and 4,000 psi at 28 days in Quikrete's tests. Light foot traffic the next day is usually fine. Wait at least a week for a car, a hot tub or a loaded shed. Pull the forms after a day or two, once the edges are firm.

What a DIY slab costs

The bag option for a 10 x 10 slab, 4 inches thick on a 4-inch gravel base, at typical October 2026 big-box prices:

Add a plate compactor rental, a curing compound and a broom if you do not own them. Hiring out, a plain slab usually runs about $4 to $8 per sq ft installed, roughly half material and half labor, with $2 to $3 per sq ft more for a rebar grid. Many contractors have a minimum charge near $1,000, so a small patio costs more per square foot than a big one. Removal of an old slab, poor access and decorative finishes add to that.

Safety

Wet concrete is strongly alkaline. Quikrete's label warns that it causes severe skin burns and serious eye damage, and that breathing the dust over time can cause lung damage or cancer. Wear nitrile or rubber gloves, long sleeves, safety glasses and rubber boots if you will stand in it, and a dust mask when you dump bags. Wash wet concrete off your skin right away; burns often show up hours later. An 80 lb bag is heavy: lift with your legs and get help.

Common mistakes

• No base, or a soft one. Loose fill and topsoil settle, and the slab cracks over the low spot.
• Adding water to make it flow. It is the fastest way to a weak, dusty surface.
• Finishing with bleed water on top. Wait for the sheen to go.
• Joints too far apart or too shallow. The slab picks its own crack lines.
• No slope. Water pools on the slab or runs to the foundation.
• Running short. Order 5 to 10% extra. A gap between batches leaves a cold joint.
• Skipping the cure. A slab that dries in a day in the sun loses strength it never gets back.

When to call a pro

A small patio or shed pad is within reach for a careful DIYer with two helpers. Hire a concrete contractor for driveways and garage floors, slabs over about 2 to 3 cubic yards, anything that carries a building or a wall, footings, expansive clay or wet sites, and pours in hot or freezing weather. The work goes fast with an experienced crew, and the timing of finishing and jointing is where most DIY slabs go wrong. If you are on the fence about the material, a raised deck is the other common backyard floor; compare with the deck calculator, and use the fence calculator if the project includes posts set in concrete.

Common questions

Sources

Prices are typical US prices as of October 2026 and vary by region and supplier. Code requirements change by edition and by local amendment: your building department has the final word.

• Quikrete, Concrete Mix No. 1101 data sheet, revised October 2022 (yields, mixing water, slump, slab method, 6–8 ft joints, curing times, cold-weather and safety warnings, 2,500 psi at 7 days and 4,000 psi at 28 days)
• NRMCA, CIP 6: Joints in Concrete Slabs on Grade (spacing 24–36 times thickness, 15 ft max, 1.5:1 panels, 1/4 depth, saw-cut timing, isolation joints)
• NRMCA, CIP 11: Curing In-Place Concrete (keep above 50°F, strength loss without moist curing)
• Portland Cement Association, Curing in construction and Contraction/control joints in concrete flatwork
• EVstudio, Code requirements for concrete slabs on grade (IRC R506: 3.5 in minimum, 4 in base, 6-mil vapor retarder, reinforcement position) and IRC Table R402.2 as adopted in Minnesota Rules 1309.0402 (3,500 psi air-entrained for exposed slabs in severe weathering)
• 811 Before You Dig, Common Ground Alliance
• Inch Calculator, concrete slab cost guide (installed cost per sq ft, rebar cost, minimum charges), accessed October 2026
• Quantities and DIY prices: HomeOutlay concrete calculator defaults (80 lb bag $6.48, ready-mix $165 per cubic yard, $100 short-load fee under 4 yards, $75 delivery, mixer $65 per day, #4 rebar $12 per 20 ft stick), October 2026