How to Lay a Concrete Slab

A person using a bull float to smooth a wet concrete slab.

The Spruce / Debbie Wolfe

Project Overview
  • Working Time: 6 - 8 hrs
  • Total Time: 2 - 4 days
  • Yield: 20-foot by 12-foot concrete slab
  • Skill Level: Advanced
  • Estimated Cost: $1,000 to $1,500

A concrete slab provides a solid, level, dry base for sheds, pergolas, gazebos, patios, outdoor kitchens, and other large projects around the home. Learning how to lay a concrete slab isn't difficult, but because it's physically hard and speed is important, it's helpful to have an assistant or two.

Before You Begin

A significant amount of concrete—3 cubic yards—is required to produce a concrete slab 20 feet long, 12 feet wide, and 4 inches thick.

Concrete for small slabs or countertops can be mixed by hand in a wheelbarrow. But for larger projects, it's difficult to manually mix and deliver a consistent flow of wet concrete to the form before the previously poured concrete hardens.

For large concrete batches like this, it's better to use a portable concrete mixer or order a volumetric (metered) concrete delivery to the site.

Tip

Some of the tools required can be expensive, so consider renting them. Bull floats can be rented easily for this project.

Using a Portable Concrete Mixer

Rental yards and some home centers rent portable concrete mixers on a daily or weekly basis. These gas-powered mixers can churn up to 9 cubic feet of wet concrete. A vehicle equipped with a tow package is required since these larger mixers must be towed.

Use either bagged ready-mix concrete or loose ingredients (cement, rock, water, and sand) to create concrete.

Tip

Yard, a term commonly used with concrete, is 1 cubic yard. One cubic yard is 3 feet long, 3 feet wide, and 3 feet tall. One cubic yard contains 27 cubic feet. Conversely, 1 cubic foot equals 0.037 cubic yard.

Pre-Mix Concrete

When ordering bagged pre-mix concrete, have the concrete delivered on pallets close to the pour site.

  • If 80-pound bags of ready-mix concrete, order 134 bags
  • If 60-pound ready-mix concrete, order 178 bags

Loose Ingredients

For general purpose concrete slabs with 2,500 pounds compression strength, like patios, use a 1:2:4 concrete mix ratio.

Estimated dry weight of concrete materials
 Batch Amount  Cement  Sand Rock
1 cubic foot of concrete 21 pounds 42 pounds 84 pounds
27 cubic feet (1 yard) of concrete 567 pounds 1134 pounds 2268 pounds
3 yards of concrete 1701 pounds 3402 pounds 6804 pounds

Using Volumetric Concrete Delivery

Barrel or tank concrete truck delivery is where concrete is mixed at a plant and then sent out in mixing trucks. While good for large professional concrete work, it isn't best for do-it-yourselfers because batches can be too large and working times too short.

Volumetric (or metered) concrete delivery brings the concrete plant to the project site. Proportions of dry materials and water are determined with an onboard computer system and then mixed and poured on the spot.

Shorter loads can be delivered and the concrete is fresh since it's made on the spot. By contrast, barrel or tank concrete sometimes can arrive hot—that is, wet concrete that's already starting to set up by the time the truck arrives.

When to Lay a Concrete Slab

Temperature extremes can affect concrete. So, avoid freezing conditions that may cause the mix to freeze and expand. Also avoid temperatures that rise to 90° F or higher, as the water will evaporate and set too quickly, weakening the concrete. Ideally, pour the concrete slab in the early morning or when temperatures are around 60° F.

Safety Considerations

Wet concrete is caustic, possibly resulting in a condition known as cement burns. Cement burns may cause blisters, scars, and hardened or dead skin. Wear waterproof gloves and wear rubber boots and long sleeves when working with wet concrete. Dry concrete mix is a breathing irritant, so always wear breathing protection when mixing concrete.

Before digging, call the 811 Call Before You Dig line to have the project site marked for vital hidden services.

What You'll Need

Equipment / Tools

  • Pull tie wire twister (for rebar)
  • Hammer
  • Tape measure
  • Circular saw or miter saw
  • Bubble level
  • Shovel
  • Bull float
  • Screed tool
  • Groover tool that can attach to bull float handle
  • Edging tool
  • Tamper
  • Cordless drill
  • Garden hose

Materials

  • 3 yards wet concrete
  • Gravel for base (if needed)
  • Steel rebar mesh (6-inch) or rebar rod
  • Rebar ties
  • 4 two-by-fours, each 12-foot
  • 4 two-by-fours, each 8-foot
  • Twine
  • Concrete release agent
  • Screws, 3-inch
  • 25 Dobies, 2-inch high OR 2 1/4-inch high rebars

Instructions

  1. Stake Out the Location

    Measure out the project site with the tape measure. Then use a hammer to drive a stake at each corner. Drive the stakes deeply so that you'll be able to tightly draw the twine around the stakes to define the slab area.

    A person wrapping string around a stake in the ground.

    The Spruce / Debbie Wolfe

  2. Dig Down

    If the site already is solid, with well-draining sand and/or gravel mixed in, you may not need to lay a gravel base. If so, dig down 4 inches.

    If you need to add a gravel base, dig down 8 inches to account for 4 inches of gravel and 4 inches of the concrete slab.

    If the slab area is covered with grass, remove the turf. Turf removal itself will take off about 3 or 4 inches.

    A person using a shovel to dig down into grass.

    The Spruce / Debbie Wolfe

  3. Clear and Level the Site

    By hand, remove rocks, sticks, and other debris. Rake down the site. Then lay a straight 8-foot two-by-four with a bubble level on top at various angles around the site to check for level.

    Tip

    Organic material like sticks, leaves, and pine cones left in the concrete pour will create voids after they decay.

    A person using a rack to smooth dirt.

    The Spruce / Debbie Wolfe

  4. Tamp the Base

    Use the hand tamper to tamp down the soil. Keep checking for level to make sure that you don't tamp the soil out of level.

    A person tamping soil.

    The Spruce / Debbie Wolfe

  5. Pour the Gravel and Grade

    If the site needs an extra gravel base, pour it and be sure to tamp it down well and level it off.

    Tip

    If desired, with a circular saw or electric miter saw, cut 24 pieces from two of the two-by-fours, each piece 1 foot long. Then, cut each 1-foot section diagonally to produce triangle-shaped stakes.


    A person pouring gravel out of a bag and onto the ground.

    The Spruce / Debbie Wolfe

  6. Drive the Stakes

    Drive two stakes at each corner, one stake on each side. Following the string, drive stakes about every 2-1/2 feet along the perimeter. Save two stakes out; do not drive them yet. You can remove the string once the stakes are set.

    Tip

    If you can not find heavy-duty grade stakes, you can cut your own form stakes. With a circular saw or electric miter saw, cut 24 pieces from two of the two-by-fours, each piece 1 foot long. Then, cut each 1-foot section diagonally to produce triangle-shaped stakes.

    A person hammering a stake into the ground.

    The Spruce / Debbie Wolfe

  7. Build the Form

    With 3-inch screws, attach the two-by-fours to the stakes:

    • A 12-foot two-by-four on each short side
    • A 12-foot two-by-four and an 8-foot two-by-four on each long side

    Raise the form sides 1/2-inch to create a slab that is a full 4 inches thick. Check for level on all sides.

    The stakes must be on the outside of the two-by-four forms and the screws must be driven from the outside toward the inside.

    Add the last two stakes at the junction between the 12-foot and 8-foot boards on the long sides to prevent hanging seams.

    A person attaching a wood frame to a stake with screws.

    The Spruce / Debbie Wolfe

  8. Coat the Concrete Form

    Use vegetable oil, motor oil, mineral oil (not mineral spirits), or a concrete-form release agent to coat the inside of the concrete form for easier removal.

    A person spraying a wood from with concrete spray.

    The Spruce / Debbie Wolfe

  9. Grade Again

    Grade the gravel or base again so that it is packed firm and level. Laying a screed board across the tops of the forms, measure downward with the tape measure to check for a 4-inch depth all around.

    A person using an orange level.

    The Spruce / Debbie Wolfe

  10. Set the Rebar

    Rebar mesh: Cut and lay 6-inch rebar mesh across the site, covering all areas except for a 3-inch band on the outside. Exposed rebar will rust, compromising the concrete's strength. Tie pieces together with rebar ties. Elevating blocks called dobies are helpful but not necessary on rebar mesh. Not only do they elevate the mesh but they help to hold down springy mesh.

    Rebar rod: If you choose to use rebar rod, create a grid of rods spaced every 2 feet. Use the tying tool to add ties at every spot where rebar crosses rebar. Elevate the rebar grid on 20 to 25 evenly spaced dobie blocks. Tie the grid onto the dobies with the dobies' own ties.

    Tip

    All work up to this point should be completed well before the concrete truck arrives. Waiting on any of these items will delay the pour.

    A person laying a sheet of rebar mesh onto a level gravel base.

    The Spruce / Debbie Wolfe

  11. Wet the Gravel Base

    When the concrete truck is on its way, wet down the gravel base with the garden hose.

    A person spraying a gravel base with water.

    The Spruce / Debbie Wolfe

  12. Pour the Concrete

    Take delivery of concrete from the concrete truck, pouring concrete evenly throughout the form. Work quickly.

    Tip

    If you earlier chose to leave the rebar mesh unelevated, this is the time to manually raise it to the center of the wet concrete.

    Concrete pouring from a mixer onto a gravel base.

    The Spruce / Debbie Wolfe

  13. Screed the Concrete

    Before screeding, be sure to cut the stakes flush to the top of the concrete. Use the screed tool or a scrap 8-foot-long piece of two-by-four to screed the top of the concrete. Move the screed back and forth to remove excess concrete and to level the surface.

    A person using wood board to screed concrete.

    The Spruce / Debbie Wolfe

  14. Float the Concrete

    Run the bull float over the surface three or four times to remove screed marks and to force large pieces of aggregate lower into the slab.

    A person using a bull float to smooth concrete.

    The Spruce / Debbie Wolfe

  15. Work the Edges

    Wait until the concrete has firmed up a bit, then run the hand edger in long passes along the perimeter of the slab.

    Attach the groover tool to the bull float handle. Create at least one (preferably two or more) contraction (control) joints from one long side of the slab to the other by resting the grooving tool at the far edge of the form and pulling towards you.

    A person using an edge tool to smooth concrete.
  16. Add Texture

    Gently drag a push broom backwards over the wet surface to create a light texture, if desired.

    A person using a push broom to add texture on a concrete slab.

    The Spruce / Debbie Wolfe

  17. Let the Concrete Cure

    Keep the concrete misted with water for the first 24 hours. After 48 hours, you should be able to walk on the concrete slab. The concrete will be fully cured in about a month.

     A person misting concrete.

    The Spruce / Debbie Wolfe

When to Call a Professional

House foundations, footers, and other structural concrete should be poured by concrete professionals.

If you're having mixed concrete delivered, keep in mind that drivers only transport and set up the truck for the initial pour. They cannot help with pouring concrete in the form or finishing it.

The Spruce uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Preventing Skin Problems from Working with Portland Cement. OSHA