How to Lay a Concrete Slab
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.
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.
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
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|
|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.
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)
- Tape measure
- Circular saw or miter saw
- Bubble level
- Bull float
- Screed tool
- Groover tool that can attach to bull float handle
- Edging tool
- Cordless drill
- Garden hose
- 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
- Concrete release agent
- Screws, 3-inch
- 25 Dobies, 2-inch high
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.
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.
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.
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.
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.
Cut the 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.
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.
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.
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.
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.
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.
Wet the Gravel Base
When the concrete truck is on its way, wet down the gravel base with the garden hose.
Pour the Concrete
Take delivery of concrete from the concrete truck, pouring concrete evenly throughout the form. Work quickly.
Screed 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.
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.
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.
Add Contraction Joints
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.
Gently drag a push broom backwards over the wet surface to create a light texture, if desired.
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.
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.