A concrete slab patio is hard to beat. It's flat and smooth, so it's suitable for all sorts of furniture and outdoor activities. It's easy to keep clean and doesn't leave grit on your shoes to be tracked into the house (like gravel patios). It's virtually impervious to weeds and does not shift with seasonal changes, unlike paver and stone patios. Perhaps best of all, if you build the patio yourself, you can save on some building costs.
The challenge of building with concrete is, not surprisingly, the concrete itself. Once concrete is mixed, there's no turning back as it hardens, no matter what. The key to success is preparation: make sure the forms are well-secured and that all of your tools (and helpers) are ready to work for you. After the concrete is poured and screeded—the initial process of leveling and smoothing the surface, done with a long 2x4 board—it's important to let the concrete set up properly before you start the final finishing. When the finishing begins, don't overwork the concrete. Bringing up too much moisture weakens the finished surface.
When to Build a Concrete Patio
You can pour concrete within a fairly wide temperature range, but for beginners it's best to wait for dry, warm weather. Rain can ruin a concrete finish, and freezing temperatures can ruin an entire concrete slab. To slow the curing process in very hot, dry weather, use a shade to keep the concrete out of direct sunlight, and mist the concrete with water as needed to prevent premature curing, which weakens the finished product.
Codes and Regulations
In most areas, large concrete slabs require approval from the city's building and/or zoning department. Slabs are permanent structures, and may be subject to zoning restrictions. Local building code rules may dictate several elements of the design, including the thicknesses of the gravel base and the slab, the type of concrete and its internal reinforcement, and the need (or not) for a moisture barrier under the slab. Contact your city's building department for recommendations specific to your project.
Equipment / Tools
- Tape measure
- Mason's line (braided nylon string)
- Line level
- Carpenter's level
- Straight 2x4 board
- Plate compactor
- Steel rake
- Wood saw
- Drill and screwdriver bit
- Concrete mixer
- Mason's trowel
- Concrete groover
- Concrete edger
- Wood float
- Medium-bristle push broom (optional)
- Compactible gravel
- 2x4 lumber
- 3 1/2-inch and 2-inch deck screws
- Vegetable oil
- Bagged concrete (or ready-mix)
- Plastic sheeting
- Concrete sealer (optional)
- 1/2-inch x 3 1/2-inch expansion joint strips (as needed)
- Spray adhesive or concrete nails (as needed)
Set Up the Guide Strings
Set up guide strings to represent the edges of the patio, using stakes and mason's lines. Start by driving two stakes at each corner of the patio area, placing the stakes about 1 foot beyond the edges of the patio. Tie mason's lines between pairs of opposing stakes to create a square or rectangular layout. The points where the strings intersect mark the patio corners. The strings should be at least 6 inches above the ground.
Square the String Layout
Check the string layout for square by measuring diagonally between opposing corners where the strings intersect. If the two diagonal measurements are equal, the layout is square. If they are not equal, adjust the stake positions as needed until the measurements are equal.
Slope the Strings
Slope the strings so that the patio will slope away from the house at 1/8 inch per foot. For example, if the patio measures 8 feet from one end to the other, it should slope downward by 1 inch over its length. Measure down 1 inch (in this example) from the strings at the low end of the patio, and mark the stakes, then move the strings to the marks.
Excavate the Patio Area
Remove all vegetation in the patio area, extending the edges about 6 inches beyond all sides of the patio (to make room for setting the concrete form). Excavate the soil to a depth of 8 inches. As you work, measure down from the layout lines to gauge the excavation depth. It's usually easiest to dig out the sides to full depth, then dig out the interior area, using a long, straight 2x4 and level to make sure the entire area is level (from side to side; it will slope in the other direction). Tamp the soil thoroughly with a rented plate compactor.
Install the Gravel Base
Fill the excavated area with 2 inches of compactible gravel. Rake the gravel smooth, then compact it thoroughly with the plate compactor. Add 2 more inches of gravel, and rake it smooth. Measure down from the layout strings and use the 2x4 with a carpenter's level on top to make sure the gravel is level side-to-side and slopes end-to-end to follow the strings. Compact the second layer of gravel.
Build the Concrete Form
Construct a concrete form with 2x4 lumber and 3 1/2-inch screws. The interior dimensions of the form should equal the final dimensions of the concrete slab. Set the form onto the gravel base. Measure the diagonals (as with the string layout) to make sure the form is square.
Secure the Form
Drive wood stakes all around the outside of the form, spacing them about 2 feet apart. The stakes should extend about 3 1/2 inches above the ground. Lift the form so its top surface is 4 inches above the ground and secure it to the stakes with 2-inch deck screws driven through the stakes and into the form boards. The top of the form should be about 1/2 inch above the top of the stakes. Maintain even spacing between the form and the guide strings. Secure the form at all of the stakes, then remove the guide strings and stakes.
Note: If the patio slab will abut an existing concrete structure, such as a garage slab, driveway, or house foundation, install strips of expansion joint material onto the existing structure, using spray adhesive or concrete nails. Align the top of the expansion joint strip with the tops of the form boards. This expansion joint will isolate the new concrete from the old to help prevent cracking.
Pour the Concrete
Coat the interior surfaces of the form with vegetable oil so the concrete won't stick to them. Mix fiber-reinforced concrete (or as specified by building code) in a rented concrete mixer, following the manufacturer's directions. Transfer the mixed concrete to a wheelbarrow, then dump the concrete inside the form in piles. Distribute the concrete with a shovel to fill the form. Repeat until the form is filled to the top.
Screed the Concrete
Use the long, straight 2x4 board to screed the top of the concrete. Rest the board on top of both sides of the form, and move the board back and forth in a sawing motion while pulling it backward. Use a shovel to remove excess concrete, or add concrete to fill in low spots, as needed, so the top of concrete is flat and level. If the slab is larger than 8 feet in either dimension, cut control joints at 8-foot intervals, using the 2x4 and a mason's trowel. Position the board across the form, then follow the edge of the board with the trowel, slicing down into the concrete to separate the gravel inside and create a control joint (to control cracking).
Finish the Concrete
Let the concrete cure until the "bleed water" (moisture that rises to the surface after screeding) disappears, then smooth the surface with a darby. Allow the bleed water to disappear again. If you made control joints, go over all of the joints with a groover tool, using the 2x4 to ensure straight lines. Round over the edges of the slab with an edger tool. Smooth out any blemishes in the surface with a wood float.
Add a Broom Finish (Optional)
"Broom" the surface of the concrete, if desired, to add slip-resistance. Drag a push broom backwards across the surface of the concrete in straight, parallel lines. After the entire slab is broomed, you can smooth the edges and control joints again with an edger and trowel or leave the broom lines extending to the edges of the slab.
Cover and Cure the Patio Slab
Cover the concrete with plastic sheeting. Keep the concrete moist while curing by lifting the plastic and misting the concrete with water each day. Otherwise, keep the concrete covered. Repeat this process for one to two weeks.
Complete the Job
Remove the plastic from the concrete. Disassemble the wood form, and remove all stakes from the ground. Backfill around the slab with soil or other material. If desired, seal the concrete with concrete sealant, following the manufacturer's directions.
Tips for Building a Concrete Patio
Buying concrete in dry, premixed bags makes sense for relatively small patio slabs. To get an idea of how many bags you would need, a 50-square-foot slab at 4 inches thick needs about 28 80-pound bags or 38 60-pound bags. You can buy bagged concrete at a local home center or lumberyard, and you can rent a concrete mixer at any large rental outlet.
For large slabs, consider ordering ready-mix concrete delivered by a concrete truck. Ready-mix is more expensive than bagged concrete, but it's far more convenient, and you don't have the labor of mixing the concrete or the concern of getting the mix just right. Discuss your plans with local concrete companies to compare costs and to make sure your site is suitable for ready-mix delivery.