What You Need to Do Before You Pour

Building a concrete patio slab means preparing the ground first, then setting up forms, pouring concrete, and finishing the surface. The work takes three to five days if you do it yourself, though the concrete needs seven days to cure before you can walk on it regularly. Most homeowners rent a concrete mixer and a tamper rather than buy them, which keeps costs down if this is your only project.

Start by marking out the area where your patio will sit. Use stakes and string to outline the shape — square or rectangular patios are easiest to build. Measure the diagonals; if they are equal, your corners are square. Most residential patios are four inches thick, which is deep enough for foot traffic but not for vehicles.

Check your local building codes before you start. Some areas require a permit for patios over a certain size, and some have rules about setbacks from property lines or drainage. Call your city or county building department; they can tell you in one call whether you need a permit and what the rules are for your address.

Key Takeaways

  • Prepare the ground by removing grass and topsoil, then compact the soil with a tamper so the concrete does not settle unevenly later.
  • Build wooden forms around the perimeter using two-by-fours, level them carefully, and brace them so they do not bow outward when you pour.
  • Pour concrete in one day, work it into corners and under the forms with a shovel, then strike off the excess with a straight board.
  • Finish the surface while the concrete is still wet by broom-finishing it for traction or troweling it smooth, depending on the look you want.
  • Keep the concrete damp for seven days after pouring so it cures properly and does not crack.

Clearing and Leveling the Ground

Remove all grass, weeds, and topsoil from the patio area. Topsoil is dark and crumbly; you want to dig down to the subsoil underneath, which is usually lighter in color and more compact. Go down about six inches total — four inches for the concrete slab itself, plus two inches for a gravel base that helps with drainage.

Spread a two-inch layer of gravel over the cleared area and compact it with a hand tamper or a plate compactor. Rent a plate compactor from a tool rental shop if the area is larger than about 100 square feet; it does the job much faster than hand tamping. The gravel base prevents water from pooling under the concrete and keeps the slab from settling unevenly as the ground shifts.

Check the ground level as you work. Use a long straight board and a level to see if the ground slopes. A slight slope — about one-eighth inch per foot — helps water run off the patio. If your ground is very uneven, you may need to add more gravel to low spots or remove soil from high spots.

Building and Setting the Forms

Forms are wooden frames that hold the concrete in place while it sets. Use two-by-four lumber for the forms; they should be the same thickness as your finished patio. Nail or screw the boards together at the corners, or use metal corner brackets if you prefer. Drive stakes on the outside of the forms every four feet to keep them from bowing outward when you pour the concrete.

Level the forms carefully — this is where most mistakes happen. If the forms are not level, your patio will slope in the wrong direction or have high and low spots. Use a four-foot level and adjust the stakes and shims underneath the forms until they are level or have the slight slope you want for drainage. Check the level in both directions: along the length of the patio and across the width.

Make sure the forms are tight against the ground with no gaps underneath. Concrete will seep out through gaps, and you will end up with a thin edge that breaks easily. Use soil or sand to fill any gaps between the forms and the ground, and tamp it down firmly.

Mixing and Pouring the Concrete

Calculate how much concrete you need by multiplying the length times the width times the depth in feet, then dividing by 27 to get cubic yards. For example, a 10-by-12-foot patio that is four inches deep needs about 1.5 cubic yards. Most concrete suppliers deliver in increments of half a cubic yard, so order a little extra rather than run short.

You can mix concrete yourself with a rented mixer if the patio is small, but most people order ready-mix concrete delivered by truck. The truck arrives with the concrete already mixed; you direct the chute into your forms and the concrete flows out. Have helpers on hand — concrete sets quickly, and you need people to spread it and work it into place.

Pour the concrete in one continuous operation if possible. Start at one end and work toward the other, filling the forms completely. Use a shovel to work the concrete into corners and under the forms, tapping the forms with a rubber mallet to help the concrete settle and release air bubbles. Do not let concrete sit in the mixer or truck longer than 90 minutes after mixing, or it will begin to set and become difficult to work.

Striking Off and Finishing the Surface

Once the forms are full, use a straight board — a two-by-four works well — to strike off the excess concrete. Two people hold the board flat on top of the forms and drag it across the surface in a sawing motion, moving from one end to the other. This levels the concrete and removes the excess. You may need to do this two or three times as concrete settles.

Let the concrete sit for 30 minutes to an hour until the surface water disappears and the concrete becomes firm enough to walk on without sinking. Then finish the surface. For a non-slip surface, drag a stiff broom across the concrete in straight lines or a circular pattern — this is called broom-finishing. For a smooth surface, use a steel trowel, working in overlapping circular motions. A smooth finish shows footprints and can be slippery when wet, so broom-finishing is more practical for a patio.

If you want decorative edges, you can use an edging tool to round the corners while the concrete is still soft. This prevents the corners from chipping later. Work the edging tool along the perimeter while the concrete is firm but not fully set.

Curing and Protecting the Concrete

Concrete needs moisture to cure properly. For the first seven days, keep the surface damp by misting it with water two or three times a day, or cover it with plastic sheeting to trap moisture. Do not let it dry out completely, or it will crack as it shrinks. If the weather is hot or windy, you may need to mist more often.

Do not walk on the patio for at least three days, and do not put heavy objects on it for seven days. The concrete reaches about 70 percent of its final strength after seven days and full strength after 28 days, but it is safe for normal use after a week.

Remove the forms after two or three days. Loosen them carefully with a pry bar and pull them away from the concrete. If the concrete has set properly, the edges should be firm enough to handle. Fill any gaps left by the forms with concrete patching compound or caulk.

Common Problems and How to Avoid Them

Cracking is the most common problem. Cracks usually happen because the concrete dried too fast, the ground underneath was not compacted enough, or the concrete was not mixed with the right water-to-cement ratio. Prevent cracking by keeping the concrete damp for seven days, compacting the gravel base thoroughly, and ordering concrete from a supplier who can control the mix.

Uneven settling happens when the ground underneath is not level or not compacted. This is why leveling the forms and compacting the gravel base matter so much. If your patio does settle unevenly after it is done, you can grind down high spots or fill low spots with a concrete overlay, but prevention is much easier.

Spalling — where the surface flakes or peels — usually happens in cold climates where freeze-thaw cycles damage the concrete. Use a concrete sealer after the patio cures to protect it from water and salt. Reapply the sealer every two to three years.

Frequently Asked Questions

Do I need a permit to build a patio?

Most areas require a permit for patios over 100 or 200 square feet, though the threshold varies by city. Call your local building department with your address and the size of the patio you plan to build. They can tell you whether you need a permit and what inspections are required.

Can I pour concrete in cold weather?

Concrete sets much more slowly in cold weather and can be damaged by freezing before it cures. Do not pour concrete if the temperature is below 50 degrees Fahrenheit or if freezing temperatures are expected within 24 hours. Spring and fall are the best times to pour a patio.

What is the difference between broom-finishing and troweling?

Broom-finishing creates a textured, non-slip surface by dragging a stiff broom across the wet concrete. Troweling creates a smooth, polished surface using a steel tool. Broom-finishing is more practical for patios because it provides better traction when wet. Smooth surfaces look nicer but can be slippery.

How long before I can seal my patio?

Wait at least 28 days after pouring before sealing the concrete. The concrete needs this time to cure fully and dry out. Sealing too early can trap moisture inside and cause damage. After 28 days, clean the surface and explore a concrete sealer according to the product instructions.

What should I do if my concrete cracks after I pour it?

Small hairline cracks are normal and do not affect the patio's durability. Fill them with concrete caulk if you want to prevent water from getting in. Larger cracks may indicate a problem with the ground underneath or the concrete mix. If cracks are wider than one-quarter inch, consult a concrete contractor about whether the patio needs repair.