What You Need to Know Before You Start
Pouring a cement patio slab is a project you can do yourself if the slab is small (under 100 square feet) and relatively flat. The basic process is: prepare the ground, build a wooden frame, mix and pour concrete, then finish the surface. For anything larger, sloped, or requiring reinforcement, hire a concrete contractor—the cost of a mistake (cracking, settling, or water pooling) usually exceeds what you save by doing it yourself.
The work takes two to three days if you pour a small slab in one session. You will need to rent or buy a concrete mixer, wheelbarrow, shovels, a level, and finishing tools. The concrete itself hardens in 24 to 48 hours, but you should not walk on it or place weight on it for at least a week.
Key Takeaways
- The ground must be compacted, level, and sloped slightly (about 1/8 inch per foot) so water drains away from your house.
- Build a wooden frame (called formwork) from 2-by-4 lumber, staked firmly in place, because the concrete will push against it as it sets.
- Order ready-mix concrete delivered to your site, or rent a mixer and buy bags—ready-mix is faster and more reliable for slabs over 50 square feet.
- Pour the concrete, spread it level with the frame, then finish the surface with a trowel or broom for traction.
- Keep the concrete damp for three to seven days after pouring so it cures evenly and does not crack.
Preparing the Ground and Building the Frame
Start by marking out the area where your patio will go. Use string and stakes to outline the edges. Dig out any grass, roots, or soft soil. The ground should be compacted—if you have loose soil, rent a plate compactor (a vibrating tool that flattens earth) or tamp it down with a hand tamper. Loose ground will settle unevenly and crack your concrete.
Check the slope with a long level and a measuring tape. Concrete should slope away from your house at about 1/8 inch per foot so rainwater runs off instead of pooling. If your patio is 10 feet from the house, the far edge should be about 1.25 inches lower than the edge nearest the house. Mark this slope on your stakes.
Build the frame from pressure-treated 2-by-4 lumber. Cut the boards to length, lay them on the ground along your marked edges, and drive wooden stakes on the outside of the frame every 3 to 4 feet. Use 3-inch deck screws or nails to attach the boards to the stakes. The frame must be rigid—push on it hard; it should not move. The top of the frame is your concrete surface, so make sure it is level side to side and sloped correctly front to back.
Ordering and Preparing Concrete
For a slab under 50 square feet, you can mix bags of concrete in a rented mixer or by hand. For anything larger, order ready-mix concrete delivered by truck. Ready-mix is worth the cost because it arrives wet and ready to pour, and you do not have to mix dozens of bags.
To calculate how much concrete you need, multiply length × width × depth (in feet), then divide by 27 to get cubic yards. A 10-by-12-foot patio that is 4 inches deep needs 1.48 cubic yards. Order 10 to 15 percent extra to account for spillage and uneven ground. Tell the concrete company your slab thickness (usually 4 inches for a patio) and ask them to deliver it on a day when the ground is dry and your frame is ready.
If you are mixing bags, you will need a concrete mixer (rent one for $40 to $60 per day) and a wheelbarrow. Each 60-pound bag makes about 0.45 cubic feet of concrete. Mix according to the bag instructions, usually one part cement powder to two parts sand to three parts gravel, plus water. The concrete should look like thick oatmeal—wet enough to pour but stiff enough to hold its shape.
Pouring and Spreading the Concrete
Pour the concrete into the frame, starting at one corner and working toward the opposite corner. Do not dump it all in one spot. Spread it roughly with a shovel as you go, pushing it into corners and against the frame. The concrete should fill the frame to the top.
Once the frame is full, use a straight board (a 2-by-4 works well) laid across the top of the frame to screed the concrete level. Two people work best: one on each end of the board, pulling it toward them in a sawing motion while walking backward. This removes excess concrete and leaves the surface flush with the frame. You may need to do this two or three times as concrete settles.
After screeding, the surface will be rough. Let the concrete sit for 20 to 30 minutes so excess water rises to the top. Then use a float (a flat tool with a handle) to smooth the surface. Push the float in overlapping strokes, keeping it nearly flat. If you want a non-slip finish, drag a stiff broom across the surface instead of floating it smooth.
Finishing and Curing
Once the surface is finished, remove the wooden frame. You can do this after 24 hours, or wait until the concrete is fully hard (three to seven days). If you remove it early, fill the gap with concrete or caulk.
Curing is the most important step after pouring. Concrete hardens through a chemical reaction that requires moisture. For the first three to seven days, keep the surface damp by misting it with a hose two or three times a day, or covering it with plastic sheeting. Do not let it dry out completely, and do not let water pool on it. If the weather is hot or windy, mist it more often.
Do not walk on the patio for at least a week, and do not place heavy objects on it for at least four weeks. Concrete reaches about 70 percent of its strength in a week and full strength in 28 days.
Common Mistakes and How to Avoid Them
The most common mistake is not compacting the ground. Loose soil settles unevenly, and your patio will crack or develop low spots where water pools. Spend time tamping the ground before you build the frame.
The second mistake is pouring concrete in cold weather (below 50 degrees Fahrenheit). Cold slows the curing process and can cause cracking. If the temperature drops below freezing within 24 hours of pouring, cover the concrete with blankets or plastic to keep it warm.
The third mistake is not sloping the surface. A flat patio collects water, which freezes in winter and cracks the concrete. Even a small slope (1/8 inch per foot) makes a big difference.
Finally, do not skip the curing step. Concrete that dries too fast cracks. Keep it damp for at least three days, longer if the weather is hot or dry.
When to Call a Professional
Call a concrete contractor if your patio is larger than 200 square feet, if the ground is sloped or uneven, if you need the slab to support a structure (like a shed or hot tub), or if you are not confident in your ability to get the frame level and the slope correct. A contractor can also add reinforcement (rebar or wire mesh) if the ground is poor or the slab will bear heavy weight.
If your patio cracks after you pour it, a contractor can assess whether it is a surface crack (which is cosmetic) or a structural crack (which may need repair). Most hairline cracks are normal and do not affect the slab's durability.
Frequently Asked Questions
How thick should a patio slab be?
A standard patio slab is 4 inches thick. This is thick enough to support foot traffic and resist frost heave (the upward pressure from freezing ground). If your patio will support a hot tub or other heavy structure, use 5 or 6 inches and add reinforcement.
Can I pour concrete in winter?
You can pour concrete in temperatures as low as 50 degrees Fahrenheit, but it cures slowly. Below 50 degrees, do not pour. If the temperature will drop below freezing within 24 hours of pouring, cover the concrete with blankets or plastic sheeting to keep it warm while it sets.
What is the difference between ready-mix and bagged concrete?
Ready-mix is delivered wet in a truck and is faster and more consistent for large slabs. Bagged concrete requires mixing and is better for small projects or repairs. Ready-mix usually costs less per cubic yard but has a delivery fee; bags cost more per yard but have no delivery charge.
How long before I can walk on the patio?
You can walk on the patio after 24 to 48 hours, but it is still soft. Wait at least a week before placing heavy objects on it, and do not use it heavily for four weeks. Concrete reaches full strength at 28 days.
Why is my concrete cracking?
Cracks usually happen because the concrete dried too fast, the ground was not compacted, or the slab was not sloped correctly. Hairline cracks are cosmetic and do not affect durability. Wider cracks may indicate a problem with the ground or the mix. Keep the concrete damp while it cures to prevent drying cracks.