What You Need to Do Before You Pour
A concrete patio slab starts with the ground, not the concrete. You need to mark out the area, remove grass and topsoil, compact the soil, and build a frame. The concrete itself comes last. Most homeowners spend two to three days on prep work and one day pouring, though the concrete takes a week to cure enough to walk on.
The first step is to decide on size and location. Mark the corners with stakes and string. Check that the ground slopes slightly — about one-eighth inch per foot — so water runs away from your house. Use a level to confirm the slope as you work.
Remove all grass, weeds, and topsoil from the marked area. Dig down 4 to 6 inches. The depth depends on your climate: colder regions need deeper slabs to sit below the frost line, which varies by location. Check your local building code or ask a concrete contractor what depth is standard in your area.
Key Takeaways
- Prepare the ground by removing grass and topsoil, then compact the soil firmly so the concrete does not settle unevenly.
- Build a wooden frame (called a form) around the perimeter using 2-by-4 lumber, and brace it so it stays square and level while you pour.
- Lay a 4-inch gravel base inside the frame, compact it, and add rebar or wire mesh for reinforcement before pouring concrete.
- Mix concrete to a thick peanut butter consistency, pour it into the frame, spread it level, and finish the surface with a broom or trowel.
- Keep the concrete moist for seven days after pouring so it cures properly and does not crack.
Building the Frame and Base
Once the soil is dug out, compact it firmly with a hand tamper or plate compactor. Loose soil will shift under the weight of the concrete and cause the slab to crack or settle unevenly. Tamp in layers — every 2 inches of soil — until the ground is hard.
Build a wooden frame around the perimeter using 2-by-4 lumber. Nail the boards together at the corners or use corner brackets. Drive stakes on the outside of the frame every 4 feet and nail them to the boards so the frame cannot move when you pour. Check that the frame is level and square using a level and a measuring tape (measure corner to corner — both diagonals should be equal).
Spread 4 inches of gravel inside the frame and compact it. The gravel allows water to drain under the slab and prevents frost heave in cold climates. Lay rebar (steel reinforcement bars) or wire mesh on top of the gravel. Rebar is stronger and better for larger slabs; wire mesh works for smaller patios. Tie rebar pieces together with wire so they do not shift when you pour.
Ordering and Mixing Concrete
For a patio, you have two choices: buy bags of dry concrete mix and mix it yourself, or order ready-mix concrete delivered by truck. Bags work for small patios (under 50 square feet) and cost less upfront. Ready-mix is faster for larger slabs and produces more consistent results, but requires a truck to access your yard.
To calculate how much concrete you need, multiply length by width by depth (in feet), then divide by 27 to get cubic yards. A 10-by-12-foot patio that is 4 inches deep needs about 1.5 cubic yards. Add 10 percent extra for waste and uneven ground.
If you mix bags, follow the instructions on the bag. Add water slowly — the mix should look like thick peanut butter, not soup. Overly wet concrete is weak and cracks easily. Mix in a wheelbarrow or concrete mixer. You will need many bags for a large patio, so mixing by hand is tiring; rent a mixer if you are pouring more than 500 square feet.
Pouring and Spreading the Concrete
Pour the concrete into the frame and spread it roughly with a shovel. Work in sections and fill the frame completely. Tap the sides of the frame with a hammer to release air bubbles trapped in the concrete — these weaken the slab.
Use a straight board (called a screed board) to level the concrete flush with the top of the frame. Lay the board across the frame and drag it toward you in a sawing motion, moving from one end to the other. Fill any low spots and screed again. The surface should be level and smooth.
Once the concrete is level, you can finish the surface. A broom finish (dragging a broom across wet concrete) creates texture and prevents slipping. A trowel finish (smoothing with a metal trowel) creates a sleek look but is slippery when wet. Most patios use a broom finish for safety.
Curing and Finishing
Concrete hardens in 24 to 48 hours but takes seven days to reach full strength. Keep the surface moist during this time by misting it with water twice a day. Dry concrete cracks because it shrinks as it cures. Cover the slab with plastic sheeting or burlap to slow evaporation, especially in hot or windy weather.
Do not walk on the patio for at least three days. Do not place heavy objects on it for seven days. After seven days, remove the wooden frame by prying it away with a pry bar. Fill the gap with soil or sand.
If the edges are rough, smooth them with a concrete edger or grinder. Seal the concrete after 28 days with a concrete sealer to protect it from stains and weather. Reapply sealer every two to three years.
Common Mistakes and How to Avoid Them
The most common mistake is not compacting the soil properly. Loose soil shifts under the weight of the concrete, causing the slab to crack or settle unevenly. Spend time tamping the soil in layers until it is hard.
The second mistake is mixing concrete too wet. Wet concrete is easier to spread but is weak and cracks easily. Mix to a thick peanut butter consistency and add water only if the mix is too dry to work with.
The third mistake is not keeping the concrete moist while it cures. Concrete that dries too fast shrinks and cracks. Mist it twice a day for seven days, even if it rains.
The fourth mistake is pouring concrete in cold weather. Concrete does not cure properly below 50 degrees Fahrenheit. If temperatures drop below 50 degrees at night, wait for warmer weather or cover the slab with insulation blankets.
Tools and Materials You Will Need
| Item | Purpose |
|---|---|
| Shovel and wheelbarrow | Dig out soil and move concrete |
| Hand tamper or plate compactor | Compact soil and gravel |
| 2-by-4 lumber and stakes | Build the frame |
| Level and measuring tape | Check that frame is level and square |
| Rebar or wire mesh | Reinforce the concrete |
| Concrete (bags or ready-mix) | The patio surface |
| Concrete mixer (if using bags) | Mix concrete evenly |
| Screed board | Level the concrete |
| Broom or trowel | Finish the surface |
| Plastic sheeting or burlap | Keep concrete moist while curing |
Frequently Asked Questions
How thick should a concrete patio be?
Most patios are 4 inches thick. This is thick enough to support foot traffic and light furniture. In cold climates where the ground freezes, the slab must sit below the frost line, which varies by region — check your local building code or ask a concrete contractor what depth is required in your area.
Can I pour concrete on top of an old patio?
Not directly. Old concrete must be broken up and removed, or the new concrete will crack as the old slab shifts underneath. If the old patio is in good condition and you want to raise the height, you can pour new concrete on top if you first clean the surface and explore a bonding agent, but this is not recommended for patios that will see heavy use.
What if it rains after I pour the concrete?
Light rain in the first 24 hours is usually not a problem — the concrete is still setting. Heavy rain can wash the surface and create pits. If heavy rain is forecast, cover the slab with plastic sheeting. After 24 hours, rain will not damage the concrete, though you should still keep it moist for the full seven-day cure.
Do I need a permit to pour a concrete patio?
Requirements vary by city and county. Some places require a permit for any structure, while others do not require one for a straightforward patio slab. Check with your local building department before you start. A permit usually costs between $50 and $200 and takes a few days to process.
How long does concrete take to cure?
Concrete is hard enough to walk on after three days and reaches full strength after 28 days. For a patio, you can place light furniture after seven days. Heavy objects and vehicles should wait until 28 days have passed. Keep the surface moist for the first seven days to prevent cracking.