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How to Lay Concrete Slabs?

Concrete slabs have become so popular for different applications because they’re affordable and accessible to build. Plus, they’re suitable for several projects around the home, whether you need a sturdy, level, dry base for your patio or gazebo.

Although hiring experts to handle this job is always a good idea, many people decide to do it themselves. If this is what you have in mind, you should learn how to lay a concrete slab properly.

Although the process isn’t complex, you need physical effort and enough speed to complete this project and achieve excellent results.

Are you ready to learn how to lay concrete slabs? Read on to find all the information you need!

Key Considerations

Before laying a concrete slab, it’s essential to consider the following key points to make the process easier, avoid costly mistakes, and achieve the best possible results.

  • If you want to build a concrete slab with the following dimensions: 20 feet long, 12 feet wide, and 4 inches thick, you need at least three cubic yards of concrete.
  • If you want to save, you can use a ready-mix product and mix the concrete yourself, but it isn’t the best option for large projects.
  • A wheelbarrow may be ideal for mixing concrete for small slabs and countertops, but it isn’t easy to mix by hand. That means it’ll also be challenging to maintain a constant flow of wet concrete to prevent the poured material from hardening. Therefore, using a portable concrete mixer for larger projects is better. You can also order and deliver volumetric or metered concrete to your house.
  • Since the tools you may need for this project are expensive, you can consider renting them.
  • Extreme temperatures can affect concrete, as freezing conditions can cause the mixture to freeze and expand while you’re working on the project.
  • Also, it would be best to avoid temperatures above 90°F, as this can cause the concrete to set too quickly because water will evaporate, weakening the material.
  • If you want to choose the perfect time to pour the concrete slab, consider doing it in the early morning or when the temperature is 60°F.

Safety Considerations

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Since wet concrete is caustic, it can cause cement burns and blisters, hardened or dead skin, and scars. Therefore, if you use this material, please don’t forget to wear waterproof gloves, long sleeves, and tall rubber boots.

You should also follow some safety recommendations if you use a dry concrete mix because it’s breathing irritating. Ideally, when mixing concrete, make sure you’re wearing breathing protection.

It’s also important to make sure you’re digging safely. Before starting this process, call the 811 Call Before You Dig line, and utility operators in your area will mark buried utility lines on the project site.

What You’ll Need 

  • Three cubic yards of wet concrete
  • Steel rebar mesh (6-inch) or rebar rod
  • Rebar ties
  • Concrete release agent
  • Gravel for the base (if needed)
  • Twine
  • 4 two-by-fours, each 12-foot
  • 4 two-by-fours, each 8-foot
  • Screws, 3-inch
  • 25 Dobies, 2-inch high or two 1/4-inch high rebars
  • Hammer
  • Tape measure
  • Groover tool attachable to bull float handle
  • Pull tie wire twister (only for rebar)
  • Shovel
  • Bull float
  • Bubble level
  • Garden hose
  • Circular saw, or miter saw
  • Edging tool
  • Screed tool
  • Cordless drill
  • Tamper

How Much Concrete Do You Need?

Before laying your own concrete slab, ensure you have the materials on hand. While the list above can help you know exactly what you need, the following information is essential for calculating quantities. Check it out!

Pre-Mix Concrete

Do you plan to order pre-mix concrete? Since it comes in bags, you can ask the supplier to deliver them on pallets near the site where you’ll pour concrete. Additionally, it would be best if you consider these:

  • You need 134 80-pound bags of ready-mix concrete.
  • If they’re 60-pound bags, you need 178 units.

Loose Ingredients

As a rule of thumb, for concrete slabs with 2,500 pounds of compression strength that will be used for general purposes, you need a 1:2:4 concrete mix ratio. You’ll use cement, sand, and rock.

This is an estimate: For one cubic foot of concrete, you need 21 pounds of cement. Multiply that amount by 2, and you’ll have the amount of sand you need, which would be 42 pounds. Finally, multiply the amount of cement four times to calculate the amount of rock you’ll use, which would be 84. 

How Are Slab Foundations Built?

As mentioned, slab foundations are widely popular because they’re easy to build. You can build the most straightforward type by simply pouring concrete directly onto prepared soil. It’ll feature a large concrete block with no space under the floor.

However, some slab foundations have additional components, such as insulating foam and supporting concrete feet, which make the construction process slightly different.

Although constructing a concrete slab requires site preparation, it only needs minimal excavation. After completing these first two phases, you must install formwork to define and shape the slab.

The next step is to place reinforcing bars and other structures and compact the sand layer or gravel immediately. Once this step is completed, water, plumbing, and power lines should be laid down. You should pour, finish, and cure concrete when these preparations are finally set.

Next, you must remove the frameworks once the concrete has been hardened. This process is so simple, so costs are usually low.

Contractor pouring a concrete mix to reinforced foundation

Steps to Lay a Concrete Slab

Below is a detailed explanation of the process to teach you how to lay concrete slabs step by step.

Step 1: Set the Location

Have you already defined the project site? Measure it out using the tape measure and a hammer to stake it out, driving a stake at each corner.

It’s important to drive the stakes so deep that you can define the slab area by tightly drawing the twine around all the stakes.

Step 2: Start to Dig Down

The next step depends on your project site. You don’t need gravel fill to make a base if it’s solid and has well-draining sand or gravel mixed in. In that case, you can start digging down right away, 4 inches.

However, things are slightly different if you have to add a gravel base. In this scenario, dig down 8 inches, as you’ll need 4 inches of gravel and 4 inches of concrete slab.

Is the project site covered with grass? Remove the turf from the slab area. This process will take off about 3 or 4 inches.

Step 3: Clear and Level the Project Site

Now, it’s time to remove debris, including rocks and sticks. Use your hands to do this. Ideally, you should rake down the site. After this, lay straight a two-by-four of 8 feet with a bubble level on top. Remember that you should do this at multiple angles around the area to ensure it’s level.

Step 4: Tamp the Base and Pour the Gravel

With a hand tamper, tamp down the soil. Don’t forget to check for levels throughout the process.

If the site needs gravel fill for an extra base, this is the perfect time to pour it. After that, tamp it down well, leveling it off.

Step 5: Drive the Stakes

Drive two stakes into each corner, keeping one on each side about every 2-½ feet along the area and always following the string. Don’t drive all stakes! Save two out. Once they’re set, remove the string.

Step 7: Build the Form

Attach a two-by-four of 12 feet to each short side of the stakes using 3-inch screws. On each long side, you also need a two-by-four of 8 feet.

After that, raise the form sides ½ inches to create a 4-inch slab. Don’t forget to check for level on all sides.

Add the remaining two stakes at the junction between the 12-foot and 8-foot two-by-fours you attached on the long sides.

Step 8: Coat the Concrete Form

Coat the inside of the form to make removal easier. You can use vegetable, mineral, or motor oil.

Step 9: Grade Again

Make sure the gravel is packed firm and level by grading it again. Also, lay a screed board across the tops of the forms and measure downward to check the depth, which should be 4 inches.

Step 10: Set the Rebar

This process depends on the option you choose and is as follows:

  • Rebar mesh: Cut 6-inch rebar mesh and lay it across the site, exposing only a 3-inch band on the outside. Use rebar ties to tie the pieces together.
  • Rebar rod: Create a grid of rods with 2 feet of space between them. With the tying tool, add ties. There should be one at each point where rebar crosses rebar. Raise the grid on 20-25 dobie blocks, which should be evenly spaced, and tie the grid onto them with their ties.

These ten steps must be completed before the concrete truck arrives at the project site if you aren’t using a ready-mix product but concrete delivery.

Contractor using a finishing tool to flattened a wet concrete surface

The Next Steps

After completing the first steps, it’s time to pour concrete straight into the area you just prepared. These are the steps you must complete:

  • Wet the gravel base with a garden hose while you wait for the concrete delivery truck or before you prepare the mix
  • Pour the mixed concrete evenly through the form, working quickly
  • Manually raise the rebar mesh to the center if you left it unelevated
  • Cut the stakes flush to the top of the concrete by screeding it with a screed tool or a scrap 8-foot-long piece of two-by-four
  • Move the screed tool back and forth to level the concrete surface by removing excess
  • Run the bull float over the concrete surface three or four times to remove the marks left by the screed tool
  • Rerun the bull float to make large pieces of aggregate go down into the slab
  • Run the hand edger in long passes along the slab once the concrete has firmed up a bit
  • With the groover tool attached to the bull float handle, rest the grooving tool at the far edge of the form and pull it towards you to create one or two contraction joints from one long side to the other
  • Drag a push broom backward over the concrete surface (it should be wet) to add texture if desired
  • Keep the concrete misted with water throughout the first day
  • Let the concrete cure for about a month, although you can walk on the slab after the first 48 hours

Final Thoughts: When to Call an Expert

Pouring a concrete slab requires expertise, suitable materials and tools, and patience. Essentially, it’s a job you could try to do, but it’s always better to let professionals handle house foundations, footers, and other concrete projects used for structural purposes.

Experts can ensure the concrete slab is well-constructed and know all the tricks and tips to achieve the best results.

Frequently Asked Questions

Ideally, you should avoid pouring concrete directly onto the dirt. Instead, you can use a layer of gravel underneath the slab. Otherwise, this porous material will absorb water, eroding the pad’s bottom.

Since you’ll need at least 4 inches of compacted stone to build your concrete slab, you need to dig out a space of at least 8-10 inches deep. In this case, 4 inches will be for the gravel base and 4 inches for concrete. The other 2 inches will allow for mortar and brick. However, this may vary depending on the type and thickness of slab you choose.

It isn’t necessary in all cases, especially for slabs that are built for general use and have a thickness of 4 inches. However, these requirements may vary based on several factors, including the weight it’ll support. Therefore, if possible, you should assess your needs and speak with an expert to determine if your slab needs rebar.

Reinforcement, including rebar or welded wire mesh, isn’t mandatory to pour a concrete slab on a residential property. However, it’s essential to determine if it’ll support more weight than usual. In those cases, the slab might need reinforcement to withstand it.