A freshly poured slab of concrete, held in place by a wooden frame.

A Guide on Preparing Ground For Concrete Slab

A stable, sturdy concrete slab is essential to create a level outdoor surface, like a driveway, as well as to safely erect any type of structure, including barns, storage sheds, and carports. If you don’t place concrete properly, you run the risk of damaging the structure you place on it and diminishing the quality and lifespan of your slab. To keep your property pristine and ensure everyone’s safety, here’s what you need to know about prepping the earth for a concrete slab on your property.

Determining the Proper Depth

First and foremost, you have to identify and prepare the site where you plan to put down the slab. You must figure out the dimensions of the slab, including its depth. If you want the slab to be level with the surrounding earth, you’ll have to dig into the ground to compact the soil and lay down a base before putting it down. 

The right depth depends on the height of your slab, local climate, the condition of your soil, and what you plan to use the slab for. Additionally, there may be rules governing not only the depth of the base, depending on where you live, but there could also be laws dictating new construction in your area. Be sure to follow these regulations and obtain any necessary permits or licenses before starting work. 

You’ll likely need to dig down several inches, and potentially up to half a foot. It’s worth talking to a professional in your area who can make recommendations based on your soil and is familiar with any local laws.

Once you know how deep to go, you can begin digging. Make sure you dig out an area with the right dimensions and that the bottom of your hole is smooth. Use extra dirt or soil to fill in any divots. This will make the ground as level as possible so it can support whatever structure you place on top of the slab.

Dampening the Earth

Next, you need to dampen the soil. Loose, dry dirt won’t make a good foundation for a slab or the base that goes under it. You can use a garden hose with a nozzle attached to get an even, light spray across the dirt.

To test things out, gather a handful of dirt and squeeze it into a ball. When it keeps its shape, the dirt is sufficiently damp. If it falls apart in your hand, then you need to add more water.

Of course, the dirt can’t be too wet, either. If water begins to pool above the soil, it’s likely too wet. You’ll have to wait for it to dry before moving forward with your project. 

Laying Down the Base

Before you can put down a concrete slab, you have to lay down a base. A solid base protects the concrete from settling dirt, water runoff, and other natural hazards. Though all outdoor fixtures are subject to natural wear and tear, concrete will break and deteriorate more quickly without a base to support it.

That being said, you don’t need steel, high-quality wood, or anything particularly fancy for your base — just crushed rock or gravel. You’ll need gravel of various sizes to build a reliable foundation and further help with drainage. The exact amount of gravel you need, again, depends on your soil conditions, climate, and how you’ll be using the slab; be sure to use an amount that appropriately matches your needs.

Once you’ve got your gravel, simply lay it down, as evenly as possible, in the hole you’ve dug. You will likely need to work by adding layers, one at a time, until you’ve completed your base. You can always work with a professional to ensure you do this correctly if you aren’t familiar with the process. 

Compacting Each Layer of the Base

Beginning with the soil itself, take the time to tamp down each layer of your base. Compacting the soil reduces the amount of pore space that air and water can take up in the dirt, making it denser and more stable to build on. 

Use either a hand or mechanical tamper to compact the dirt itself. When your footprints are almost invisible or you cannot puncture the soil with a sharp tool, the dirt has been fully compacted.

Then, begin working on your gravel base. Generally, you shouldn’t put down more than about two inches worth of gravel before compacting that layer. After each layer is smooth and fully compacted, you can add the next. Repeat this process until you’ve completed your base.

Building Up the Concrete Formwork

With your foundation finished, you can start getting ready to pour your concrete. Create a secure formwork that indicates where, and to what depth, the concrete needs to be poured. Form boards will help you outline the appropriate area and keep the viscous concrete properly contained while it sets.

Place stakes every few feet along the form boards for additional reinforcement. Secure them to the form boards using screws, not nails, to keep everything properly aligned. If they surpass the top of the form board, remove the tops of the stakes to make them flush with the rest of the formwork. This way, they won’t interfere with the concrete when you pour it and even out the surface.

Don’t forget to coat the formwork with some kind of release agent, such as silicone rubber or even cooking oil. This will make the formwork significantly easier to remove once the concrete has been poured. 

Reinforcing With Rebar

Even a well-constructed formwork needs to be reinforced if it’s going to stand up to concrete. Place rebar in a grid across the entire site. Use rebar ties to bring the pieces together. To ensure it fits properly, you can also use a saw to the rebar to create smaller pieces, or a rebar bender to mold it into the appropriate shape.

You can also use chairs to lift the rebar above the foundation. This will ensure it stays level, but it may not work for you if you need to use wheelbarrows or bring other pieces of equipment to the site. Exercise additional caution if you do lift your rebar; it’s all too easy to trip over.

Pouring Concrete

With all that taken care of, you can then pour concrete. You can do this on your own, as ready-mix concrete can be found at many hardware stores, or hire a professional. Self-sustainability is important, especially if you like to DIY your projects, but don’t hesitate to enlist professional help for this part of the process. 

If you use ready-mix concrete, follow the provided instructions as closely as possible. Pour your concrete into the space you’ve prepared. Use a straight board to screed, or smooth out, the top of the concrete while it’s still wet.

Again, be sure to follow instructions closely while waiting for the concrete to cure. Though concrete usually sets within two days, you’ll probably need to keep it wet or covered with plastic for several weeks until it’s reached full strength. Go ahead and remove the frame when the concrete is set and it’s safe to do so.

Building on the Slab

Once enough time has passed, you can use the slab for its intended purpose. If it’s going to be a driveway or the foundation of your outdoor living area, then your work is done. If you plan to put a structure on the slab — especially a living structure, like a tiny home or a barndominium — then your work is just getting started.

Put just as much care and energy into building your structure as you did into the slab itself. Working on these types of projects can be tiring, but in the end, they’re well worth the effort.