can you pour concrete directly on dirt

Can You Pour Concrete Directly on Dirt? (Proper Technique)

Homeowners often take on DIY projects, responding to contractors who overcharge for even the simplest construction tasks. Pouring your own concrete is a relatively simple process and a great way of saving money, but you want to do it correctly. Pouring the concrete is a critical step, but can you pour concrete on unprepared ground?

You should not pour concrete directly on the ground before preparing a proper concrete base. The minimum best practice is to thoroughly prepare the ground by leveling and compacting the subgrade. Then add a subbase of gravel for additional load support of the final slab if needed. 

In this article, we’ll explain more about the different layers used in creating a solid concrete slab and other ways you can ensure your concrete cures so that it stays firm and can weather the elements.

A Brief Look at Concrete

Concrete slabs are frequently used as a foundation or base level because they provide a sturdy platform. They make excellent surfaces for carports and barbecue areas and can help connect two areas as a walkway.

They are also a trendy finish for modern DIY projects, but you must treat the area where you will pour the concrete correctly to ensure you get a smooth finish.

In cases where you’re laying a concrete slab to support other building projects, it’s also imperative that you do so correctly for the sake of protecting your other buildings. Unstable concrete can crack and give way when it’s unable to support the weight place upon it.

Concrete was created in 1824, though its early roots in mortar date all the way back to Ancient Egypt (source). This means that concrete has been a critical part of many important buildings throughout history in some form or another.

Concrete is the name given to a mixture of cement and aggregate, making it a composite construction material (source).

It is a common misconception that just about anything can be added to this mixture, such as dirt, based on the rough-looking aggregate that plays such an important role in creating concrete’s structural rigidity.

This misconception is most prevalent when it comes to setting pavement or concrete tiles through a garden. Any additional elements added to the concrete mixture can negatively impact how it cures and the setting’s long-term integrity.

For this reason, loose dirt must be removed or compacted to avoid interference or mixture with the concrete as it would alter the delicate ratio of cement, water, and aggregate — all of which bond together, resulting in concrete.

What to Do Before Pouring Concrete

There are many wrong ways to pour concrete, while construction experts agree that there is only one correct way to pour concrete. Following these steps will result in a well-cured concrete slab.

Clear the Area

When laying concrete outdoors, it is important to be meticulous about clearing the area thoroughly. Be on the lookout for natural debris like leaves and grass, insects, and make sure no animals or children will interfere with the concrete setting (source).

It may be wise to cordon off the area where you’ll be pouring concrete if you feel as though there may be external interference. This may also require setting up an umbrella or shade awning to protect your concrete from the elements while it cures.

As you prepare the ground to become the subgrade for your concrete, be on the lookout for potential obstacles that may interfere with it.

Tree roots, electrical cables, conduits, and water pipes are major concerns that may affect how you lay your concrete, whereas smaller obstacles like ant nests can be dealt with so that the ants relocate and do not tunnel into your concrete while it cures.

Prepare the Subgrade and Subbase

Preparing your subgrade and subbase is easily the most important step as it plays the most significant role in how your concrete sets.

As mentioned earlier, concrete is made by mixing cement with aggregate. Aggregate is essentially rocky material, ranging from fine to coarse, which the cement binds to while it cures. The aggregate consists of two levels: the subgrade and the subbase.

Subgrade Concrete Preparation

This is the name given the prepared ground, normally compacted and improved natural soil or infill (source).

Loose dirt is undesirable as it impacts the composition of the concrete. This is why the foundation level is rolled and compacted to provide a stable base to place the subbase.

Construction experts consider the subgrade layer to be the most important when it comes to determining the load that the concrete slab will be able to withstand — in short, concrete can only be as strong as its subgrade layer.

The subgrade preparation is the first opportunity to grade the area where you will be laying concrete. Leveling your slab is critical, so ensure that, when compacting, you are leveling the ground as well so that the layers to come will also be even.

The subgrade layer can be improved by adding stabilizing components, such as Portland cement, calcium chloride, or lime. These components would be added to the soil and then compacted, aiding to create a dry and stable environment ready to receive the subbase.

Subbase Concrete Preparation

There are various subbases to choose from, where the coarser materials are used in heavy-duty, industrial applications. As the thickness of the subbase layer increases, so does the concrete slab’s load-carrying capacity.

The subbase plays an important role in preventing water from moving upwards through the soil and into the concrete. This would deteriorate the concrete’s integrity in time and provides a solid argument as to why one shouldn’t pour concrete directly onto the dirt.

Image by Bing Dang Nam via Unsplash

Construct Your Concrete Form

Many DIY concrete layers move from the second step straight to pouring concrete. There are still two further steps that are equally important in producing a well-cured concrete slab.

First, a form would be incredibly helpful when pouring, as it helps direct the pouring, funneling the concrete as it lands. It will also keep your edges square as the concrete cures. The form’s edges are also useful when leveling and screeding the top layer of concrete (source).

Forms also fulfill another function, keeping loose dirt from the ground surrounding the slab you’re laying out. As discussed, this is important since it would change your concrete’s composition and affect its durability.

Add a Wire Mesh

A final measure to increase your concrete slab’s structural integrity is to add a wire mesh to it. This is essentially a “skeleton” onto which the concrete clings.

Concrete is able to bind to itself while curing, and, for smaller tasks, one may get by without using a mesh. In most situations, construction experts recommend your concrete is reinforced regardless so as to keep your structure strong.

It is a common misconception that the steel bars used in the mesh are tack-welded together, but construction experts advise against this since it reduces the cross-section of the steel and therefore offers less integrity than when they are unjoined.

For the mesh to be optimally effective, it is important to set the steel rods at a suitable height, dependent on the thickness of concrete you’re looking to lay.

This is another reason why it is important to follow these four steps as opposed to pouring the concrete directly onto dirt since the subgrade preparation provides a flat surface to get the mesh ready for pouring.

Things to Keep in Mind When Pouring Concrete

After following these four steps, you’ll be ready to pour your concrete. There are several subtle details to consider when preparing your concrete, such as mixing the right amount of water to your aggregate. We cover this topic specifically in a separate article, “Can I Pour Dry Concrete?

Safety gear is also highly recommended when mixing and pouring concrete to protect yourself from the splashback.

Spreading the concrete evenly is important right from the pouring stage, which is why it is recommended that you enlist help when pouring. Using shovels, rakes, and come-along rakes to assist with spreading the concrete evenly across your form.

Screeding will help you get an even finish at the top layer of concrete and will allow you to put an adequate slope on your concrete to direct rainfall. Floating the surface is also a good idea to further compact the concrete and help it set.

This picture shows the scene of a construction site. In the foreground a hand wearing work gloves is carrying a bucket. In the background a cement truck is visible. Cement is flowing from the truck into the bucket.

Image by Life of Pix via Pexels

Final Thoughts

Laying concrete requires more steps than simply pouring concrete onto the dirt. One needs to prepare the ground surface, making a subgrade and subbase, add the correct amount of aggregate, construct a form to mold the concrete into the desired shape, and reinforce the slab with a wire mesh.

The composition of your concrete is important, as well as following these steps accurately, as they all play a part in the overall strength of the concrete once it sets.

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