You’ve decided to get yourself a concrete patio to expand the relaxing area of your house. But a part of your mind is wondering why you should bother with a gravel base. It’s not like you’re building a five-story building, right? It’s just an inch or two.
So, does a concrete patio need a gravel base? Or can you get away with pouring your concrete directly on the soil?
It’s highly recommended to have a gravel base under your concrete patio for optimum support and stability. Skipping the gravel base will still allow for the successful construction of the patio, but the longevity will be compromised over time.
In this post, we’ll explain why and how that happens, and why it’s better to have a gravel base.
Here’s how a gravel base can make your patio a lot more durable:
If you were asked to pour some liquid over uneven ground, do you think it would be easy to attain a flat-looking finish of the liquid?
The same concept applies to pouring concrete directly on the ground. It’ll be much harder to obtain a flat concrete level when the base underneath is uneven.
A gravel base solves that issue by providing a flat surface that facilitates pouring and installing the concrete patio.
Concrete is a porous material and will absorb most of the fluids that fall over it. Even if you take complete care of your concrete patio, some weather elements like rain and wind will wet the patio at one point.
Without dissipating gravel underneath your patio, the water will retain and gradually erode the concrete from underneath.
Have you ever wondered why the leaning tower of Pisa is leaning? The leaning wasn’t intended during construction. It happened because of the uneven hardness of the foundation under the tower.
A gravel base acts as a stable foundation for your concrete patio. It provides enough stability to the concrete and prevents it from resting on a potentially deformed ground.
Did you know that most structures expand and contract depending on the temperature? This is known as thermal expansion and contraction.
Let’s take a metal rod, for example. If you leave it in the sun, it’ll gain a few inches depending on its size and the heat. The same concept applies to the ground beneath us.
In areas with weather extremes, the constant shift in the size of the base under your patio could cause damage to the concrete.
A gravel base mediates that seasonal ground movement and prevents it from damaging the concrete structure.
Gravel sizes and types can vary, but it’s best to use crushed rocks of sizes ranging between ¾–1 inch. This will allow for the best compatibility while providing proper drainage to prevent water damage.
The appropriate size isn’t the only thing you should consider here.
The edges of the gravel should be smoothened out to avoid causing any cracks in the concrete. Crushed limestone is a good material that ticks all of these boxes.
While there’s no absolute thickness you should use under every concrete patio, 4–6 inches seems to be the most common thickness.
Various factors can affect that thickness. For example, it may increase if you’re planning to pour a larger amount of concrete than usual. You’d then require a stronger base and hence, more thickness.
However, the thickness shouldn’t be too much to prevent compromising the compactability of the concrete.
While it’s not recommended a construct a concrete patio without a gravel base, it’s not impossible.
You can directly pour the concrete onto the soil. However, the integrity of the patio may be compromised depending on the nature of the soil.
Compacted, clay soil often provides better stability than softer and sandier soils. Neither can stabilize your patio as a gravel base would, but clay soil is the next best thing.
Still, regardless of the soil type, you should expect water pooling under your concrete slab. You don’t have to worry about this if you have gravel, but if you don’t, you’ll need a vapor barrier.
Even with that barrier, the longevity of your concrete patio will be markedly less than if you had used gravel.
The best base should contain a mix of coarse and fine crushed rock aggregate.
The difference in size and texture allows the gravel base to be compactable while still maintaining the porosity required for water drainage.
Do your best to avoid having sand under concrete.
Whether it’s sandy soil or just a sandy sub-base, sand will compromise the stability of your concrete structure because of how unevenly soft it can be.
A concrete patio will often need a gravel base, especially if it’s going to be used heavily, like by vehicles, for example.
The ground alone won’t be enough to sustain the durability of a concrete patio for extended periods.
A compacted gravel of coarse and fine stone will support your gravel, protect it against moisture damage, and prevent any damage resulting from thermal contraction and expansion.
Ben has a bachelor’s degree in construction engineering. When not constructing or remodeling X-Ray Rooms, Cardiovascular Labs, and Pharmacies, you can find him at home with wife and two daughters. Outside of family, He loves grilling and barbequing on his Big Green Egg and Blackstone Griddle, as well as working on projects around the house.
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