Is Gravel Necessary Under Concrete?

Posted on: July 23, 2022

When pouring a concrete slab on top of gravel, you need to be careful to ensure that the bottom layer is smooth and free of jagged edges. Otherwise, repeated freeze/thaw cycles could rip the concrete. Instead, use a smooth layer of gravel that is impermeable to water and resistant to freeze-thaw damage. This way, you can rest easy knowing that the slab you’re pouring on top of gravel will last for many years.


A basic explanation of why sand is needed under concrete is that it increases the volume of concrete, thereby reducing the presence of air. This, in turn, makes the finished product more durable and stronger. Crushed sand, also known as manufactured sand, is a more sustainable option than natural sand. Crushed sand is produced in quarries, rather than riverbeds. This means that it is less susceptible to erosion and is therefore a more durable option than natural sand.

The angular granules of concrete sand differ from those of white sand, which has a uniform microscopic surface. It is therefore suitable for concrete mixing, but it can also be used in bedding pipes. Crushed sand is produced by crushing high-quality industrial quartz. This sand is often coarse and contains small particles of shells, decomposed rock, corals, and minerals.


A slab of concrete that sits on a layer of gravel provides a solid base. Gravel provides the required drainage and helps to prevent water from pooling under the slab. When slabs are poured directly on the ground, they may be uneven and may have holes or other defects in their base. If the slab is reinforced with mesh or fiberglass, the underlying concrete may also crack or fracture. Gravel is the preferred material for backfilling beneath concrete.

The best base material for your concrete slab is crushed stone or a gravel mix. Both materials are equally suitable for this application. Besides, they can help improve drainage. Once your concrete is ready, you can place the gravel layer. A layer of gravel will keep the slab level and prevent cracks from forming. You can use whichever gravel material you want, but be sure to prepare your subgrade thoroughly. Crushed stone is the most common type of gravel.

Form boards

If you’re laying concrete on a patio, the first step is to place form boards and gravel under the slab. A piece of lumber long enough to span the width of the concrete slab should be laid across one stake at each corner and nailed into place. Place 2×4 kickers every two feet and nail them in place as well. This will prevent the forms from moving while the concrete cures. You may also need to place stakes every two feet to prevent the forms from bending out of alignment.

When installing a concrete form, make sure that the boards are flush with the top of the forms and slightly below. If they’re sticking out, they’ll interfere with the smooth surface of the finished concrete. You can use plywood to secure long sides of the boards by nailing them to the form using duplex nails. Once the boards are in place, you can place the gravel under them to check whether they’re level.

Vapor barrier

Using a vapor barrier under concrete is a great way to keep moisture out of your building. While moisture is natural, it can contribute to the growth of mold and mildew. These contaminants are harmful to your health and can lead to allergies. By placing a vapor barrier under your concrete foundation, you can avoid these problems and create a drier building. If you want to add a vapor barrier to your concrete foundation, you should hire a professional home inspector.

A vapor barrier should meet ASTM E-1745 standards. It should have a water vapor permeance of less than 0.3 perms. A vapor barrier with a 10-mil thickness is good enough for residential construction but 15-mil is best for construction projects involving heavy equipment and machinery. In addition, a vapor barrier will allow a concrete slab to dry faster, which is a good thing for radon, the second leading cause of lung cancer in the U.S.

Sizing of gravel

When sizing gravel for your project, make sure to consider the size of the area you intend to cover with it. While gravel sizes tend to be consistent from one area to another, you should never select a gravel type based on the numerical code on the bag. Rather, read the bag description and talk to a sales representative about your project requirements. If you’re not familiar with gravel sizing specifications, talk to a contractor or distributor for assistance.

Although a slab of concrete can be placed over dirt, it is much more stable to place a layer of gravel underneath it. Because gravel is compatible with the subgrade, it will hold the slab in place and support it. When sizing gravel for your project, make sure that it matches the grading requirements of the area. There are several factors to consider, including climate, soil type, and drainage system. To prevent concrete slabs from sinking, make sure the gravel is compatible with the subgrade.