Sub-base is the term we use to describe the area that lies directly underneath the artificial grass. It is most commonly made up of a sand/gravel mixture, although in some instances it consists of concrete, asphalt or even a wood base.
A common question is “Why do I need to install a sub-base? Why can’t I just install my artificial grass on top of dirt?” The answer is that native soil is expansive (particularly soil with clay in it) and when it gets wet, the soil expands; when it dries, the soil contracts. This expansion and contraction can result in wrinkles, dimples and/or soft spots and can create a very uneven surface. A sub-base is used to replace the expansive native soil with a non-expansive surface which will help to ensure an artificial grass installation will continue to look good for the duration of its 15- to 20-year useful life.
The most common types of sub-base are a crushed miscellaneous base (CMB), a Class II road base, or a decomposed granite (DG). CMB typically comes from recycled concrete and/or recycled asphalt and consists of a sand/gravel mixture of 3/4” to fine. Class II road base is similar but is subject to more stringent testing which makes it more expensive, and in most artificial grass applications, it is not necessary. Decomposed granite is granite rock that has weathered to the point that it readily fractures into smaller pieces of weak rock and/or into mixtures of gravel, sand, and silt-size particles of clay.
CMB compacts better and is less expensive than DG which is why it is the more common choice for landscaping applications.
DG is more permeable and can be used to achieve a smoother finish than CMB, but it is substantially more expensive. This makes it more suitable for putting green applications.
Use the guide below to determine how much CMB or DG to use for your application:
1. When sold by the ton:
a. 2” of base – 1 ton of CMB for 100 sq. ft.
b. 3” of base – 1 ton of CMB for 70 sq. ft.
c. 4” of base – 1 ton of CMB for 50 sq. ft.
2. When sold by the yard:
a. 2” of base – 1 yard of DG for 150 sq. ft.
b. 3” of base – 1 yard of DG for 100 sq. ft.
c. 4” of base – 1 yard of DG for 75 sq. ft.