A proper foundation beneath artificial grass is necessary primarily to manage water and ensure the long-term integrity of the installation. This engineered sub-base is a system of permeable materials designed to move water away from the surface quickly, preventing standing water, puddling, and odors. Without a well-designed drainage layer, moisture can become trapped, leading to problems that shorten the turf’s lifespan. The layers placed under the synthetic surface must provide both stability for heavy foot traffic and a highly porous medium for efficient water run-off. This multi-layered approach ensures that water filters through the turf’s drainage holes and then through the base materials below.
Site Preparation and Initial Grading
The first step involves excavating the existing surface to a depth typically between three and four inches to make room for the sub-base materials. Removing the native soil and organic matter is necessary because these materials hold moisture and can decompose, causing the turf surface to settle unevenly. Once the area is cleared, the ground must be graded to establish a specific slope, which is the most important element for directing water flow. A minimum grade of one to two percent is required, ensuring that water is channeled away from structures and towards designated discharge points.
After the initial grading, a geotextile weed barrier fabric is often laid directly over the native soil. This fabric functions as a separation layer, preventing the aggregate base materials from mixing with the underlying soil while still allowing water to pass freely. This separation maintains the integrity and permeability of the base layers placed above it.
The Primary Drainage Base
The bulk of the drainage system is the primary sub-base layer, which typically consists of two to three inches of crushed aggregate material. This layer must be constructed using angular materials, such as crushed granite, limestone, or Class II Road Base. Angular aggregate is favored over rounded river rock because the sharp edges interlock when compacted, creating a firm, stable foundation that resists shifting. This interlocking action is essential for stability but still leaves sufficient void space between the particles for water to rapidly percolate downward.
The stone size is usually a three-quarter inch minus material, meaning it includes pieces up to three-quarters of an inch and smaller fines. Compacting this layer thoroughly is necessary to achieve maximum stability and prevent future settling, which is usually done in multiple passes with a plate compactor. For deeper installations or areas with poor native soil drainage, the aggregate may be placed in successive layers, known as lifts, with each lift being compacted before the next is added. This primary layer provides the structural support and the necessary volume to manage significant water run-off.
The Final Leveling Layer
A final, finer layer is placed atop the primary drainage base to create the surface upon which the artificial turf will rest. This leveling layer must be fine enough to provide a smooth finish, yet still maintain high permeability so it does not impede water flow to the base below. Materials commonly used are decomposed granite (DG), crusher dust, or fine crushed fines, typically around one-quarter inch minus in size. This material is spread to a depth of about one-half to one inch.
This fine layer covers the rougher stones of the primary base and provides a perfectly flat plane. A smooth surface is necessary to ensure the artificial turf lies flat without bumps, wrinkles, or visible imperfections. This layer is compacted and then carefully screeded, a process of using a straight edge to shave the material down to a precise, final grade. This meticulous leveling guarantees the turf will feel firm and even underfoot.
Perimeter Containment and Run-Off Management
The entire sub-base system needs structural components around the edges to keep the aggregate layers contained and prevent them from migrating laterally. Perimeter containment is commonly achieved using bender boards, plastic edging, or concrete curbing, which physically hold the crushed stone and fines in place. Without these restraints, the sub-base could wash out or shift, especially during heavy rainfall events.
The final step involves managing how water exits the system once it has passed through all the base layers. In areas with highly permeable native soil, the water is simply allowed to soak into the ground beneath the sub-base and geotextile fabric. If the underlying soil has poor drainage, such as dense clay, the system must be designed to direct the water to external collection points. This can involve routing the water towards a low point or integrating external drainage solutions like French drains or catch basins to carry the excess water away.