An impervious surface is defined as any ground cover that blocks or significantly retards the natural infiltration of rainfall into the underlying soil. Surfaces like concrete, asphalt, and rooftops prevent water from soaking into the ground, leading to increased stormwater runoff. Whether artificial turf is impervious does not have a simple answer because the turf itself is only one part of an engineered drainage system. Permeability is determined by the material’s design, the quality of the installation, and local regulatory definitions.
The Design of Permeable Artificial Turf Systems
Modern artificial turf is manufactured to manage water and is engineered to avoid being an impervious surface. The initial defense against water pooling is the turf backing, typically made from latex or polyurethane. This material is perforated with small drainage holes, allowing water to pass through the synthetic carpet layer into the foundation beneath.
Some advanced systems utilize a fully permeable, “flow-through” backing that allows water to drain uniformly across the entire surface. Perforated backings commonly drain at an impressive rate of 20 to 30 inches of water per hour. The infill material, such as sand or rubber crumb, is spread between the synthetic blades and also aids drainage. It helps keep the fibers upright, ensuring a clear path for water to flow down to the permeable backing and the sub-base below.
Installation Factors Determining Drainage
Even the most permeable artificial turf can become an impervious system if the ground preparation and installation are flawed. The sub-base, the layer directly beneath the turf, is the most crucial component for vertical drainage. This base is typically a compacted layer of porous aggregate material, such as crushed stone, gravel, or decomposed granite, often installed to a depth of three to four inches for residential use.
The sub-base must be compacted for stability but must remain porous enough to allow water to filter rapidly into the native soil. Proper grading is also essential, requiring a slight slope of approximately one to two percent. This incline directs water laterally, preventing ponding and ensuring water moves away from the surface and adjacent structures.
Placing a non-perforated weed barrier or vapor barrier beneath the sub-base can negate the turf’s design. These barriers create a completely impervious layer, trapping water and preventing it from reaching the native soil, which forces runoff. A truly permeable system relies on every component, from the turf backing to the prepared ground, to function as a unified water management solution.
Regulatory Status and Stormwater Management
The categorization of artificial turf as impervious or permeable often depends on local building codes and municipal zoning ordinances, which vary widely. Historically, many jurisdictions treated artificial turf as impervious because it did not absorb water like natural soil. However, some regions are beginning to acknowledge the drainage capabilities of modern turf systems.
Certain states have passed legislation that exempts compliant artificial turf from impervious surface limits, provided it meets specific permeability standards. Even with permeable designs, the construction process—which involves compacting the sub-base—can cause regulators to categorize the system as low-impact impervious. This classification acknowledges that the system allows some infiltration, but not at the rate of undisturbed natural ground.
To manage potential stormwater runoff volume, local requirements often demand integrated drainage solutions alongside the turf installation. These systems might include French drains or catchment areas designed to collect and slowly release water. A properly installed artificial turf system can often handle intense rainfall more effectively than natural grass on compacted clay soil. However, its performance remains dependent on the maintenance of the entire engineered drainage structure.