Impervious Surfaces

An impervious surface is a hard surface that does not let water soak into the ground or greatly reduces the amount of water that soaks into the ground. Examples include:
  • roofs
  • solid decks
  • patios
  • sidewalks
  • driveways
  • parking areas
  • roads
  • Compacted gravel
Impervious surfaces are the single most important factor affecting the amount of water flowing off a property, how quickly that water will flow, and the amount of pollution that will be picked up and carried with that water.

Law Revision
A recent change to state law redefines “built-upon area” so that it does not include gravel, the water area of a swimming pool, or wooden slatted decks. Note: Scientists generally agree that compacted gravel is an impervious surface. This means water does not travel down through the gravel material into the ground.

The revised law sets a new state minimum standard for built-upon area. As in most parts of environmental regulation, cities and towns may pass local laws with higher standards than the state minimum. The section of the state laws changed by this provision clearly allows cities and towns to exceed state minimum standards to implement stormwater programs (G.S. §143-214.7(d)).

Calculating Stormwater Measures
Cities and towns generally study whether a surface is impervious when calculating needed stormwater measures for new development and redevelopment projects. Classifying gravel as impervious generally results in additional stormwater control devices for a property. This classification is a higher standard than the new state minimum created by this law. A higher standard such as this is allowed for a local program. This means that any existing municipal ordinances that label gravel as impervious do not conflict with the new state law. Therefore, they do not need to be changed.

Another context in which cities and towns consider whether gravel is impervious is when a city or town calculates the stormwater fee for a property. The calculation is generally based on the amount of impervious surface on the property. The new law applies only to the implementation of state stormwater quality programs authorized by G.S. § 143-214.7. Therefore, it does not affect local stormwater programs. Local stormwater programs are authorized under a different statutory section (G.S. § 160A-314). This means that a city or town with a stormwater fee that counts gravel as impervious surface does not need to change that practice.