Overview of Impervious Surface Requirements

Impervious Surface means a surface that because of its composition and/or its use impedes the natural infiltration of water. It includes but is not limited to buildings, roofs, solid decks, driveways, parking areas, patios, sidewalks, and compacted gravel areas. It does not include areas that are part of permitted stormwater controls; the open surface water such as swimming pools; a surface of 57 stone, as designated by the American Society for Testing and Materials, laid at least four inches thick over a geotextile fabric; a trail as defined in G.S. 113A-85 that is either unpaved or paved as long as the pavement is porous with a hydraulic conductivity greater than 0.001 centimeters per second (1.41 inches per hour); or landscaping material, including, but not limited to, gravel, mulch, sand, and vegetation, placed on areas that receive pedestrian or bicycle traffic or on portions of driveways and parking areas that will not be compacted by the weight of a vehicle, such as the area between sections of pavement that support the weight of a vehicle.

Portions of Durham County are located within a Watershed Protection Overlay Zoning district, which limits the extent of impervious surface coverage allowed on a lot in order to protect water supply lakes and rivers.

How can I find out if there is a limit on the amount of impervious surface on my lot?

Contact the Development Services Center at 919.560.4137 (choose option 3) or at dscticket@dsc.mojohelpdesk.com. Impervious surface limits on single-family lots are based on when the lot was created. Parcels created before the adoption of the watershed overlay districts (original districts in 1984 and expanded districts in 1994) are exempt from impervious surface restrictions. Planning staff will research the history of your property and advise whether impervious surface limits apply to a specific property.

Why is there a limit on the amount of impervious surface on my lot?

The lakes and rivers Durham uses for its water supply (Falls Lake, Lake Jordan, Eno River, Lake Michie, Little River) are protected by watershed overlay districts that limit the amount of impervious surfaces which affect the water runoff into the streams feeding the lakes and rivers. Roughly three fourths of Durham City and County fall under a watershed overlay district. The closer a property is to a water source the greater the restriction of impervious surfaces.

What are the regulations that govern this?

Watershed regulations, including impervious surface limits, are part of the Unified Development Ordinance (http://www.codepublishing.com/NC/Durham/) and are covered by Section 4.11 (http://www.codepublishing.com/NC/Durham/#!/4.11.html#4.11 ).

Can I increase the impervious surface limit on my lot? If so, how? If your lot is part of a subdivision, you can transfer impervious surface from one lot to another within the same subdivision via a new plat. Since the subdivision was approved for a certain maximum amount of impervious surface, the transfer must be between lots within that subdivision.Another option would be to remove existing impervious surface to account for proposed additional impervious surface. You can contact the Development Services Center for more information.

Are there types of structures/surfaces that I could use that would not be considered impervious?

Slatted wooden surfaces or other slatted material surfaces

Under Durham’s Unified Development Ordinance, slatted wood decks and walkways (such as a handicap ramp) do not count as impervious surfaces. If, however, a deck is enclosed to create a screened porch or sunroom, the roof means it now becomes impervious and must be counted towards any maximum allowance.

Number 57 stone

While the State law allows Number 57 stone for “implementing stormwater programs,” this is only for large projects like a new subdivision or non-residential development like a shopping center. Because this use must be engineered and monitored, Durham does not allow it for single family lots.

Paver stones and other types of permeable pavement

Currently, these would not be allowed to be installed within City limits to avoid exceeding impervious limitations on a parcel. In order to be counted as pervious area, surfaces paved with these materials would have to be installed as a stormwater control measure per the requirements in Chapter C-5, Permeable Pavement of the NC DEQ Stormwater Design Manual. These Permeable Pavement standards have detailed design specifications and soil testing requirements. However, the City does not currently allow stormwater control measures on single-family residential lots. Properties located outside of City limits and within Durham County are allowed to use Permeable Pavement on single family lots, if designed in accordance with the NC DEQ Stormwater Design Manual.

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