An impervious surface limits water from soaking into the ground, and its area is an input to drainage analysis and some stormwater requirements.
Definition
For a civil engineering firm, impervious surface typically includes mapped roofs, pavement, concrete, and other areas treated as limiting infiltration under the applicable method. Work may include reviewing site plans, classifying surfaces, measuring areas, and documenting assumptions for drainage calculations or permitting. The result depends on the project boundary, surface condition, mapping scale, and governing criteria. Firms may classify areas differently or use different measurement methods, so the basis should be clear in the deliverable. An impervious area estimate is not a direct measurement of runoff volume, and it does not by itself determine a drainage design. Runoff also depends on factors such as rainfall, soils, slopes, and drainage connections. Owners can use the estimate to understand design scope and coordinate site layout. Stormwater approaches that manage runoff near its source are described by EPA.
How owners use it
- Check whether the site plan takeoff matches the areas used in drainage calculations.
- Track revisions when paving, roof footprints, or other site features change during design.
- Example: 18,000 square feet of roof + 12,000 square feet of pavement = 30,000 square feet of mapped impervious area.
- Define the measurement method and project boundary in the scope or technical memorandum.
