Roofs, pavement, sidewalks, etc. are impervious. if you're trying to come up with a Curve Number for the drainage area, you calculate a Weighted Curve Number based on the Impervious areas (typically CN 98) and the Hydrologic Soil Group (A B C or D) of the soils, along with the ground cover type (bare ground, grass, woods, etc) and condition (poor, fair, good, etc.)
See instructions for TR-55 or TR-20 for calculating Weighted Curve Number.