Storm drainage systems shall be permitted to be sized using the roof area served by each of the drainage system. Maximum allowable roof areas with various rainfall rates are listed in Table 110 1.8 and Table 1101.12, along with the required pipe sizes. Using this method, it shall be permitted to interpolate between two listed rainfall rate columns (inches per hour) (mm/h). To determine the allowable roof area for a listed pipe size at a listed slope, divide the allowable square feet (m2) of roof for a 1 inch per hour (in/h) (25.4 mm/h) rainfall rate by the listed rainfall rate for the local area. For example, the allowable roof area for a 6 inch (150 mm) drain at ⅛ inch per foot (10.4 mm/m) slope with a rainfall rate of 3.2 in/h (81 mm/h) is 21400/3.2 = 6688 square feet (621.3 m2).