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 1/8 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).