Roof drains and storm drainage piping are sized based on which two factors?
Not quite. The answer is A: Roof area (or projected horizontal area) and the local rainfall rate for the design storm
Storm drainage is a hydrology calculation, not a fixture-count calculation: leaders and drains are sized from the maximum projected roof area and the rainfall rate for the design storm (the UPC bases primary drainage on a 60-minute, 100-year storm unless the authority having jurisdiction says otherwise). DFU-based sanitary sizing measures a completely different quantity.
Checked against the 2021 UPC text. Washington's exam is written against the 2015 edition, which we have not read for this section, so confirm it against your 2015 book.
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