Flat and low-slope rowhome roofs.
Many Philadelphia rowhomes, especially older ones on the city's south and north sides, were built with flat or low-slope roofs instead of a pitched roof. That means membrane roofing (EPDM rubber, modified bitumen, or a built-up system) rather than asphalt shingles, and drainage has to be planned carefully since there's no slope to shed water on its own.