Can I use shingles on a flat roof?
Usually no, unless there are multiple layers. Please see the code below for reference.
Minimum Roof Slope (Roof Pitch) | R905.2.2
Asphalt shingles can only be installed on roof slopes of 2:12 or greater. This means for every 12 horizontal units, the roof must rise a minimum of 2 vertical units.
Also if Asphalt shingles are installed on a roof slope of 2:12 to 4:12, it shall be provided with a double underlayment application.
Modified Bitumen Roofing
Minimum Roof Slope (Roof Pitch) | R905.11.1
Modified bitumen membrane roofs must have a design roof slope of 1/4:12 or greater (2% slope). This means for every 12 horizontal units, the roof must rise a minimum of one-fourth vertical unit.