A roof generally isn’t designed to double as a reservoir.
Without an appropriate roof drain system, standing water in large volume is a serious problem because most roof decks aren’t designed to bear extreme water weight for long periods. It’s estimated that a 20-ft. by 20-ft. section of roof covered in water 1-in. deep weighs approximately a ton. Too much water pooled on a commercial roof can mean leaks, structural damage, roof collapse and mold.