(1) Except as provided in Sentences (2) and (3), every drainage pipe that has a size of 3 in. or less shall have a downward slope in the direction of flow of at least 1 in 50.
(2) Sentence (1) does not apply to a force main.
(3) Where it is not possible to comply with Sentence (1), a lesser slope may be used if it will produce a gravity flow of not less than 0.6 m per second.