A "water line" usually refers to a pipe supplying pressurized water ready for use while a sewer line refers a pipe carrying water with human waste at atmospheric pressure. My understanding is that a further distinction is made between drain lines from sinks, showers, toilets and washing machines to the perimeter foundation of a house which are then referred to as sewer lines from the house perimeter to the street and under the street. Roof drains and patio drains are not permitted to enter the sewer system but must instead drain to the street and the storm drain system.

Otherwise they will cause dilution and process problems at the waste treatment plant and may also flood it resulting in raw sewage reaching the nearest body of water such as river, lake or seashore.