Several outlet drains can be connected to a single collector pipe. This means that the pipe can be laid without fall directly under the roof covering, with numerous outlet drains discharging into it. The collector pipe is routed to a single downpipe. When the pipes fill, the water in the downpipe wants to fall. This action causes the water in the horizontal collector pipe to be pulled through the downpipe to replace the water flowing out. The water pressure in the system falls below atmospheric pressure and water on the roof is sucked into the connected drains. On reaching the ground, the water travels into a vented manhole or inspection chamber where it is discharged at atmospheric pressure into the storm sewer.