<p>A drainage piping system for discharging soil water and waste water from various plumbing fixtures comprises a vertical main pipe and lateral branch pipes connected to the fixtures. To provide a single-pipe confluence system for installation in medium-sized or high buildings without employing a vent pipe to be arranged along the vertical main pipe, the main pipe incorporates at least one twisted pipe portion having an axis helically deviating from the axis of the main pipe.</p>
