A plumbing diagram shows the central vent line, the various individual drainpipes of the house's water fixtures and one or more cleanouts. The vent line extends from the base of the house to the roof. All drainpipes flow from their fixtures into the vent line. There are typically several cleanouts.
The vent line is the easiest of all the different pipes to identify. It is the largest pipe, extends for the entire height of the building, and is the only pipe that opens up above the home's roof. The vent pipe opens above roof level to allow volatile and poisonous sewer gases to easily escape the home.
Every water fixture in the home, including sinks, bathtubs, showers and toilets have drainpipes that flow into the vent line. All drainpipes are graded with a slight downward slant, allowing water to easily flow into the vent line and preventing clogs.
Cleanouts are Y-shaped pipe fittings that have removable caps. Plumbers usually fit a cleanout in any area prone to clogs, such as bends and pipe junctions. The cleanout allows a plumber to remove the cap and snake all of the pipes in the surrounding area. This permits removing stubborn or difficult to access clogs without having to cut out and replace entire sections of pipe. Typical locations for cleanouts include attics, crawlspaces and basements.