A diagram for a typical house will show the cold water supply lines, which permit the flow of water from the municipality's water main into your home. It indicates where the cold water passes through the meter that measures how much water the household uses. The schematic should also show the branch lines that take the cold water throughout the home and where the water exits through the various plumbing fixtures.
The schematic also points out the branch lines that distribute hot water throughout the residence. The starting point of this part of the diagram is the home's hot water heater. Once it exits the heater, hot water usually flows next to the cold water in a separate pipe and runs to faucets and fixtures. The diagram may also show hot water going from the water heater to household appliances such as washers and dryers.
A key component of the home's plumbing system, the drainage system takes waste water and sewage and directs it out of the home and into the main sewer drain. A sound schematic should indicate all of the pipes and drains that carry waste. A residential drainage system will typically consist of both vertical stacks and horizontal drains. The diagram may also display the slight downward slant of the horizontal drains, which facilitates the flow of the waste water.
In addition to detailing the incoming and outgoing water pipes, a good plumbing diagram will point out where the traps, or seals, occur. Traps, the U-shaped pieces of pipe that help prevent dangerous sewer gases from backing up into the home, can be found under every fixture. The traps always hold a small amount of water, which the gases cannot pass through. All traps need to connect to a vent, which you can determine by examining the schematic.