Plumbing Definitions, Materials, and Standards
Definitions that drive design
The code defines a building drain as the lowest piping receiving soil and waste inside the building and ending 2 feet outside the wall; the building sewer begins there and runs to the public sewer or private disposal system. A fixture unit is a load measure, not a pipe size. Developed length is the measured length along the pipe centerline including fittings, not the straight-line distance. A trap is a fitting that maintains a 2- to 4-inch water seal.
Materials and standards
Materials must be listed for the application and installed per the manufacturer and the referenced standard. Common DWV materials include cast iron soil pipe (ASTM A74), hubless cast iron (CISPI 301), PVC DWV (ASTM D2665), ABS DWV (ASTM D2661), and copper (ASTM B88). Copper and CPVC are used for hot and cold distribution; PEX must be listed to the applicable ASTM standard and protected from UV and freezing. Solvent-cement joints must use the proper cement for the material; mechanical joints must be accessible where the code requires.
Worked example
A 4-inch drain exits a wall and continues 6 feet to a septic tank. The pipe inside the building and the first 2 feet outside the wall is the building drain; the remaining 4 feet is the building sewer. Calling the entire 6-foot run the building drain is a definition error that can affect cleanout and slope requirements.
Traps
Do not treat a vertical stack as a vent, do not size pipe by fixture count alone without the tables, and do not mix PVC cement with ABS. Confirm that a material is approved for the specific service and temperature.