Plumbing Code Basics: IPC, UPC, and How to Look Things Up

What the work looks like

The United States has two major model plumbing codes: the International Plumbing Code (IPC, published by the International Code Council) used in most of the country, and the Uniform Plumbing Code (UPC, published by IAPMO) used mostly on the West Coast and parts of the Southwest. Every state and city adopts one as the base and then amends. The rule that applies to your job is always the locally adopted code as amended.

Safety and tools

Most plumbing code looks at the same things:

• Fixture units (DFU for drainage, WSFU for supply): numeric values assigned to each fixture so you can size pipes by summing the units on the line.
• Drain sizing: minimum pipe diameter by total DFU per the code tables. Typical minimums: 1-1/4 inch (lavatory), 1-1/2 inch (kitchen sink, shower), 2 inch (washing machine), 3 inch (toilet, building drain up to certain DFU load), 4 inch (building drain above that threshold).
• Vent sizing and placement: every trap needs a vent within the maximum trap-to-vent distance (table in the code, usually 5 feet for 1-1/4 inch trap through 15 feet for 3 inch). The vent terminates above the roof with an approved cap, or joins a stack.
• Trap rules: every plumbing fixture has a trap (P-trap or similar) with a water seal of 2 to 4 inches. No double-trapping. No S-traps (they siphon).
• Minimum slope on gravity drainage: 1/4 inch per foot for 2-1/2 inch and smaller; 1/8 inch per foot for 3 inch and larger. Too little slope and waste stalls. Too much slope and liquids outrun solids.
• Cleanouts: required at building drains, at changes of direction greater than 45 degrees, at set intervals along horizontal runs.
• Backflow prevention: required wherever a cross-connection exists (see Introduction to Backflow Prevention).

Tools: a copy of the code (printed or digital), a highlighter, and a scale ruler for reading plan drawings. IAPMO and ICC both offer free read-only access on some products. Every plumber keeps tabs in their code book for the tables they use most.

Your first exercise

Find whether your state uses the IPC or UPC as its base. Google "state of [your state] plumbing code". Then look up the fixture-unit table for the code you found. Note the DFU values for a toilet, lavatory, shower, and kitchen sink. Plug those into a sample house and calculate the building drain size required.

Where to go next

Build on Plumbing Code Basics (Introduction to Plumbing Code) plus Plumbing Fundamentals (Introduction to Plumbing), Pipe Fitting (PEX/Copper/PVC), Fixture Installation & Repair, Backflow Prevention (Introduction to Backflow Prevention), Water Heater Installation, Gas Line Installation Basics (Introduction to Gas Line Installation). Cross-trade literacy: Codebook Navigation (Trade Foundations).