Two Codes, One Profession

Plumbing engineers in the United States work under one of two dominant model codes: the International Plumbing Code (IPC), published by the International Code Council (ICC), and the Uniform Plumbing Code (UPC), published by the International Association of Plumbing and Mechanical Officials (IAPMO). Understanding which code applies to your project — and how the two differ — is foundational to compliant, defensible design.

Adoption Map: Which States Use Which Code

As of 2025, the IPC has the broader footprint. States that have adopted the IPC (often with local amendments) include New York, Florida, Texas, Illinois, Georgia, Virginia, North Carolina, and most of the Southeast and Mid-Atlantic. The UPC dominates the West Coast and several Mountain states: California, Oregon, Washington, Arizona, Nevada, and Hawaii all reference the UPC as their base plumbing document.

Some states maintain their own state-specific plumbing codes that draw from one or both model codes — Massachusetts (248 CMR), New Jersey (NJAC 5:23), and Wisconsin (Comm 82–84) are notable examples. Always verify the current adopted edition and any local amendments through the Authority Having Jurisdiction (AHJ) before starting design. ICC and IAPMO both maintain online adoption maps, but state-published amendment documents supersede them.

Key Technical Differences

Venting Methods

The IPC (Chapter 9) and UPC (Chapter 9) share many vent types but diverge on details. The IPC explicitly recognizes air admittance valves (AAVs) per ASSE 1051 as an approved venting method with specific location restrictions (IPC §918). The UPC also permits AAVs per ASSE 1051, but California (which amends the UPC) has historically restricted their use — always check state amendments. Wet venting under the IPC (§912) allows a single pipe to serve simultaneously as a drain and a vent for a limited number of fixtures in a vertical arrangement, whereas the UPC wet vent provisions (§908) are structured differently and limit the allowable fixture drain size.

Drainage Fixture Units (DFU)

Both codes use DFUs to quantify sanitary load, but the assigned values differ for several fixtures. Under the IPC (Table 709.1), a lavatory carries 1 DFU; a water closet (private use, tank-type) carries 3 DFU; a kitchen sink carries 2 DFU. The UPC (Table 702.1) assigns similar values for most fixtures but defines "fixture unit" slightly differently for combination fixtures. When sizing drainage systems, use the DFU table from the specific code edition adopted in your jurisdiction — do not mix tables between codes.

Trap Requirements

The IPC (§1002) prohibits S-traps because they are subject to self-siphonage — fixture discharge can pull the water seal out of the trap, allowing sewer gas to enter the occupied space. P-traps are standard. The UPC (§1007) similarly prohibits S-traps. Both codes require a water seal of 2 inches minimum and 4 inches maximum (IPC §1002.2). Crown-vented traps and bell traps are prohibited by both codes.

Horizontal Drain Slope

IPC §704.1 requires horizontal drainage piping to slope at ¼ inch per foot (2%) for pipe 2½ inches and smaller, and ⅛ inch per foot (1%) for pipe 3 inches and larger. The UPC §708.0 uses identical slope requirements. Both codes permit steeper slopes with engineering justification, subject to AHJ approval.

How to Look Up Your Jurisdiction's Adopted Code

Start with the state's department of labor, department of licensing, or building codes division website. Search for "adopted plumbing code" and the current effective date. Download the state amendment document — these often run 20–40 pages and modify specific sections of the model code. For local municipalities (particularly major cities), check the city's building department website for any additional local amendments layered on top of the state adoption.

The ICC's Code Adoption Status page and IAPMO's Codes Adoption Tracker are useful starting references but lag official state publications by months or years.

Impact on Design Decisions

Code selection affects pipe sizing tables, acceptable materials, vent configuration, water supply sizing methods, and fixture count calculations. A system designed to IPC standards in an office building may be sized differently than one designed to UPC standards for the same occupancy, particularly in multi-story stacks where vent stack sizing tables diverge. Pipe materials also differ: the IPC Table 702.1 lists acceptable drain, waste, and vent materials; the UPC Table 701.1 has a comparable list but includes some materials — such as certain ABS formulations — with differing testing standard references (ASTM D2661 vs ASTM F628).

Common Code Violation Areas

Inspectors and plan reviewers most frequently flag: (1) improper vent termination height above roof (IPC §903.1 requires 6 inches minimum; UPC §906.1 requires 6 inches); (2) missing cleanouts at the base of stacks and at changes in direction exceeding 45° (IPC §708); (3) AAVs installed in locations without adequate air supply or below fixture flood rim; (4) improper trap-to-vent distances (IPC Table 909.1 specifies maximum distances by pipe diameter — e.g., 1¼-inch trap arm limited to 2.5 feet, 2-inch limited to 5 feet); and (5) undersized building drain at the point of connection to the public sewer. Reviewing these five areas during design QC will catch the majority of plan check comments before submission.