The model plumbing codes in the United States began adopting a common code format, developed as a unified chapter structure to make the codes easier to navigate and compare when multiple model codes were in use. This unified chapter structure came about during the 1990s when there was discussion of merging the model codes into one.

In 1994, the Building Officials and Code Administrators (BOCA), the International Council of Building Officials and the Southern Building Code Congress International agreed to merge as the International Code Council (ICC). They had developed the common code format during discussions comparing the three model codes in an effort to create a unified code series. 

The newly formed International Code Council (ICC) used the common code format when it published the 1995 draft of the International Plumbing Code (IPC) after a few rounds of code hearings, resulting in the publication of the 1996 edition of the IPC, the first of the major International Codes (I-Codes) to be released, officially establishing the new common code format for plumbing.

By the year 2000, the ICC had completed development of the first complete edition of its full family of I-Codes, including the International Building Code (IBC), and the legacy organizations ceased development of their individual regional codes but continued to support the existing documents through 2003.

Prior to the consolidation of the codes, national corporations and industries built buildings across the country with the same floor plan, adapted to each location. When owners, engineers, designers and contractors dealt with the electrical codes, there was only one code, and everything was the same from one location to another, but the plumbing codes were drastically different from one jurisdiction to another. 

This required a redesign every time the same plan was installed in a different location to meet the colloquial requirements. Many of these national code users pushed for at least having all the model codes use the same chapter numbering format so it would be easier to compare codes. 

The key details regarding transitioning the codes to a common code format included:

1. Reorganizing the chapters to reduce confusion. Developers, owners, builders and designers previously had to navigate multiple, differently structured/numbered codes (such as the BOCA Basic Plumbing Code, the National Standard Plumbing Code (NSPC), the Southern Standard Plumbing Code and IAPMO’s Uniform Plumbing Code (UPC)).

2. Adoption of the common code format. Most model codes reorganized their chapter order to follow this shared format, resulting in a consistent structure in which the chapters had the same numbers for administration, definitions, materials and specific plumbing systems. 

3. The resulting common code chapter structure was adopted in all model codes by the year 2000 editions. The common code format structure was firmly in place when the ICC consolidated the older regional or legacy codes into the I-Codes. 

While the common code format refers to the organizational structure of the chapters, the content within those chapters (such as materials and standards) continued to be developed and managed separately by different organizations, such as IAPMO and the ICC.

Today, model plumbing codes, such as the IPC and the UPC, still follow the chapter numbering format that reorganized their chapters. The IPC and UPC are the two main model codes in the United States. There is also the NSPC, which is mostly used in New Jersey, Maryland and a few other local jurisdictions in the northeastern United States. 

The NSPC recently reached an agreement with IAPMO to work together going forward. So, it appears the NSPC will be replaced by the UPC in areas it serves in the coming years. 

Each model code is arranged following the common code format for the first 11 chapters. The common code format was proposed by interested parties and implemented about 30 years ago. Since then, many new chapters and appendices have been added to each code for various topics. After the first 11 chapters, the codes differ. 

For example, many years ago, fuel gas was covered in a chapter in the UPC, which was a copy of the Uniform Fuel Gas Code (UFGC). When the model building codes were consolidated into a single organization around 2002 and became the ICC, IAPMO chose not to follow, leaving it without a building code. 

It teamed up with the National Fire Protection Association (NFPA) to develop a building code, but NFPA already had its own fuel gas code. Many of IAPMO’s members were plumbing and mechanical contractors who were familiar with the layout of the UFGC, and they were happy to have a fuel gas chapter in the plumbing code. With the merger to develop a building code, they had to compromise and switch to the NFPA’s National Fuel Gas Code. 

Contractors didn’t want to buy an extra book to do fuel gas installations, so they tried to keep the fuel gas chapter in the plumbing code. This has created an issue because both organizations had a fuel gas code, and the NFPA code was superior. IAPMO members have been working on wholesale changes to the fuel gas chapter in the UPC and the Uniform Mechanical Code (UMC). The UPC committee did not always agree with the NFPA fuel gas code committee. 

So, there is now an agreement that NFPA is the controlling document, and it gets reprinted in the UPC and the UMC as a separate chapter in those books. This creates a couple of extensive chapters in the UPC and UMC. The resistance to removing the fuel gas code language from the plumbing and mechanical codes has generally been led by the plumbing and mechanical contractors. 

For years, they would look up the fuel gas piping and venting information in the plumbing or mechanical code book when installing a fuel-fired appliance. The fuel gas piping and the flue venting information are covered in the fuel gas code book. 

Many of the other chapters and appendices are informative and not written in mandatory code language.

2024 IPC vs. 2024 UPC

IPC Chapter 1: Scope and Administration

Covers the limits of the code, authority of code officials, permitting processes, inspection requirements and violation penalties. 

UPC Chapter 1: Administration

Covers the application, enforcement and administration of subsequent requirements of the code. The administration chapter also establishes the scope of the code and is responsible for enforcing the requirements set out in its body. 

My comments on code administration and contractor/engineer communications: The code is intended to be adopted as a legally enforceable document to safeguard health, safety, property and public welfare. The code cannot be effective without satisfactory provisions for its administration and enforcement by an ordinance adopted by the local jurisdiction. The authority having jurisdiction (AHJ) should have inspectors and plan reviewers to review the work and decide whether a plumbing system conforms to the code requirements. As public servants, AHJs should enforce the code in an unbiased, proper manner. 

The design professional is also responsible for researching the applicable code and for providing a safe plumbing system that meets or exceeds it. The contractor is responsible for installing the plumbing system to comply with both the plumbing code and the contract documents (plans and specifications) for the project. If the contractor notices a discrepancy between the contract documents and the code, he should immediately notify the engineer of record and the owner to resolve the issue and document the resolution in writing for his own liability protection. 

I have seen engineered plans that were wrong and violated the code, and the contractor blindly followed the plans, which was troublesome. For example, in one forensic investigation involving the expansion of a boiler plant, a contractor added to an existing power plant building that had a 4-inch sanitary drain. The engineer added many systems, equipment and fixtures to the building; the new fixture unit count required a minimum 8-inch building drain. 

The engineer’s plans called for connecting an 8-inch sanitary drain from a blow-down tank to the existing 4-inch sanitary drain under the floor. The contractor simply followed the engineer’s plans in violation of the code section that does not allow a reduction in the direction of flow for sanitary drains. 

The plumbing codes strictly prohibit reducing the size of any sanitary drain in the direction of flow, a rule designed to prevent blockages and ensure proper flow, where the sanitary drain should never exceed 50% capacity; otherwise, traps could siphon or blow out. This code requirement is commonly found in Chapter 7 of each model code (Change in Size), which states that “drainage piping shall not be reduced in size in the direction of the flow.”

The contractor simply connected the 8-inch building drain to the existing 4-inch building drain, which led to a backup and flood of the mechanical room during the boiler blowdown operation. The boiler blowdown effluent exceeded 200 gallons per minute and 200 F. Records indicate that during boiler start-up and testing, they noticed the blowdown flooding the building drain, and hot effluent and cooling water flowed from all floor drains in the room. 

The solution was to not use the aftercooler and cooling water; even without the cooling water, the drains overflowed with super-hot effluent. So, they decided to pre-drain down the blowdown tank and shorten the blowdown operation as a “workaround” rather than replace it with a few hundred feet of 8-inch building sewer to a nearby manhole. 

This workaround was dangerous and a code violation. Eventually, a high-water alarm caused the blowdown to last longer than its limited blowdown timing, and a worker was scalded to death when he slipped and fell into the hot effluent from the overflow. Both the contractor and the engineer shared fault. 

Chapter 2: Definitions

Each code provides precise technical definitions for terms used throughout the code to ensure consistent interpretation with the terms used in each code.

Chapter 3: General Regulations

Each code covers general regulations applicable to all areas of the code and general conditions that do not fit into a specific section. It includes fundamental requirements such as structural safety (cutting/notching), piping protection (freezing/corrosion), trenching, support and testing methods. 

Chapter 4: Fixtures, Faucets and Fixture Fittings

Each code specifies the minimum number and types of fixtures required, along with installation standards and product standards for fixtures, fittings and materials. It includes water consumption requirements and temperature limits at various fixture outlets.

Chapter 5: Water Heaters

Each code regulates installation requirements, safety devices and controls for water heaters, listing requirements, venting and the efficiency of water-heating equipment. It does not address storage or distribution temperatures.

Chapter 6: Water Supply and Distribution

Each code details requirements for potable water pipe sizing, materials, backflow prevention and installation of water service piping. Discusses backflow prevention and hot water system requirements.

Chapter 7: Sanitary Drainage

Each code specifies pipe sizing, materials and installation and slope requirements for sanitary waste systems, including building drains and sewers.

Chapter 8: Indirect/Special Waste

Each code governs waste connections that do not directly discharge to the drainage system, such as food-handling equipment, sterilizers and appliance drains.

Chapter 9: Vents

Each code provides regulations for venting systems to protect trap seals from siphonage or backpressure. Some codes restrict venting methods based on politics, favoring labor-saving devices and methods over science and engineering. However, alternative-engineered venting methods are used extensively under an engineer’s seal. 

Chapter 10: Traps, Interceptors and Separators

Each code requires traps for fixtures and sets standards for interceptors designed to remove grease, oil or sand from waste.

Chapter 11: Storm Drainage

Each code regulates the sizing and installation of systems that convey rainwater or subsurface water. After this chapter, there is no common code format.

IPC Chapter 12: Special Piping and Storage Systems

Covers nonpotable water systems, medical gases and specialized piping systems.

UPC Chapter 12: Fuel Gas Piping (copied from NFPA 54)

A duplication of the piping requirements of the NFPA National Fuel Gas Code. The fuel gas provisions are copied here for the convenience of plumbers installing gas piping. However, a separate code book exists for fuel gas piping in both model code organizations.

IPC Chapter 13: Nonpotable Water Systems

Focuses on the types of systems and their installation for greywater, rainwater harvesting and reclaimed water.

UPC Chapter 13: Healthcare Facilities and Medical Gas and Medical Vacuum Systems

Lists general medical gas and vacuum system requirements, references to NFPA 99, Health Care Facilities Code, for piped medical gas systems design and sizing and installation requirements.

IPC Chapter 14: Sub-Surface Graywater Soil Absorption Systems

Lists general requirements for sub-surface soil absorption systems, design and sizing, and installation. 

UPC Chapter 14: Firestop Protection 

This appendix lists firestopping standards for pipe penetrations through walls and floors. It is not all-inclusive. Some projects require each trade to do firestopping for their system, while other projects have a separate firestopping contractor responsible for the entire project. This chapter provides guidance for plumbers who do their own firestopping. 

IPC Chapter 15: Referenced Standards

Lists the accredited standards (such as ASSE, NSF, ASTM or ANSI) that products, materials and installation methods must meet. 

Comparison of IPC and UPC appendices

The appendices are generally not part of the plumbing code unless the local AHJ specifically adopts the appendices. The IPC and UPC use their appendices differently to supplement their main regulations. While the IPC focuses more on administrative and environmental data (such as rainfall and temperature), the UPC’s appendices are independent documents that provide more technical support, sizing and installation information for specific systems. 

It appears that some of the appendices were added to pad the code book and were written by smaller, nonconsensus groups in unenforceable language that would pose problems if a jurisdiction were to adopt these appendices and try to enforce certain provisions.

IPC Appendix C — Structural Safety: The installation of plumbing systems frequently requires piping to pass through building framing members. Appendix C provides the regulations for limits cutting and notching on structural members and covers the sizes and locations of holes that can be drilled or punched in various types of framing members. 

Note: The IPC committee does not hear and consider changes to the IPC Appendix C for limits cutting, notching and drilling holes in structural members. Code change proposals to the IPC Appendix C are heard and voted on by the IBC Structural Code Development Committee during the (Group B) Code Development Cycle.

These two model codes are developed in the United States and updated on three-year cycles using a consensus code development process. Code changes for the model plumbing codes are due in early January 2027 for the 2030 edition. Check with the code organization for the code change deadlines.