Plumbing plan review is thorough in a way that surprises engineers new to the process. Reviewers don't just check that pipes are shown. They check fixture unit counts, pipe sizing methodology, vent termination heights, trap locations, and the presence of required devices that are easy to forget when they don't appear on every project.
These are the IPC violations that generate the most correction notices on commercial and multi-family projects.
1. Fixture unit count errors
IPC Table 709.1 assigns drainage fixture unit (DFU) values to every plumbing fixture. The DFU load determines drain pipe sizing per IPC Table 710.1(2). When the fixture unit count is wrong, the pipe sizing that follows from it is also wrong, and the reviewer will flag both.
| Fixture | DFU Value | Water Supply Units | Common Error |
|---|---|---|---|
| Water closet (public) | 4 | 2.5 (flush valve) | Using residential DFU of 3 |
| Lavatory (commercial) | 1 | 0.5 | Combining with kitchen sink DFU |
| Kitchen sink (commercial) | 2 | 1.5 | Using residential value of 1 |
| Dishwasher (commercial) | 2 | 1.4 | Omitting from fixture unit count |
| Floor drain (2-inch) | 2 | N/A | Omitting from drainage count |
| Drinking fountain | 0.5 | 0.25 | Missing from drawings entirely |
The most persistent error is using residential fixture unit values on commercial projects. A water closet with a flush valve (commercial) carries 4 DFUs. A water closet with a tank (residential) carries 3 DFUs. On a floor with 20 fixtures, that difference adds up to a pipe size discrepancy.
2. Fixture count minimums (IPC Section 403)
IPC Table 403.1 establishes minimum fixture counts based on occupancy type and occupant load. The calculation is: occupant load from IBC divided by the fixture ratio for the occupancy, rounded up. The drawings need to show that the provided fixture count meets or exceeds the minimum.
The gender split assumption also matters. IPC 403.1 Note c requires equal distribution between male and female when gender is not known. For occupancies where the gender distribution is clearly unequal, documentation is required to support the deviation.
3. Vent termination height and location
IPC Section 903 governs vent termination. Vents must terminate at least 6 inches above the roof, at least 10 feet horizontally or 2 feet above any opening within 10 feet, and away from mechanical air intakes per Section 903.2. On commercial projects with rooftop mechanical equipment, the proximity to air intakes is the most common correction.
Drawings that show vent stacks terminating through the roof without indicating their location relative to rooftop mechanical equipment are flagged routinely. The reviewer can't confirm compliance without dimensions or a roof plan that shows both the vents and the equipment.
4. Water hammer arrestor requirements
IPC Section 1002.1 (2021 IPC; previously in the appendix) requires water hammer arrestors at quick-closing valves, including solenoid-operated valves, dishwashers, washing machines, and similar equipment. The arrestor must be sized per PDI WH-201 and located within the branch supply to the fixture.
The 2021 IPC moved water hammer arrestor requirements into the body of the code rather than the appendix, making them mandatory rather than advisory. Projects designed under 2018 IPC in jurisdictions that have adopted 2021 IPC are particularly prone to this omission.
5. Trap requirements and trap-to-vent distance
Every fixture requires a trap per IPC Section 1002.1. The trap must be within the maximum distance from the trap weir to the vent connection per IPC Table 909.1. That distance is a function of pipe diameter: 3.5 feet for a 1.25-inch pipe, 5 feet for 1.5-inch, and 8 feet for 2-inch.
| Pipe Diameter | Max Trap-to-Vent Distance | Typical Fixture |
|---|---|---|
| 1-1/4 inch | 3.5 feet | Lavatory |
| 1-1/2 inch | 5 feet | Bathtub, shower, laundry sink |
| 2 inch | 8 feet | Floor drain, shower |
| 3 inch | 10 feet | Water closet |
| 4 inch | 12 feet | Large drain |
The correction appears on drawings where the vent connection is shown too far from the trap, or where the trap-to-vent distance isn't dimensioned at all. Reviewers check this on every floor plan that shows a horizontal drain run.
6. Backflow prevention
IPC Section 608 requires backflow prevention on any connection between the potable water supply and a non-potable source. The type of backflow preventer required depends on the degree of hazard: air gap, reduced pressure zone (RPZ) assembly, double check valve assembly, or atmospheric vacuum breaker.
The most common error: specifying a double check valve assembly on a connection that requires an RPZ because of the hazard type. Boiler makeup water connections, chemical feed systems, and irrigation systems with fertilizer injection are all high-hazard connections requiring an RPZ. Drawings that show a double check valve at these locations get corrected.
7. Grease interceptor sizing and placement
IPC Section 1003.3 requires grease interceptors for food service establishments. The interceptor must be sized per PDI G101 or equivalent method, and located outside the building or in a location accessible for maintenance without requiring access through food preparation or storage areas.
Drawings that show a grease interceptor under the kitchen sink without a sizing calculation, or located in a mechanical room accessible only through the walk-in cooler, are flagged on both counts. The sizing calculation needs to appear on the drawings or in the submittal documents.
8. Hot water system design temperature documentation
IPC Section 607 requires domestic hot water to be maintained at or above 110°F to inhibit Legionella growth in the storage tank and distribution system. It also requires mixing valves where the storage temperature exceeds the delivery temperature limit for the occupancy. Drawings need to state the storage temperature, the delivery temperature, and the mixing valve locations.
Why plumbing corrections are harder to resolve late
Plumbing corrections are expensive to resolve after the permit is issued because they often require coordination across disciplines. Adding a backflow preventer changes the pipe routing. Moving a vent stack changes the ceiling coordination. A missed grease interceptor may require a floor penetration that conflicts with the structural slab.
Catching these in the drawing review phase, before the set goes to the AHJ, is worth the time. The corrections are on paper, not in the field.
Callout reviews plumbing drawings against IPC 2021, IPC 2018, and UPC 2021, including fixture unit counts and backflow prevention requirements. Upload a drawing set and get findings with exact code citations in under a minute. Try it with 50 free credits →