Plumbing plan reviewers check every fixture, every pipe run, and every connection against the applicable plumbing code. Depending on jurisdiction, that means the International Plumbing Code (IPC), the Uniform Plumbing Code (UPC), or a local amendment to either. A plumbing plan review covers drainage, waste, and vent (DWV) systems, domestic water supply, storm drainage, gas piping, and fixture requirements. The review also checks for code-required backflow prevention, cross-connection control, and accessibility compliance.
This checklist covers the 10 plumbing plan review issues that generate the most correction comments. They are organized by system, starting with drainage and venting, then water supply, then fixtures and coordination.
Drainage, waste, and vent
1. Drainage fixture unit (DFU) calculations and pipe sizing
Every plumbing fixture has a drainage fixture unit (DFU) value that represents its load on the drainage system. The IPC assigns DFU values in Table 709.1, while the UPC uses Table 702.1. Drain pipe sizing is based on the cumulative DFU load and the slope of the pipe. A 3-inch drain at 1/4 inch per foot slope handles 42 DFUs in the IPC. Undersized drain pipes cause slow drainage, backups, and cross-contamination between fixtures. Plan reviewers trace each branch, stack, and building drain, total the DFU load at each segment, and verify the pipe size matches the code table for the segment slope. Missing DFU calculations or unstated pipe slopes are flagged on almost every first review.
2. Vent sizing and termination
Every trap must be vented to prevent siphonage and maintain the water seal. The IPC sizes vents based on DFU load and developed length in Table 916.1 (individual vents) and Table 918.1 (circuit vents). Vent pipes must terminate at least 6 inches above the roof surface per IPC 903.1, at least 10 feet horizontally from any openable window or air intake, and must not terminate under an overhang. In cold climates, vent terminals must be increased to at least 3 inches to prevent frost closure per IPC 903.2. Reviewers check that every fixture has a compliant vent path (individual, common, circuit, or combination waste and vent), that vent sizes match the tables, and that the vent termination height and location are shown on the roof plan. Island sink venting (loop vents or air admittance valves where permitted) is frequently detailed incorrectly.
3. Cleanout locations
The IPC requires cleanouts at the upper end of each horizontal drain, at each change of direction exceeding 45 degrees, at intervals not exceeding 100 feet in pipes 4 inches or smaller (or 150 feet for larger pipes), and at the junction of the building drain and building sewer. The UPC has similar requirements but specifies maximum 100-foot intervals for all pipe sizes. Cleanouts must be accessible and must provide clearance for rodding equipment (18 inches for pipes up to 3 inches, 36 inches for larger pipes). Plan reviewers check the plumbing plan for cleanout locations at every required point and verify they are accessible, not buried behind finished walls without access panels.
Water supply
4. Water supply sizing and pressure
The domestic water supply system must deliver adequate flow and pressure to the most remote and highest fixture. The IPC uses the water supply fixture unit (WSFU) method from Table 604.3 to size supply pipes, converting fixture units to gallons per minute using Table 604.3.1. The design must account for static pressure loss from elevation (0.433 psi per foot of height), friction losses through piping and fittings, and losses through the water meter, backflow preventer, and pressure-reducing valve. The minimum pressure at the most remote fixture must be at least 8 psi for most fixtures or 20 psi for fixtures requiring flush valves. Reviewers check the water supply calculation for completeness: available street pressure, meter loss, elevation loss, friction loss, and residual pressure at the critical fixture.
5. Backflow prevention and cross-connection control
The IPC requires backflow prevention at every cross-connection where potable water could contact non-potable water or contaminants. The level of protection depends on the hazard: high-hazard connections (chemical feed lines, boiler feed, irrigation with chemical injection) require a reduced-pressure zone (RPZ) backflow preventer. Low-hazard connections require a double-check valve assembly (DCVA) or atmospheric vacuum breaker. IPC 608.1 requires a backflow preventer at the water service entrance in many jurisdictions. Reviewers check every connection to non-potable systems (boilers, cooling towers, fire sprinklers, irrigation, lab equipment) and verify the correct device type is specified. They also check that air gaps are maintained at indirect waste connections. Missing backflow preventers at fire sprinkler connections and irrigation systems are the most common omissions.
6. Water heater sizing and safety
Water heater selection must match the building's hot water demand, and the installation must comply with safety requirements. The IPC requires temperature and pressure (T&P) relief valves on all storage water heaters, with the discharge piped to within 6 inches of the floor or to an approved indirect waste receptor. T&P discharge pipes must not be reduced in size, must not have valves, and must not discharge where they can cause personal injury or property damage. Expansion tanks are required on closed-loop systems (systems with a backflow preventer on the cold water supply). Seismic strapping is required in seismic design categories C and above. Reviewers check the water heater detail for the T&P valve discharge routing, expansion tank, seismic strapping (where required), and combustion air supply for gas-fired units.
Fixtures and coordination
7. Fixture count and accessibility
The IPC requires minimum plumbing fixture counts based on occupancy type and occupant load per Table 403.1. The fixture count includes water closets, lavatories, drinking fountains, and service sinks as applicable. Separate counts are required for male and female occupants. Reviewers check the fixture schedule against the occupancy classification and occupant load to verify adequate fixtures are provided. They also check that accessible fixtures comply with ADA and ICC A117.1 requirements: accessible water closets with proper clearances, accessible lavatories with knee clearance, and accessible drinking fountains at the required mounting heights. At least one of each fixture type in each toilet room must be accessible.
8. Trap seal protection and indirect waste
Every fixture trap must maintain a water seal of at least 2 inches and not more than 4 inches under normal operating conditions. The trap must be within the maximum developed length from its vent (IPC Table 909.1) to prevent siphonage. Floor drains and similar fixtures that receive infrequent use can lose their trap seal to evaporation. For floor drains in mechanical rooms, elevator pits, and similar spaces, reviewers check for trap seal primers (either waterline or electronic) to maintain the seal. Equipment condensate, food waste grinders, sterilizers, and similar sources must discharge through an indirect waste pipe terminating with an air gap above an approved receptor, not connected directly to the drainage system. Reviewers check for indirect waste connections on HVAC condensate drains, commercial kitchen equipment, and medical equipment.
9. Storm drainage and overflow
Roof drainage systems must be sized for the 100-year, 1-hour rainfall intensity for the project location. The IPC sizes roof drains and leaders using Tables 1106.2 and 1106.3 based on the rainfall rate and the horizontal projected roof area served by each drain. The building code requires a secondary (overflow) drainage system designed to handle the full rainfall rate, activated when the primary system is blocked or overwhelmed. Secondary drainage is typically provided by scuppers or overflow drains set at a higher elevation than the primary drains. Reviewers check the roof plan for primary and secondary drain locations, verify the sizing against the local rainfall rate, and confirm the secondary system discharges to a visible location (not tied into the primary storm system). Missing secondary/overflow drains are a common correction item, especially on flat roofs with parapet walls.
10. Gas piping sizing and coordination
Gas piping must be sized to deliver the required volume to each appliance at the minimum supply pressure. The IPC (Chapter 12) and the International Fuel Gas Code (IFGC) provide pipe sizing tables based on pipe length, inlet pressure, specific gravity, and total BTU/hour demand. Reviewers check the gas piping plan for pipe sizes, verify the riser diagram shows the total demand and longest run, and confirm the sizing matches the applicable table. They check for shutoff valves at each appliance (within 6 feet, accessible), a manual shutoff at the meter, and earthquake-actuated shutoff valves where required by local code. Gas piping materials must be suitable for the application: CSST requires bonding per the NEC, and piping through concealed spaces must be protected from physical damage. Reviewers cross-check the gas piping layout with the mechanical drawings to verify all gas-fired equipment is served.
Catching these before submittal
Plumbing plan review touches the IPC or UPC, the building code, the fuel gas code, and accessibility standards simultaneously. A first-pass check that verifies DFU calculations, vent sizing, water supply pressure, backflow prevention, and fixture counts catches the majority of review comments before the plans reach the building department. An automated review that checks plumbing drawings against all applicable codes in a single pass can identify cross-code conflicts between the plumbing, mechanical, and fire protection drawings that are difficult to spot on paper.