Drains, Traps and Vents: Rules in Phoenix-Area Cities
Drain, waste and vent rules: slope, cleanouts, traps, venting, floor drains and storm drainage. City by city, then the Arizona law and model code behind it.
By City
| City | What the rule says |
|---|---|
| Valley-Wide | Retention Basin and Drywell Rules by Valley City Every Valley city makes commercial sites hold their own stormwater and empty it within 36 hours, but the details differ. Most size basins for the 100-year, 2-hour storm; Gilbert uses the 50-year, 24-hour storm and Tempe the 100-year, 1-hour. Drywells are a last resort in Mesa, Chandler and Peoria, and required in every Avondale basin. |
| Maricopa County | Maricopa County Zoning 1205: Retention Basins and the 36-Hour Rule In unincorporated Maricopa County, a commercial site must retain the 100-year, 2-hour storm falling on the whole site and empty it within 36 hours, under Zoning Ordinance Section 1205.7.6. Drywells must be ADEQ-registered, tested one by one, and cleaned by the owner at least every three years. |
| Phoenix | Phoenix City Code 32A-24: On-Site Stormwater Retention for Commercial Sites Phoenix City Code 32A-24 requires every development to retain its own stormwater, sized by the city manual for the 100-year, 2-hour storm and emptied within 36 hours. Parking lots can pond only six inches, drywells default to 0.1 cfs each, and the owner cleans them at least every three years. |
| Phoenix City Code 32C-103: Can You Wash a Parking Lot Into the Storm Drain? No. Phoenix City Code 32C-103 lets only stormwater and a short list of exceptions reach streets, retention basins and storm drains. Commercial pressure washing, dumpster pad rinse water and mop water aren't on the list, so they go to the sanitary sewer, get collected, or need a city discharge permit. | |
| Phoenix City Code 32C-103: Who Maintains Retention Basins and Drywells Under Phoenix City Code 32C-103, the property owner must keep retention basins, drywells and underground storage working as designed and keep maintenance records for five years. On projects of one acre or more built since mid-2022, the city also inventories those controls, inspects them within a year of completion and asks for yearly self-inspections. | |
| What rainfall rate does Phoenix use to size roof drains and storm piping? Three inches per hour. Phoenix amends IPC 1106.1 so vertical conductors, leaders, building storm drains, building storm sewers and their horizontal branches are all sized on 3 inches of rain per hour, in place of the base code's 100-year rate from its rainfall figures. | |
| Mesa | What rainfall rate does Mesa use to size roof drains and storm drains? IPC 1106.1 Mesa sizes vertical leaders, building storm drains, storm sewers and horizontal branches for 3 inches of rain per hour. The city's plan checklist gives the roof drain equation: roof area times 0.0104 times inches of rain equals gallons per minute. Scuppers must be at least 4 inches high. |
Arizona Law
All Arizona law- State LawUpdated Sep 2026AAC R18-9-A902: Industrial StormwaterArizona's 2024 Multi-Sector General Permit, effective January 16, 2025, covers industrial stormwater from 29 business sectors, including warehousing and trucking, when it reaches a protected surface water. Operators write a SWPPP, file a Notice of Intent in myDEQ and pay an acreage-based yearly fee. Fully covered sites can certify no exposure.
- State LawUpdated Sep 2026AAC R18-9-I652: Drywell Inventory RulesArizona drywells are now Class V injection wells under ADEQ, which took over the federal UIC program on October 15, 2025. Under A.A.C. R18-9-I652 the owner files a Class V inventory in myDEQ, and existing wells get one year from the program start. The old drywell registration statute, A.R.S. 49-331, was repealed in 2022.
Uniform Plumbing Code
All UPC sections- Model CodeUpdated Jul 2026UPC 1003: Which Fixture Traps Are Allowed (and Banned)UPC Section 1003 describes which fixture traps are approved and which are banned. The standard P-trap is allowed. S-traps, bell traps, drum traps, and traps with moving or hidden parts are prohibited. Each fixture gets one approved trap, and double-trapping is not allowed.
- Model CodeUpdated Jul 2026UPC 705: Joints and Connections in Drainage PipingUPC Section 705 sets the approved ways to join drain, waste, and vent piping for each material. That includes solvent-welded ABS and PVC, no-hub cast iron, and soldered copper. It also bans joints that are prone to leak.
- Model CodeUpdated Jul 2026UPC 706: Changes of Direction in Drainage PipingUPC Section 706 controls how a drain line turns corners. Every change of direction has to use an approved fitting with the right sweep. Tight fittings are only allowed where a horizontal line drops into a vertical stack. Turns that stay horizontal or flatten out at the base of a stack need gentle long-sweep fittings so waste keeps moving and does not clog.
- Model CodeUpdated Oct 2026UPC 712: Testing Drain, Waste and Vent PipingNew drain, waste and vent piping must pass the applicable leak test before it is covered. Test method, pressure or water head, duration and inspection requirements depend on the adopted code, local amendments and manufacturer instructions. Charlotte Pipe prohibits compressed-air or gas testing of ABS, CPVC and PVC pipe and fittings.
- Model CodeUpdated Jul 2026UPC 905: How Vent Pipes Must Be Graded and ConnectedUPC Section 905 sets how a vent pipe must slope and where it may tie back in. A vent has to grade back toward the drain so no water pools inside it, and it must reconnect above the flood level of the fixture so waste can never rise into a dry vent.
- Model CodeUpdated Jul 2026UPC 907: Vent Stacks and Relief VentsUPC Section 907 sets when a tall or busy drainage stack needs its own vent stack and relief vents. A vent stack runs beside the soil stack to keep air pressure near normal, and relief vents cross-connect the two so falling water cannot pull trap seals dry on upper floors.
- Model CodeUpdated Jul 2026UPC 908: Wet Venting a Bathroom GroupUPC Section 908 allows wet venting, a method where one drain pipe also vents the other fixtures in a bathroom group. The drain doubles as the vent for the upstream fixtures, so the group needs far less vent pipe.
- Model CodeUpdated Jul 2026UPC 909: Island Fixture (Loop) VentingUPC Section 909.0 covers special venting for island fixtures like a kitchen island sink that has no wall behind it. The vent rises to drainboard height, loops over, and drops to a foot vent below the floor. A cleanout must be provided so the loop can be rodded clear.
- Model CodeUpdated Jul 2026UPC 910: Combination Waste and Vent SystemsUPC Section 910 lets one oversized horizontal drain carry both the waste and the air for certain fixtures. It is used where a normal vent is hard to run, like a floor drain in the middle of a room or an island sink. The drain is sized larger than usual so air moves freely above the flowing water and vents the fixtures.
- Model CodeUpdated Jul 2026UPC Floor Drain Requirements: Traps, Size, and Keeping the SealThe Uniform Plumbing Code requires a floor drain to have a trap, to be sized for the water it must carry, and to keep its trap seal so sewer gas cannot rise. Because floor drains sit unused for long stretches, code addresses drying-out with trap primers or approved seal devices.
- Model CodeUpdated Jul 2026UPC Section 1101: Storm Drainage and Roof RunoffUPC Section 1101 governs how rainwater is carried off roofs and paved areas. It requires storm drainage to be kept fully separate from the sanitary sewer, and it sizes roof drains, gutters, and leaders based on the roof area and local rainfall so heavy rain drains without flooding.
- Model CodeUpdated Oct 2026UPC Section 707: Where Drain Cleanouts Are RequiredUPC Section 707 requires access points, called cleanouts, on a building's drain system so a plumber can clear a clog. Code calls for cleanouts near where the drain leaves the house, at each long horizontal run, and at sharp changes of direction, sized to match the pipe.
- Model CodeUpdated Oct 2026UPC Section 708: How Much Slope a Drain Pipe NeedsUPC 708.1 starts at 1/4 inch per foot for building drains and other horizontal drainage piping. Building drains 4 inches and larger may use 1/8 inch per foot where the baseline is impractical because of sewer depth, structural features or building arrangement, with prior approval from the authority having jurisdiction.
- Model CodeUpdated Jul 2026UPC Section 710: When Phoenix Code Requires a Backwater ValveUPC Section 710 requires a backwater valve on any drain line serving a fixture whose flood-level rim sits below the elevation of the next upstream public-sewer manhole cover. That valve blocks sewage from backing into the building when the main surcharges. Fixtures above that line must never drain through the valve.
- Model CodeUpdated Jul 2026UPC Section 807: Indirect Waste and the Air BreakUPC Section 807 requires certain equipment, like a dishwasher, ice maker, or food-prep sink, to drain indirectly rather than being hard-piped to the sewer. The waste line ends above a receptor with an air gap or air break, so sewer water can never back up into food or clean equipment.
- Model CodeUpdated Jul 2026UPC Section 906: Where a Plumbing Vent Can TerminateUPC Section 906 controls where a plumbing vent pipe can end so sewer gas leaves safely and cannot re-enter the building. A vent through the roof must rise a set height above the roof and stay a set distance from any door, window, or air intake.
- Model CodeUpdated Jul 2026UPC Table 1002.2: Maximum Trap Arm Length by Pipe SizeUPC Table 1002.2 caps how far a fixture's trap can sit from its vent, so the vent can break suction before it siphons the trap seal dry. The limit rises with pipe size: 30 inches for 1-1/4 inch pipe up to 10 feet for 4 inch pipe. Self-siphoning fixtures like toilets are exempt.
Need The Work Done To Code?
Call a licensed plumber who pulls the permit in your city and passes inspection the first time.