Does a grease interceptor need a vent?

Yes, a grease interceptor needs a vent. The 2024 International Plumbing Code (IPC), Section 1003.9, says interceptors and separators shall be designed so as not to become air bound. It adds that they shall be vented in accordance with one of the methods in Chapter 9. Section 1003.10 says access must be provided to each interceptor for service and maintenance. The same rule covers the sand and oil separators a car wash needs and the lint interceptors on laundry drains; see whether a car wash needs a sand and oil interceptor and whether a commercial laundry needs a lint interceptor.

That is the whole rule for gravity interceptors in the model code. It has no drawing, no pipe size and no route. It points you to the general vent chapter.

### How do Valley cities word it?

Scottsdale's guidelines excerpt the 2021 version of the code with amendments. That text says interceptors and separators shall be designed so as not to become air bound where tight covers are utilized, and that each one shall be vented where subject to a loss of trap seal.

Phoenix uses the 2024 IPC with its own amendments. The Phoenix amendments to Chapter 10 that we read, Sections 1003.2 through 1003.3.7, do not add a separate vent rule for interceptors. Chapter 9 of the base code still applies.

### What does Chapter 9 add?

Chapter 9 is the general vent chapter. A few of its rules matter to a grease line. They apply to vent piping in general, not to interceptors alone.

• Purpose. Section 901.2 says the vent system must let air in and out so that a fixture trap seal is not subjected to a pressure difference of more than 1 inch of water column.
• Outdoor termination. Section 903.1 says the vent pipe ends outdoors, through the roof or a side wall.
• Where the terminal can sit. Section 903.5 says an open vent terminal shall not be directly beneath a door, openable window or other air intake, and shall not be within 10 feet horizontally of one unless it is 3 feet or more above the top of it.
• Grade. Section 905.2 says vent pipes must be graded and connected to drain back to the drainage pipe by gravity.
• Size. Section 906.2 says individual vents are not less than half the required diameter of the drain served, and no vent pipe is smaller than 1 1/4 inches. Vents longer than 40 feet developed length go up one nominal pipe size.
• Testing. Section 901.5 says the vent system is tested under Section 312.

There is also a line aimed at this exact case. Section 905.4 says every dry vent shall rise vertically to a point not less than 6 inches above the flood level rim of the highest trap or fixture being vented. Then it gives an exception: "Vents for interceptors located outdoors." The code does not say why. A reasonable reading is that a buried tank has no indoor fixture rim to clear. That is a reading, not text, so ask your plan reviewer how they read it for your layout.

### What does the code not say?

Chapter 10 does not give one vent pipe size, height or route for every interceptor. It sends you to Chapter 9, and your tank's manufacturer instructions and the city's plan review fill in the rest. That is a real gap, and it is why the vent belongs on the plumbing plans from the start.

Small hydromechanical traps, the ones usually inside the kitchen, have a second vent rule.

IPC Section 1003.3.5.2 says grease interceptors shall be equipped with devices to control the rate of water flow so that flow does not exceed the rated flow. It then says the flow-control device shall be vented and terminate not less than 6 inches above the flood rim level, or be installed in accordance with the manufacturer's instructions.

The city codes echo it:

• Mesa's code says a flow restriction valve shall be installed upstream of the grease trap and vented properly. If the trap is below floor level, the valve must be installed so it can be inspected and maintained.
• Scottsdale's amended text says the same thing about the valve upstream of the hydromechanical unit.

Phoenix's amendment carries the 1003.3.5.2 flow-control wording through as well. So a trap has two things to check: the building vent on the drain, and the vented flow control ahead of it. For the difference between traps and tanks, see the trap versus interceptor page.

Because vents and odors go together. The National Precast Concrete Association says odor issues with outdoor interceptors can be mitigated when a properly designed interceptor is part of the building's plumbing and venting system. It says most building codes require the interceptor to be vented back through the inlet plumbing and to a roof vent, and that in almost all cases odor problems come from improper venting of the building's plumbing system. It also points to gastight manhole covers and seals as part of keeping odors in the tank and letting them escape through the roof vents.

Mesa's code backs the cover side: where bolted covers are required, they shall be gastight and watertight.

A manufacturer's guide tells a similar story for a small unit. It says the interceptor is for open system (vented) gravity drainage use only, that it is not a sewer gas trap, that all upstream fixtures must be trapped, and that the drain line is vented per local code. It also says to add a vent if the unit sits one or more floors below the fixtures it serves. That guide is for one 75 gallon-per-minute unit, so treat it as an example of manufacturer wording, not a rule for your tank.

If your kitchen smells like a sewer, a vent is only one possible cause. See why a restaurant smells like sewer and grease trap smell and when it is full.

• Put the vent on the drawings. Phoenix's over-the-counter checklist says an isometric drawing shows pipe size and venting whenever a fixture is rerouted or added. The vent is part of what the reviewer looks at.
• Follow the tank maker's sheet. The IPC says gravity interceptors are installed in accordance with the manufacturer's instructions.
• Check the flow-control vent on any trap. It is easy to miss, and the code sets 6 inches above the flood rim unless the maker says otherwise.
• Keep the drain sloped. Phoenix's amendment sets a slope of not less than 1/4 inch per foot on drainage piping upstream of a grease interceptor, which is a 2 percent slope.
• Keep the lids tight. Scottsdale's text ties the air bound rule to tight covers, so a sealed lid is the reason the vent matters. The lid and traffic rating page covers covers.
• Keep access. Section 1003.10 requires access for service, so a vent route should not block the manways. The location rules page covers clearances.

The full sequence of an install, from plan review to the first pump-out, is on the installation steps page. Sizing is on the worksheet page.

Call one whenever an interceptor is added, moved or replaced. The vent ties into the building's drainage system, and the plumbing plan has to show it. A licensed plumber can lay out the vent, check the flow-control vent on a trap, and draw it into the plan. Our plumbers are licensed and insured, and we give free estimates with upfront pricing. When a grease line is backing up during service, call any hour, since we run 24/7 emergency service. Start at our commercial plumbing page.