Does a parking garage need an oil separator or sump pump?

Not when cars only park there. The 2024 International Plumbing Code (IPC) sets the oil separator rule in Section 1003.4, and it names the places that need one: "At repair garages where floor or trench drains are provided, car washing facilities, factories where oily and flammable liquid wastes are produced and hydraulic elevator pits, oil separators shall be installed." Parking is not on that list. Section 1003.4.2.2 then closes the gap directly: "Parking garages in which servicing, repairing or washing is not conducted, and in which gasoline is not dispensed, shall not require a separator." It adds that "Areas of commercial garages utilized only for storage of automobiles are not required to be drained through a separator."

So the trigger is activity, not the word "garage." A few things change the answer:

• A service bay, wash bay or fuel island inside the structure. Those areas fall under the repair, wash and fuel language, and the code sizes the separator by the area drained. Our page on auto shop floor drains and the sewer covers that case.
• A hydraulic elevator pit. Many parking structures have an elevator, and the code lists hydraulic elevator pits for separators. It carries an exception: "An oil separator is not required in hydraulic elevator pits where an approved alarm system is installed."
• A fleet or valet operation that washes vehicles on site. Washing is named in the same section, so a wash bay is treated as a wash bay no matter what the building is called.

Where a separator is required, it sits ahead of the rest of the plumbing. The code says the oily wastes go into the separator "before emptying into the building drainage system or other point of disposal." It must also be "listed and labeled" or designed to the code's own sizing, with "a depth of not less than 2 feet" below the invert of the discharge drain.

In Phoenix, the city can ask for more than the model code does. Phoenix City Code Section 28-12 says "Interceptors must be installed and operated by laundries, food service establishments, service stations, auto repair shops, carwashes, commissaries, and other facilities when the Director determines they are necessary for the proper handling of liquid wastes containing grease or oil in excessive amounts, or any flammable wastes, solids, or other harmful ingredients." A garage is not named, but "other facilities" and the Director's judgment leave the door open if a site is discharging oily waste.

Phoenix also amends IPC Section 1003.2 so that approval of every interceptor and separator rests with the local authority. The amended text says "Wastes that do not require treatment or separation shall not be discharged into any interceptor or separator." That cuts both ways. You should not add a separator the city has not asked for, and you should not run unrelated waste through one. We checked the amendment list for the 2024 IPC: Phoenix amends Sections 1003.2 and several 1003.3 grease interceptor sections, but not Section 1003.4 on oil separators or Section 712 on sumps.

Other Valley cities adopt their own sewer and pretreatment rules, so a garage in Mesa, Tempe or Scottsdale should confirm with that city's plan review before design. The 2024 IPC is the starting point everywhere, and the local office has the last word.

Only if some drains cannot flow to the sewer by gravity. Below-grade levels are the usual reason. IPC Section 712.1 says "Building subdrains that cannot be discharged to the sewer by gravity flow shall be discharged into a tightly covered and vented sump from which the liquid shall be lifted and discharged into the building gravity drainage system by automatic pumping equipment or other approved method." If the lowest level sits higher than the sewer, gravity does the work and no pump is needed.

If a sump is needed, the code sets the basics. The pit must be "not less than 18 inches (457 mm) in diameter and not less than 24 inches (610 mm) in depth, unless otherwise approved," with a solid bottom and a gastight removable cover. Section 712.3.1 adds that "The sump pump capacity and head shall be appropriate to anticipated use requirements." That last line is where design work comes in, since a garage floor can take a lot of water at once. Our pages on whether a building needs a sewage ejector or lift station and on sump pumps versus ejector pumps explain the equipment side.

Rainwater and groundwater are a separate path, and a below-grade garage often has its own storm-side sump for them; our page on sump pumps versus ejector pumps explains how the IPC treats that one. Phoenix's sewer code bars "any stormwater, surface water, groundwater, roof runoff, surface drainage, or unpolluted water" from the sanitary sewer unless the Director approves it. The IPC also recognizes that parking structures drain to storm systems. In multilevel structures, a floor drain that discharges to a building storm sewer is not required to be individually trapped. Where such drains go to a combined building sewer, they must connect through a main trap instead. Check how the rain on the open upper decks leaves the building before you assume the same drains serve everything.

Plans can be wrong, and renovations change things. EPA warns, in its guidance on service-bay drains, that "Any plans for your building that show wastewater flow may not necessarily reflect the actual construction of your wastewater system." It also notes that "Your local health department or a plumber using dye or smoke tests may be able to help you determine where your drain goes."

A practical order of operations:

1. 1Walk the garage and list every drain, including elevator pits, wash areas and the lowest level.
2. 2Pull the as-built plumbing plans and the city's permit file.
3. 3Trace each drain with dye or a camera, and note where it ends: sanitary sewer, storm drain, sump or something else.
4. 4Write down any activity that puts oil, solvents or wash water on the floor.
5. 5Ask the city's wastewater or plan review office whether that activity needs an interceptor.

If a drain turns out to feed a dry well or septic system, stop and get that corrected. EPA says "Typical motor vehicle waste disposal wells are floor drains or sinks in service bays that connect to a septic system or dry well," and the same issue applies to any garage drain that handles vehicle waste.

HQ Plumbing & Air does commercial camera inspection of building drain lines, which helps show where interior garage drains actually run before a permit question comes up. If you are dealing with a low spot or a drain that backs up, see our guide to how floor drains, trench drains and area drains differ and whether a loading dock drain needs an oil separator, which follows the same code language for a different building type.