What size sewer line does a commercial building need?

The IPC uses a unit called a drainage fixture unit, or DFU. Each fixture gets a DFU value that reflects how much waste it can put into the pipe. You add up the DFU of every fixture that drains to a pipe, and then you look up the smallest pipe that can carry that total.

The lookup is in Section 710.1, on the maximum fixture unit load. It says the maximum number of DFU connected to a given size of building sewer, building drain or horizontal branch of the building drain "shall be determined using Table 710.1(1)." That table gives the maximum DFU for each pipe diameter at four slopes: 1/16, 1/8, 1/4 and 1/2 inch per foot. These values are from the 2024 IPC; confirm the adopted edition and amendments for your project.

Here are some of the values in Table 710.1(1), in DFU:

Two things stand out. A steeper slope lets the same pipe carry more, and a larger pipe carries far more. A 4-inch line can carry 180 DFU at a 1/8 inch per foot slope, while a 3-inch line at the same slope carries 36.

The table's footnote sets the floor: "The minimum size of any building drain serving a water closet shall be 3 inches."

Table 709.1 lists the DFU for fixtures. Some public-use fixtures have higher assigned loads than their private-use equivalents. A few values from the 2024 table:

• A public water closet at 1.6 gallons per flush counts 4 DFU. A public water closet flushing more than 1.6 gallons counts 6.
• A flushometer tank water closet, public or private, counts 4.
• A lavatory counts 1, and a service sink counts 2.
• A urinal counts 4. A urinal at 1 gallon per flush or less counts 2, and a nonwater urinal counts one-half.
• A commercial automatic clothes washer counts 3.
• A floor drain counts 2 in the table row for floor drains, and floor sinks and some floor drain cases point to Section 709.4 for sizing.

The code also says that for computing loads on building drains and sewers, "water closets and urinals shall not be rated at a lower drainage fixture unit unless the lower values are confirmed by testing." So a designer cannot pick a smaller number for a low-flow fixture just because it saves water.

Take a small single-story office with eight public water closets at 1.6 gallons per flush, eight lavatories, two floor drains and one service sink.

• Eight water closets at 4 DFU each is 32.
• Eight lavatories at 1 each is 8.
• Two floor drains at 2 each is 4.
• One service sink at 2 is 2.

The total is 46 DFU. Table 710.1(1) shows that a 3-inch pipe can carry 50 DFU only at a slope of 1/2 inch per foot, which is very steep for a long run. A 4-inch pipe carries 180 DFU at a slope of 1/8 inch per foot. In practice a designer would choose the 4-inch size for this load because it has plenty of room and a gentle slope. The table shows what the code allows. The designer picks a size that fits the site and the city.

This is a simplified example. Real sanitary plans include the fixtures, approved equipment discharges and pretreatment requirements. Stormwater belongs in the appropriate storm-drainage system; IPC 1101.3 prohibits stormwater drainage to a sewer intended for sewage only. Do not count roof runoff as a sanitary discharge. Change any of those and the total changes.

Yes, because the table gives a different limit at each slope. The code also sets a minimum slope for horizontal drainage pipe in Table 704.1. Pipes of 2 1/2 inches or less need 1/4 inch per foot. Pipes of 3 to 6 inches need 1/8 inch per foot. Pipes of 8 inches or larger need 1/16 inch per foot.

Phoenix amends Section 704.1. Its version says that where the drainage piping is upstream of a grease interceptor, the slope must be not less than 1/4 inch per foot. It also adds an exception. The Authority Having Jurisdiction may approve a lesser slope for building sewers in place of a sewage ejector or pumping station when a registered engineer or architect certifies the design and the owner agrees in writing under notary. That exception matters on a flat site, where a pump is the alternative. Our page on whether a building needs a lift station or sewage ejector pump covers that choice.

The plumbing code sets the minimum, and the city controls the connection. In Mesa, for example, Section 8-4-8(C) of the city code says: "The number, location, manner of connection, and size of all building connections shall be subject to the approval of the Development Services Manager." Other cities have their own rules and sometimes their own tap sizes, so a size that works on paper still has to match what the city will accept at the main.

Size also affects maintenance. The IPC puts cleanouts on a building sewer smaller than 8 inches at intervals of not more than 100 feet. A building sewer of 8 inches or larger needs a manhole within 200 feet of the junction with the building drain, and then at intervals of not more than 400 feet. Our guide to cleanout code requirements explains where cleanouts go. Who owns and maintains the line also depends on the city, so see who is responsible for the sewer line in Phoenix.

How should you use this?

• Owners and managers: ask for the DFU count on the plans, and ask what size and slope the designer chose.
• Tenants planning a build-out: adding fixtures can push a shared line past its limit. Ask the landlord for the line size.
• Anyone replacing the whole drainage system: where the entire sanitary drainage system of an existing building is replaced, the code says the existing building sewer shall be internally examined to check that it slopes correctly, is not broken, is not obstructed and is sized for the load. That text is in Section 703.4 of the 2024 IPC. Replacing a single failed line does not trigger it.

If a line is undersized or has lost its slope, the symptoms are slow drains and repeat backups. HQ Plumbing and Air can inspect the line with a camera to look for damage, obstructions and standing water. Confirming its size and grade may also require plans or additional measurements. We are licensed and insured, we give free estimates, and you get the price up front.