Why do restrooms clog during big events at churches and event venues?

Plumbing loads are worked out on the assumption that people don't all flush together. NIST describes the reasoning behind fixture units this way: "Since it would not be expected that all fixtures in a building would be used at the same time, the curves incorporate the probability of use." That holds on a normal Tuesday. A break in the program is the opposite case, and the design assumption is at its weakest. (Our page on how commercial drains are sized walks through the math.)

The fixture count has the same limit. The IPC says fixtures "shall be provided in the minimum number as shown in Table 403.1, based on the actual use of the building or space," with the number of occupants set by the International Building Code. It's a minimum. Phoenix's amendment replaces the UPC's own fixture table with IPC Table 403.1, so both codes use that table here. A sanctuary that fills for one big service can meet the minimum on paper and still have a line at the door.

The EPA's advice: "Flushing anything other than toilet paper, including disinfecting wipes, can damage internal plumbing, local sewer systems and septic systems." Wipes labeled flushable are part of that, and our page on whether flushable wipes are really safe covers the labels. INDA, the nonwovens trade group, wrote in 2018 that findings "show consumers are flushing too many products not designed to be flushed."

At an event, a lot of people who don't know your building use the restrooms in the same short window, so whatever gets flushed goes down together. PERC, an industry research group, says feminine hygiene products in particular "are known to contribute to drainline blockage problems." A bin in every stall and a sign is cheap insurance.

PERC's FAQ says the U.S. was in a market transition toward 1.28 gallon per flush toilets. It reports that its first study concluded the use of 1.28 gpf toilets "performed predictably during testing," and that the second phase "does illustrate the need for caution regarding continued reductions in toilet consumption volumes." PERC says it set out to replicate a commercial office building with no cafeteria or other feature giving long-duration flows into the building drain. It also warns that a test drain that didn't overflow doesn't mean a real one won't, since a real building drain is likely to include "snag points, such as horizontal junctions."

PERC's concern was long horizontal commercial drains with little other flow. If your venue's restrooms sit at the end of a long run like that, ask your plumber to look at it before the next event.

A branch drain that runs flat, sags or is partly blocked is the first place a surge shows up. Slope is a factor: see what slope a drain needs by code.

Venting is the part people forget. IPC Section 901.2 calls for vent piping so that the seal of any fixture trap is not subjected to a pressure differential of more than 1 inch of water column. If a vent can't pass air, the system isn't doing what the code intends, and a plumber can check the vents as part of a clog call. Our page on telling a main line clog from a branch clog helps sort that out.

• Put covered bins in every stall and a "toilet paper only" sign at eye level.
• Run a full flush test on every toilet and urinal a few days ahead, and fix slow ones now. See why a toilet keeps clogging.
• Know where your cleanouts are, and get a sewer camera inspection if the building has a history of backups.
• Keep a plumber's number handy for the day itself.

HQ does commercial plumbing, including drain clearing and sewer camera work, with 24/7 emergency service, upfront pricing and free estimates. If your venue clogs every time the crowd shows up, we'd rather find the branch or vent problem before your next event.