Every floor drain in every building in the world relies on the same 200-year-old invention to keep sewer gas out: a small pool of water sitting in a curved pipe. It is called a P-trap, and when it works, you never think about it. When it fails, you smell it immediately.

This article explains how P-traps work, why they fail far more often than most facility managers realize, and what options exist for buildings where water-based seals are not reliable enough.

A P-trap holds water in the curved section of pipe. This water blocks sewer gases from entering the building.

How a P-trap works

A P-trap is a U-shaped bend in a drain pipe. When water flows through the drain, some of it stays behind in the curved section. This standing water creates a seal between the building interior and the sewer system below.

The concept is simple: gases cannot pass through liquid. As long as water sits in the trap, sewer gas stays in the sewer. The P-trap is required by plumbing codes in every state and has been the standard method of drain sealing since the 1800s.

Every plumbing fixture has one. Sinks, toilets, showers, and floor drains all use a variation of the same water-seal principle. The difference is that sinks and toilets get used regularly, keeping the trap continuously recharged with fresh water. Floor drains often do not.

Why P-traps fail

The entire system depends on one thing: the water staying in the trap. When that water disappears, the seal is gone. There is nothing between the building and the sewer.

Evaporation

Water evaporates. In a floor drain that receives no regular water flow, the trap water can evaporate completely in 2 to 3 weeks. In hot climates, dry climates, or spaces with HVAC airflow over the drain, it can happen in under a week.

This is the most common cause of P-trap failure. It affects every building with floor drains in low-traffic areas:

  • Mechanical rooms and utility closets
  • Vacant tenant spaces in commercial buildings
  • Seasonal hotel rooms and resort wings
  • School buildings during summer break
  • Hospital corridors and storage areas
  • Warehouse and industrial floor drains
2-3 wks Typical time for a P-trap to dry out
85% of buildings have at least one dry trap
200+ years old: the P-trap design

Capillary action and wicking

Debris that accumulates in the trap (hair, sediment, cleaning product residue) can act as a wick, pulling water out of the trap faster than normal evaporation. This is especially common in drains that are cleaned infrequently.

Wind and pressure fluctuations

In tall buildings, wind loads and stack effect can create pressure differentials in the drainage system. These pressure changes can siphon water out of traps, particularly on upper floors. A gust of wind on a high-rise can empty a trap in seconds.

Infrequent use

The simplest cause: nobody uses the drain. A floor drain in a storage room, a shower in an unoccupied hotel room, a restroom in a closed school wing. If water does not flow through the drain regularly, the trap will eventually dry out. It is not a question of if. It is a question of when.

What happens when a P-trap fails

When the water seal disappears, the drain becomes an open pipe connected directly to the sewer. Three things can now enter the building:

1. Sewer gas

Sewer gas is a mixture of hydrogen sulfide (H2S), methane, ammonia, and carbon dioxide. Even at low concentrations, hydrogen sulfide causes headaches, nausea, and eye irritation. At high concentrations, it can be fatal. The characteristic rotten-egg smell is the most recognizable symptom of a dry trap, but by the time you smell it, occupants have already been exposed.

2. Pests

An open drain is an open door. Drain flies breed in the biofilm inside drain pipes and emerge through open traps. Cockroaches, mosquitoes, and even rodents can travel through sewer lines and enter buildings through failed traps. There are no registered pesticides specifically approved for drain fly control, which means pest control services cannot solve a problem that originates from an open drain.

3. Pathogens

In healthcare facilities, dry traps are a documented source of hospital-acquired infections. Peer-reviewed research has linked drain biofilms to outbreaks of carbapenem-resistant organisms (CRE), MRSA, Pseudomonas aeruginosa, and other antibiotic-resistant pathogens. When the trap seal fails, aerosolized bacteria from the drain can enter patient care areas.

From the research: A 2018 systematic review documented 23 hospital outbreaks of carbapenem-resistant organisms traced to wastewater drains. Chemical disinfection of the drains repeatedly failed. Physical barrier interventions showed the most consistent success.

Traditional solutions and their limitations

Manual flushing

The oldest approach: pour water down the drain on a schedule. This works as long as someone remembers, the schedule is maintained, and every drain in the building is included. In practice, manual flushing breaks down during vacations, staff turnover, building closures, and weekends. The traps that need attention most (in unused areas) are the ones most likely to be forgotten.

Trap primers

A trap primer is a mechanical or electronic device that periodically sends water to the trap to keep it charged. Trap primers solve the evaporation problem but introduce new ones:

  • Water consumption: A single continuous-flow trap primer can use over 52,000 gallons of water per year. Even pressure-drop models consume thousands of gallons annually.
  • Maintenance: Trap primers have mechanical components (valves, solenoids, sensors) that require regular maintenance. Mineral deposits, debris, and valve failures are common.
  • Failure modes: When a trap primer fails, it fails silently. The trap dries out and the building has no warning until the odor appears or an outbreak occurs.
  • Cost: Between water consumption, maintenance, and replacement, trap primers create ongoing operational expenses that compound over the life of the building.

Use our Water Savings Calculator to see the annual water and cost impact of trap primers in your building.

Chemical treatments

Enzyme cleaners, drain deodorizers, and chemical sealants provide temporary relief. They do not prevent evaporation. They do not create a physical seal. And they must be reapplied on a recurring basis. In school districts, many chemical treatments raise Proposition 65 compliance concerns.

The alternative: waterless trap seals

A waterless trap seal is a mechanical device that creates a physical barrier in the drain without relying on water. Instead of a pool of liquid that can evaporate, it uses a one-way valve that allows water to flow down but blocks everything from coming back up.

Green Drain is a silicone one-way valve that drops into the existing floor drain body. It installs in 30 seconds with no tools and no plumbing modifications. The valve opens when water flows through (cleaning, mopping, equipment drainage) and closes automatically when the flow stops. Because the seal is mechanical, not liquid, it never evaporates.

Green Drain creates a mechanical seal using a one-way silicone valve. Water flows down normally. Gas, odors, and pests cannot travel back up.

Key differences from traditional P-trap maintenance:

  • Zero water consumption to maintain the seal
  • No mechanical components to maintain or replace (no valves, solenoids, or sensors)
  • No evaporation risk during building closures, summer breaks, or vacancy
  • 30-second installation with no tools or plumbing modifications
  • 13 certifications including cUPC, ASSE 1072, NSF/ANSI 2, and HACCP International

Which buildings benefit most

Any building with floor drains in low-use areas benefits from addressing P-trap failure. The highest-impact applications include:

  • Hospitals and healthcare facilities — where dry traps can lead to pathogen transmission and hospital-acquired infections
  • K-12 schools — where 8-12 weeks of summer break guarantee trap dry-out
  • Restaurants — where drain flies and sewer odor affect health inspections and customer experience
  • Hotels — where seasonal rooms and low-occupancy wings create drain odor problems
  • Commercial office buildings — where vacant tenant spaces become odor sources

Frequently asked questions

How often should I check my P-traps?

If you are relying on water-based seals alone, every floor drain in the building should be checked and recharged at least monthly. Drains in low-use areas (mechanical rooms, vacant spaces, seasonal wings) should be checked more frequently. Buildings with waterless trap seals can reduce this to an annual visual inspection.

Can I just pour water down the drain periodically?

Yes, this works temporarily. The challenge is consistency. Facility teams manage hundreds of drains across multiple floors and buildings. Even one missed drain can create an odor, pest, or health issue. Waterless seals eliminate the need for ongoing manual intervention.

Are waterless trap seals code-compliant?

Green Drain is cUPC listed and ASSE 1072-2020 certified. It is accepted by plumbing authorities in all 50 states. See our state plumbing code guide for specific jurisdiction details.

How long does a waterless trap seal last?

Green Drain's medical-grade silicone valve is tested to over 2,500 open-close cycles. Under normal building conditions, this translates to years of maintenance-free operation. The device is designed to be a permanent replacement for ongoing P-trap maintenance.