If you have questions or comments, join the discussion on our LinkedIn forum.

From the 2021 UPC Illustrated Training Manual, Chapter 10, TRAPS AND INTERCEPTORS

1001.2 Where Required. Each plumbing fixture shall be separately trapped by an approved type of liquid seal trap. This section shall not apply to fixtures with integral traps. Not more than one trap shall be permitted on a trap arm. Food waste disposers installed with a set of restaurant, commercial, or industrial sinks shall be connected to a separate trap. Each domestic clothes washer and each laundry tub shall be connected to a separate and independent trap, except that a trap serving a laundry tub shall also be permitted to receive the waste from a clothes washer set adjacent to it. The vertical distance between a fixture outlet and the trap weir shall be as short as practicable, but in no case shall the tailpiece exceed 24 inches (610 mm) in length. One trap shall be permitted to serve a set of not more than three single compartment sinks or laundry tubs of the same depth or three lavatories immediately adjacent to each other and in the same room where the waste outlets are not more than 30 inches (762 mm) apart, and the trap is centrally located where three compartments are installed.

The requirement for fixtures within the building to be trapped by a water seal (see Figure 1001.2a) has been in plumbing codes for almost 100 years. The trap’s main purpose is to eliminate the possibility of sewer gas from entering the confines of the room or building. The water seal, or trap seal, effectively accomplishes this by creating a barrier of 2 to 4 inches of water that prevents sewer gas from entering the room. A secondary benefit is that it prevents access of pests or vermin to drains, as well as access to the building through the fixture by way of the sewer system.

Each fixture must be separately trapped. Some fixtures such as water closets and urinals have integral traps and do not require an additional trap since the water seal is built in its structure (see Figure 1001.2c).

The trap arm, the piping from the end of the trap to the inner edge of the vent connection, may only serve one trap (see Figures 1001.2b and 1001.2d). This eliminates the possibility of one trap being siphoned as the other fixture is used. The flow along the trap arm passing the other trap connection could siphon the unused trap. This is the reason why each trap must be protected by a separate vent as is required in Section 1002.2 and in Section 901.2, Vents Required.

The continuous waste provides a method of draining multiple compartments to a single trap. The continuous waste is limited to up to three compartments, sinks or lavatories, but only if they are within the 30 inch limitation. In a three-sink configuration the trap must be located under the
center sink. This is to provide uniform drain sizing and to shorten the distance to the trap and its drain (see Figures 1001.2e, 1001.2f and 1001.2g).

The installation of a food waste disposal in a commercial kitchen is normally under the jurisdiction of a health department. In some areas, this installation may even be prohibited. Commercial kitchen sinks will often be connected to a grease interceptor or hydromechanical device so depositing food waste into these devices will negatively affect their function. When they are allowed to be connected to the drainage system they must be installed on a separate trap. This also means that they will have a separate trap arm, vent, and fixture drain. This will effectively separate the sink from the disposal, and if the disposal line becomes plugged, the sink will still function (see Figure 1001.2h).

If a clothes washer and laundry sink are both to be installed in the same area, they must each have their own drainage connection. If a laundry sink (sometimes called a laundry tray or tub) is installed but there are no connections for a clothes washer, then the clothes washer may be set next to the sink and drain into it. The laundry sink compartment will be able to handle the pumped waste from the clothes washer because of its volume.

The fixture tailpiece should be as short as possible to limit the fall of waste into the trap. Excessive vertical lengths of the tailpiece will increase the effective head pressure above the inlet of the trap by increasing the height of the column of water, which may be sufficient to induce
self-siphonage. As the length of the column increases, so also the velocity of the moving column of water, increasing the momentum pull on the trap seal. Research has proven that limiting the tailpiece length to 24 inches will help to eliminate the possibility of the trap to self-siphon. The tailpiece may offset but the developed length of the tailpiece must not exceed 24 inches (see Figure 1001.2i).

Other methods developed to eliminate self-siphonage of the trap are the limitation of trap arm lengths (see Section 1002.2) and the provision of a vent for the trap (see Section 1002.1).

The clothes washer standpipe is not the tailpiece for a clothes washer. The standpipe for the clothes washer is considered to be an indirect waste receptor rather than a tailpiece and, therefore, is limited by Section 804.1, Standpipe Receptors, and not by this section of the code.

The 2021 Uniform Plumbing Code Illustrated Training Manual is available for purchase here.

IAPMO Memebers: An archive of the Code Spotlights is now available. Log in at iapmomembership.org and click the link in the left menu.

(This email is not to be considered the official position of IAPMO, nor is it an official interpretation of the Codes.)