- Q.My client’s kitchen sink has a shallow 6-inch-deep bowl, and the drain connection is located in the wall rather than the floor. She would like to replace the sink with one that has a much deeper bowl, but that will place the bottom of the bowl at nearly the same level as the drain opening. Is it okay to use a deeper trap and the same drain opening, or will the drain opening need to be moved to accommodate the deeper sink?
A.Mike Casey, a licensed plumbing contractor and co-author of Code Check Plumbing, responds: When a trap is too shallow, the trap seal — the slug of water that remains in the trap and blocks sewer gas from entering the house — is more likely to evaporate. When the trap is too deep, liquid may flow through it too fast, which can lead to siphoning of the trap seal and sludge buildup.
The drain connection at the wall is too high on this sink, resulting in a 9-inch-deep trap seal — 5 inches deeper than is allowed by code.
To prevent these problems, plumbing codes require that the water seal in a trap be between 2 and 4 inches deep (2006 IRC, P3201.2). If installing a sink with a deeper bowl results in a trap seal deeper than 4 inches, you’ll have to open up the wall and relocate the trap arm connection at the drain (see photo, above). Remember, too, that the trap arm on the discharge side of a trap must be at least two pipe diameters long and should slope 1/4 inch per foot toward the vertical discharge (2006 IRC, P3105.3).