- 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).