Q: On a current kitchen remodel, the sink has an S-trap instead of a P-trap, which my plumber says does not meet code. Why is an S-trap bad, and what’s the best way to retrofit a P-trap?

A: John Smith, a licensed plumber in Harwich, Mass., responds: Your plumber is correct in his assessment. In simplest terms, an S-trap is shaped like an “S” and a P-trap is shaped like the letter “P” if both are lying on their side (see S-Traps Vs. P-Traps, below). With an S-trap, the drainpipe drops down from the sink and into a conventional trap. It then loops over and exits downward. In a P-trap configuration, the drain also comes down from the sink and into the trap, but instead of looping over and back down, the drain enters a horizontal run before exiting downward.

The water in a plumbing trap creates a seal to keep out sewer gases. S-traps are not allowed because they can create a siphon that leaves that seal partially open. A P-trap works with a vent to equalize air pressure and allow the trap to remain full of water. Illustration by Tim Healey.

Before discussing the drawbacks to an S-trap, we first need to understand how sink traps operate. A plumbing trap is supposed to stay full of water to seal the pipe and block sewer gases from escaping through the drain. The trap (and drain) can’t work properly without a properly installed vent in the system (see “Plumbing Vents Explained,” Jun/99). A vent allows air to move freely in the system, which in turn allows the trap to maintain that water seal. As water flows from the sink into the drain, air is pushed out of the pipe via the vent. When the sink is empty, water flows back into the trap from the pipe and air is sucked back in through the vent, equalizing the pressure to keep the trap full of water.

The horizontal run of a P-trap allows water to drain and air to move in the pipe to maintain the trap seal. But an S-trap loops back down with no way for air to get into the pipe. If enough water (say a sinkful) drains through an S-trap, a siphon can be created that sucks water out of the trap, leaving it partially open and unable to seal out sewer gases. Another potential problem is that when draining, other plumbing units can cause a vacuum that will suck the water out of an S-trap.

Have your plumber remove the S-trap and install a P-trap that exits through the back of the cabinet and into the wall. He can then either tie into an existing vent or run a new vent up through a stud bay and through the roof. If rerouting the drain isn’t feasible, you may be allowed to install an air admittance valve. Check to make sure that these devices are allowed in your local jurisdiction.