P Trap vs. S Trap: Which Drain Configuration Wins for Safety & Code?
P Trap and S Trap are two drainpipe bends under a sink. The P Trap forms a “P” shape, looping back to connect horizontally to the wall. The S Trap forms an “S” shape, dropping straight down through the floor. Both hold water to block sewer gas, but only the P Trap meets modern plumbing codes.
Homeowners renovating older houses often spot an S Trap and think it’s just an upside-down P Trap. Contractors may reuse existing floor-drain lines to save time, creating a stealth code violation that inspectors flag later. The mix-up happens because both look like simple U-bends—until airflow and venting come into play.
Key Differences
P Trap vents to the wall, preventing siphonage and sewer odor backflow; it’s legal everywhere. S Trap drops straight to the floor, can self-siphon dry, and is banned in most jurisdictions for health safety.
Which One Should You Choose?
Choose P Trap for every new or remodel job—install a proper vent or use an Air Admittance Valve if a wall stack is hard to reach. Remove or retrofit any S Trap to stay code-compliant and odor-free.
Examples and Daily Life
Under a new IKEA vanity: P Trap with slip-joint kit, 1 ¼ in. tailpiece, and mini vent tucked in back. Under a 1950s kitchen island: old S Trap replaced by P Trap looped inside cabinet to nearby vent stack.
Can I legally keep an old S Trap if it hasn’t caused problems?
No—code inspectors will require conversion during any permit work.
Does a P Trap cost more to install?
Usually $10–20 extra for vent fittings, but it prevents future repairs.