An S-trap is a drain trap shaped like an “S”; codes ban it because it can self-siphon and let sewer gas leak indoors.
What Is An S-Trap And Why It’s Not Allowed Anymore
An S-trap is a curved section of pipe under a sink or fixture that looks like a sideways “S.” It holds a small pool of water to block sewer gas. When water rushes through, the outlet drops straight down, which can pull that seal right out of the trap. That loss of water is the whole problem.
Modern rules call for traps that keep a steady seal and work with venting. A P-trap sends the outlet horizontally to a wall or floor drain line that is vented. That side outlet and the vent keep pressure steady, so the seal stays put. With an S-trap, the outlet goes down, the flow can build suction, and the water seal can vanish after a strong drain event.
Trap Types And Code Status At A Glance
| Trap Type | How It Works | Code Status & Notes |
|---|---|---|
| P-trap | “U” bend with a horizontal outlet that ties into a vented line; the vent balances pressure. | Allowed in standard installs; default choice for sinks and similar fixtures. |
| S-trap | Looks like an “S”; outlet drops downward, which can pull the water seal during draining. | Not allowed by major codes; banned due to self-siphon risk and sewer gas concerns. |
| Drum/Bottle trap | Chambered body that stores water; can clog and hide debris. | Often not allowed except niche uses; check local rules for any narrow allowances. |
| Integral fixture trap | Trap built into the fixture (some floor drains, some appliances). | Allowed when listed for that fixture; follow the listing and instructions. |
| AAV with P-trap | P-trap plus an air admittance valve to relieve negative pressure on the trap arm. | Accepted in many places; some areas restrict or bar AAVs. Rules vary by jurisdiction. |
How An S-Trap Loses Its Seal
When you dump a basin of water through an S-trap, the column of water in the vertical leg can pull the bend dry. That is classic self-siphon. Next time air moves in the drain, gas from the stack can slip past the empty bend and into the room. You might notice a rotten-egg odor, a gurgle after the drain empties, or both.
Venting stops that chain reaction. A vent supplies air so the flow does not build suction in the trap arm. A P-trap ties into a vented branch; the trap seal sits calm. An S-trap does not have that horizontal run to a vent, so it is prone to pulling itself dry. Long story short: shape and venting decide whether the water seal stays or goes.
Is An S-Trap Allowed? What Codes Say And What To Use
Major model codes list S-traps in their “prohibited traps” sections. You can see this stated in the International Plumbing Code, and city rulebooks echo the same stance, like NYC Buildings which lists “Bottle, Bell + S Traps: Not permitted.” Federal rules for manufactured housing follow the same direction in 24 CFR 3280.606.
So what do you install? A standard P-trap connected to a vented branch. In a typical kitchen or bath sink, a trap arm runs from the P-trap to a sanitary tee in the wall. A vent rises from that tee, and the drain continues down. That layout lets air in and keeps the trap seal stable.
Where AAVs Fit In
An air admittance valve opens to admit air when the drain pulls a slight vacuum, then closes. Many areas allow AAVs for island sinks, remodels, or spots where a vent tie-in would mean heavy wall work. Some areas require a traditional vent through the roof and do not allow AAVs at all. Before you shop parts, read the local rulebook or permit guide so you match the allowed method.
Special Cases You Might See
Floor drains: Many floor drains have integral traps, sometimes paired with a trap primer to keep water in the bend. Old basements may still show S-traps on floor drains; those fail modern inspection in most places.
Old drum or bottle traps: Older baths and lavs used these for space reasons. They clog and hide debris. If you’re opening a wall, plan for a P-trap and a vented tie-in.
Mobile or factory-built homes: Federal rules match the same bans. Follow the listing on the fixture and the federal section linked earlier.
Fixing An S-Trap Under A Sink
Most swaps are straightforward: remove the S-trap, install a P-trap, and run a proper trap arm into a vented sanitary tee. If the stub-out in the wall is missing or too low, you’ll need to open the wall and set a tee at the right height, then add a vent rise. If a roof vent is tough to reach, your rulebook may allow an AAV high in the cabinet; mount it above the trap arm weir as the listing requires.
Parts You’ll Need
- Listed P-trap kit in the correct size (most sinks use 1-1/4″ or 1-1/2″).
- Trap adapter for the wall stub-out or new sanitary tee, as required.
- Trap arm pipe and slip-joint nuts with washers.
- Primer and cement for PVC/CPVC or appropriate couplings for ABS, or slip-joint parts if all parts are tubular brass/plastic.
- Optional: AAV and adapter if your area allows it, plus a riser to set it high and accessible.
Steps At A Glance
- Place a bucket under the trap. Loosen the slip-joint nuts and remove the S-trap.
- Dry-fit the P-trap: the “U” under the tailpiece, the trap arm horizontal toward the wall.
- Check the trap arm slope: a slight fall toward the wall is enough. Keep the arm short within the allowed trap arm length for your pipe size.
- Connect to a vented sanitary tee in the wall. If there’s no vented tee, install one and run a vent up. If an AAV is permitted, mount it high and accessible.
- Tighten slip-joints hand-tight, then add a small turn with pliers. Fill the sink and test for drips and gurgles.
Common Mistakes
- Using a deep seal or adding a second trap. Double traps and extra-deep seals are not allowed and cause slow drains.
- Letting the trap arm drop before it reaches the tee. Keep that section horizontal with the slight fall noted above.
- Burying an AAV. Valves need air and access for future replacement.
Costs, Difficulty, And When A Pro Makes Sense
A simple under-sink swap with a nearby vent usually calls for basic hand tools and a mid-range P-trap kit. A wall rework costs more, since you need a tee, new pipe, and patching. Island sinks, long runs to a stack, or a finished tile backsplash can push the job into pro territory fast. If gas odor persists after a DIY fix, or the drain gurgles and burps every time, bring in a licensed plumber to sort out venting and height.
Troubleshooting After The Swap
Gurgling sound: That points to negative pressure at the trap. Shorten an overlong trap arm, fix a sag, or confirm the vent is open. If you used an AAV, make sure it sits above the trap arm weir and opens freely.
Slow drain: Check that the trap arm has that slight fall and that the sanitary tee in the wall points the right way. Hair and debris in a pop-up can mimic a trap issue, so clear that first.
Odor near the sink: Look for leaks at slip-joints, a cracked trap, or a dry trap after a long vacation. Run water to refill the seal. If odor returns fast, the vent path may be blocked.
Real-World Retrofit Scenarios
Older homes and tight cabinets throw curveballs. The table below maps common setups to fixes that pass inspection in many places. Use it as a planning guide, then match the layout to your local rulebook.
| Scenario | Best Fix | Notes |
|---|---|---|
| Pedestal or wall-hung lav with S-trap | P-trap to wall stub-out with sanitary tee and vent rise | Hide the tee inside the wall; set the stub-out centerline at a standard height. |
| Kitchen sink on an island | P-trap with loop vent or listed AAV (if allowed) | Loop vent needs a return line to the stack; AAV must stay accessible and high. |
| Vanity with low floor drain connection | Raise the outlet into the wall; add a vented tee | A floor drop makes an S-trap by default; move the tie-in up to a wall branch. |
| Old drum trap in a bath | Remove drum, install P-trap and vented tee | Plan for cleanout access and new finishes after wall work. |
| Shared trap on two basins | Separate P-traps to a double fixture tee with a vent | One trap per fixture arm; use a listed double fixture tee for back-to-back bowls. |
Quick Myths And Facts About S-Traps
“My S-Trap Hasn’t Smelled In Years, So It’s Fine.”
Many go unnoticed until a large discharge or a long dry spell pulls the seal. The absence of odor today doesn’t mean the setup is safe or passable at inspection.
“I’ll Fix Siphonage With A Deeper Trap.”
Extra depth does not fix the pressure problem. You still get a strong pull on the seal, and deep seals can slow flow and trap debris.
“A Bigger Drain Pipe Solves It.”
Pipe size doesn’t cure an unvented path. The trap needs air to balance pressure. That comes from a vent or an approved valve where allowed.
“Adding A Second Trap Helps.”
Two traps in a row make things worse. You end up with a pocket that stalls flow and hammers the first seal. Model rules forbid double trapping for that reason.
Safe Alternatives That Pass Inspection
Standard P-trap to a vented tee: This is the gold standard. The trap sits under the fixture, the arm runs into a tee, a vent rises, and the drain drops. Simple layout, steady seal, clean, and serviceable.
Island loop vent: For islands, a loop runs up inside the cabinet, then returns to the drain line and ties into a vented stack under the floor or nearby. It takes planning, but it gives the trap the air it needs without a roof run at that point.
Listed AAV on a P-trap: Where allowed, a valve can save a wall tear-out. Mount it high inside the cabinet and keep it accessible. Follow the listing for height above the trap arm weir and clearance.
Trap primer for floor drains: If a floor drain dries out between uses, a primer feeds a small amount of water to keep the seal. That stops odor without oddball trap shapes.
Code-Friendly Design Tips For A Clean Install
Trap Arm Length And Slope
Keep the trap close to the wall. Stay within the allowed trap arm length for your pipe size and give it a gentle fall toward the tee. A long, sagging arm creates a mini-siphon even with a P-trap.
Correct Fittings
Use a sanitary tee for the trap arm connection in a vertical stack, not a tee-wye or a straight tee in the wrong plane. A long-turn elbow helps on direction changes downstream of the tee.
Cleanout Access
Plan a cleanout where you can reach it. Hair clogs and food bits happen. A reachable cleanout cuts future mess and keeps snakes out of the trap itself.
Health And Comfort Payoffs
A working trap does two jobs: it blocks odor and it blocks pests that travel damp piping. That water seal is your door against gas like hydrogen sulfide. When the seal holds, the room smells clean and the drain sounds quiet. When the seal fails, you get odor, gurgle, and a draft you don’t want near food or a toothbrush.
Main Points You Can Act On Today
- If you see an “S” under a sink, plan a swap. A P-trap with a vented tie-in is the standard fix.
- Use listed parts, keep the trap arm short and level with a slight fall, and set fittings the right way.
- For islands or tight remodels, check whether AAVs are allowed and mount them high and accessible.
- Keep a little water in traps you don’t use daily. Run water after trips to refill any that sat dry.
- Odor or gurgle after a fix points to venting or layout. Shorten the run, clear the vent, or have a licensed plumber realign the tee and arm.
Why This Topic Still Matters During Remodels
Many S-traps are still out there, tucked in older vanities and kitchen bases. They pass from owner to owner because the fixtures still drain. A code-compliant swap removes guesswork, stops after-flush gurgle, and sets you up for a smooth inspection when you sell or refinance. It also brings your layout in line with modern safety rules, which means fewer callbacks and fewer musty odors down the road.
Taking An S-Trap Out And Using A P-Trap The Right Way
Measure twice. Dry-fit everything before glue touches pipe. Keep the trap weir level, the arm short, and the vent path open. If the wall outlet sits below the trap, raise it. Do not loop pipe downward between the trap and wall. That shape recreates the same siphon you set out to remove.