Are S-Traps, Flexible Drains, or Sanitary Tees on Their Back Allowed? (IRC 2018)

Quick Answer

No. S-traps, flexible accordion drain tubes, and sanitary tees used as 90-degree changes in horizontal drain direction are all prohibited by IRC 2018. S-traps lose their water seal due to self-siphoning, flexible accordion drains create debris accumulation and siphoning, and sanitary tees turned on their back create turbulent flow that causes blockages. Each of these is a prohibited fitting or use under IRC 2018 Section P3005.1 and the general prohibited trap provisions of Section P3201.

What P3005.1 Actually Requires

IRC 2018 Section P3005.1 governs changes in direction in drain, waste, and vent piping. The section specifies that drainage pipe changes in direction must use appropriate fittings for the type of change being made. Specifically: horizontal to horizontal changes in direction must use long-sweep 1/4 bends, 1/6 bends, 1/8 bends, or wye fittings — sanitary tees (also called sanitary crosses or single-sweep tees) are approved only for horizontal-to-vertical changes and for vertical-to-horizontal only in a limited configuration; they are not approved for one horizontal run to another horizontal run because the short-sweep turn inside the fitting creates turbulence and is prone to blockage.

S-traps — trap configurations in which the trap outlet runs straight down through a floor and the trap arm is horizontal and short — are prohibited because S-trap configurations are prone to self-siphonage. When a fixture drains quickly (such as a bathtub or lavatory), the discharge velocity can draw the water out of the trap seal, leaving the fixture unprotected from sewer gases. The IRC requires that P-trap configurations with proper trap arm venting be used for all fixtures.

Flexible accordion drain tubes — the ribbed plastic corrugated tubes commonly sold at hardware stores for connecting a sink drain to the trap — are not approved under IRC 2018. The accordion ridges inside the tube accumulate grease, soap, and hair, creating chronic clogs. These tubes are often listed for "emergency use" only, and many plumbing codes explicitly prohibit them for permanent installations. While P3005.1 does not call them out by name, the general requirement that all drain fittings and pipe be listed and suitable for the use, combined with the prohibition on fittings that impede drainage, effectively prohibits accordion tubes in permanent installations.

A sanitary tee used on its back — turned 90 degrees from its designed orientation so the branch serves as a horizontal run rather than a vertical drop — creates turbulence at the transition point and is a prohibited use under P3005.1. The correct fitting for combining two horizontal drain runs into one horizontal continuation is a wye-and-1/8-bend combination or a long-pattern combination wye.

The drainage flow direction requirements that govern fitting selection under P3005.1 also apply to vent connections. A vent connection to the drainage stack must be made above the highest anticipated drain connection to prevent back-flooding of the vent during peak flow conditions. Connections must be made using sanitary fittings such as sanitary tee or wye rather than straight tee fittings with dead-leg branches that can trap waste. The sanitary tee is designed with a swept entry that directs flow from the branch in the direction of stack flow, not perpendicular to it, which would create turbulence and potential blockage. Straight tee fittings with flat perpendicular branch connections must not be used as drain connections because waste can collect in the perpendicular branch and solidify into a blockage over time. This fitting requirement applies to both 3-inch and 4-inch main stack connections and to 1.5-inch and 2-inch branch connections for lavatories, tubs, and showers.

Why This Rule Exists

Drain system design is based on hydraulic principles that govern how water and air flow together through a pipe. Fittings that create abrupt changes in direction, trap siphoning conditions, or accumulation zones disrupt the hydraulic balance of the system. An S-trap that loses its seal allows sewer gas — including hydrogen sulfide, methane, and potentially pathogens — into the occupied space. A sanitary tee on its back creates a turbulence zone that rapidly accumulates debris and requires frequent service calls. These prohibitions reflect lessons learned from failures in older plumbing systems.

This requirement reflects the fundamental principle of the IRC that electrical and mechanical systems must be installed in a manner that protects occupants over the life of the building, not just at the moment of installation. Proper installation documented at inspection provides future owners and service technicians with confidence that the system was built to code, reducing liability and preventing disputes about pre-existing conditions.

This requirement reflects the fundamental principle of the IRC that electrical and mechanical systems must be installed in a manner that protects occupants over the life of the building, not just at the moment of installation. Proper installation documented at inspection provides future owners and service technicians with confidence that the system was built to code, reducing liability and preventing disputes about pre-existing conditions.

What the Inspector Checks at Rough and Final

At rough inspection, the inspector checks the drain piping configuration, including all fitting types and orientations. The inspector will flag: any S-trap configuration (horizontal trap arm connecting to a straight downward pipe without a proper vent), any sanitary tee used on its side for a horizontal-to-horizontal connection, and any flexible accordion tubing in the drain system. At final inspection, the inspector checks the drain-trap-vent configuration at each fixture and may observe a flow test where suspect configurations are present.

What Contractors Need to Know

Use P-traps on all fixtures. The trap arm must extend horizontally to a vent or a vented drain before dropping vertically — never directly down. Keep trap arms at 1/4-inch-per-foot slope and within the maximum unvented length per the fixture drain size. Use wye-and-1/8-bend or long-sweep combinations for horizontal-to-horizontal changes in direction. Never use a sanitary tee for a horizontal-to-horizontal connection even if the fitting is turned to make it seem like it could work. For kitchen sink connections, use solid PVC or ABS drain tubing — never accordion flex drain.

When identifying fittings at the supply house or jobsite, look for the DWV marking on sanitary fittings versus industrial-grade fittings. Drain fittings rated for DWV service under the referenced ASTM standards include the appropriate swept radius geometry. Compare the sweep radius of a fitting against the manufacturer's specification sheet. A fitting sold as a drain tee with a sharp perpendicular branch inlet is not compliant with P3005.1 regardless of its material. For underground drain connections and sewer connections, use fittings marked with the ASTM standard and the manufacturer's listed certification on the fitting body. Unmarked or unrecognizable foreign-manufactured fittings should be rejected on the jobsite even if they appear to be the correct shape. Inspectors look for fitting markings during rough plumbing inspection and will require replacement of any fittings that cannot be verified as compliant with the applicable ASTM standard.

What Homeowners Get Wrong

Hardware stores sell accordion flex drain tubes and S-trap configurations as drain parts, which leads homeowners to believe they are acceptable. The accordion tube in particular is routinely used as the under-sink drain connector by homeowners and even some plumbers, but it is prohibited for permanent use in most code jurisdictions. Another error is installing a P-trap that immediately drops straight down without a horizontal trap arm — this creates an S-trap by default. The trap arm must extend horizontally at least 2 pipe diameters before connecting to the drain stack.

State and Local Amendments

IRC 2018 states — TX, GA, VA, NC, SC, TN, AL, MS, KY, and MO — follow the fitting and trap prohibitions of P3005.1 and P3201 without significant modification. Some states explicitly list accordion drain tubes in their prohibited fittings list; others rely on the general prohibition on fittings that impede drainage. IRC 2021 added more explicit language in P3201 (prohibited traps) calling out S-traps by configuration rather than just by historical name, making it easier for inspectors to identify prohibited configurations regardless of what a particular product is marketed as.

When to Hire a Licensed Plumber

Drain system design — determining where vents are needed, what fitting types to use at each change in direction, and how to configure multiple fixture connections — requires training that goes beyond basic DIY knowledge. A licensed plumber understands the hydraulic reasons behind each fitting prohibition and can design a drain system that will pass inspection. Homeowners who install their own drain systems using hardware store components frequently install S-traps, accordion tubes, and incorrect fittings that fail inspection and must be reworked.

Common Violations Found at Inspection

• S-trap configuration at a lavatory or kitchen sink — trap connects directly to a vertical drain drop without a proper horizontal arm and vent
• Flexible accordion drain tube used as the sink tailpiece-to-trap connection
• Sanitary tee used on its back as a horizontal-to-horizontal drain fitting
• No vent connection to the trap arm — trapped fixture relies on stack ventilation through a long unvented arm
• Trap arm slope exceeding 1/4 inch per foot — accelerates self-siphoning
• Trap arm length exceeding the maximum unvented distance for the pipe diameter and slope
• Combination wye used with the branch end connecting a horizontal run in the wrong orientation (creating a sanitary-tee-on-side equivalent)
• Crown vent (vent at the top of the trap bend) — prohibited because water is drawn into the vent during discharge