S-Trap — definition, uses, identification, and replacement

An S-trap is an older S-shaped drain trap that can siphon itself dry and allow sewer gas into a building.

S-Trap diagram — labeled parts, dimensions, and installation context

What It Is

An s-trap uses a vertical drop after the trap bend instead of a properly vented horizontal trap arm. As water rushes through, it can pull the water seal out of the trap and leave the drain open to sewer odors.

That siphoning risk is why modern plumbing codes prohibit new s-trap installations. They still show up in older sinks, floor drains, and basement fixtures that were never updated.

Types

Most residential examples are made from brass, galvanized steel, cast iron, or tubular plastic. The main distinction is not style but whether the old trap has been corrected to a vented P-trap configuration.

Where It Is Used

S-traps are usually found under older sinks, laundry tubs, and basement fixtures where the drain drops straight through the floor instead of running into a vented wall connection.

How to Identify One

Look under the fixture for a drain assembly that curves down and then turns straight into the floor, creating an obvious S profile. Sewer odor, gurgling, or a fixture that smells worse after draining are common clues.

Replacement

Replacement usually means reworking the drain into a properly vented P-trap, often with new trap arm piping and a compliant vent connection or approved venting method. A simple like-for-like swap is not the right fix because the shape itself is the problem.

S-Trap — FAQ

What does a s-trap do?

An S-trap is an older S-shaped drain trap that can siphon itself dry and allow sewer gas into a building. S-traps are usually found under older sinks, laundry tubs, and basement fixtures where the drain drops straight through the floor instead of running into a vented wall connection. In practical terms, it matters because an s-trap uses a vertical drop after the trap bend instead of a properly vented horizontal trap arm. As water rushes through, it can pull the water seal out of the trap and leave the drain open to sewer odors.

How can I tell if the s-trap needs attention?

Persistent sewer odor, gurgling after the fixture drains, or a trap that seems to lose its water seal are the main warning signs. Those symptoms matter because the trap can no longer block gas reliably. Look under the fixture for a drain assembly that curves down and then turns straight into the floor, creating an obvious S profile. Sewer odor, gurgling, or a fixture that smells worse after draining are common clues.

Can a homeowner handle s-trap work, or should I call a pro?

A homeowner can identify an s-trap, but correcting it often requires drain reconfiguration to meet code. If the fixture drains through the floor with no obvious vent, this is usually a plumber job rather than a parts-only swap. If the issue involves hidden leaks, structural support, code compliance, or specialty tools, professional help is usually the better path.

What should I match when buying a replacement s-trap?

Do not shop for another s-trap as the final solution. Match the fixture outlet size, drain location, and venting plan so the replacement can be converted to a proper P-trap assembly. Taking the old part, measurements, or a manufacturer model number with you usually saves time and return trips.