How do I tell whether I have a P-trap or an S-trap?

Look at what happens after the U. A P-trap turns horizontally into the wall. An S-trap turns downward through the floor. If it goes down, the seal can be siphoned out every time the fixture drains.

Why does the smell come back after I run the faucet?

Because the seal is being pulled out rather than evaporating. Evaporation takes weeks. If it returns in days on a fixture in daily use, the trap shape or the venting is the cause.

Are S-traps against code?

Modern plumbing code requires a properly vented P-trap, which is why S-traps are found in older work and in DIY installations rather than in new ones. We do not issue code determinations — a plumbing inspector does — but we will tell you which one you have.

Can a handyman change a trap?

Yes — replacing a trap and re-piping the trap arm to a vented P-trap is repair and fixture work. What we do not do is new drain and vent stack work, which is licensed plumbing scope and is arranged separately.

What about a basement floor drain that smells?

Same cause, and the easiest fix on the list: pour a bucket of water down it, and a little cooking oil on top slows the next evaporation. Floor drains are the fixture people forget exists.

Do you charge to come and diagnose a smell?

No. There is no diagnostic fee and no trip charge, which matters here because half of these turn out to be a dry trap and thirty seconds of running water.

P-Trap vs S-Trap: Why Your Drain Smells | Lansing, MI

P-Traps vs. S-Traps: The Ultimate Guide to Your Home's Drain Traps

The curved pipe under a sink holds a plug of water, and that water is the only thing between your house and the sewer. When a drain smells, that seal has gone — and there are only two reasons it goes.

It evaporated. A guest bathroom or a basement floor drain nobody uses will dry out in a few weeks. Run the faucet for thirty seconds and the smell goes with it. That is not a repair, and anyone charging you for one is selling you a faucet.

Or it is being siphoned out. An S-trap — the shape that turns down again after the bend, common in older Lansing houses and in anything a previous owner plumbed themselves — pulls its own seal down the drain as the basin empties. If the smell returns within days on a fixture you use daily, that is what you have. The fix is re-piping the trap arm to a P-trap with a proper vent, and it is a repair-level job, not a re-plumb.

The Critical Purpose of a Drain Trap: The Liquid Seal

P-traps and S-traps are two types of plumbing drain traps, and they serve the same critical purpose: to prevent harmful and foul-smelling sewer gases from entering your home.

They achieve this by using a U-shaped bend to hold a small amount of water, which creates a liquid seal between your sink or tub and the sewer line. Without this seal, noxious gases from the sewer could easily flow straight up into your living spaces.

Think of the water in the trap as a permanent, invisible barrier that keeps the bad air where it belongs.

What is a P-Trap? (The Modern Standard)

The P-Trap is the industry standard for modern plumbing and the design you will find in virtually every new home and professional remodel.

It gets its name because its shape resembles the letter "P" turned on its side. It features a horizontal pipe that runs toward the wall, connecting to a vertical pipe called the drain stack. This connection to the drain stack is the key to its success.

Key Advantages of the P-Trap:

Venting Success: The P-Trap works seamlessly with your home's vent system (the pipes that run through your roof). This vent system introduces air into the drain line, preventing a siphon effect.
No Siphoning: Because the P-Trap is properly vented, the water seal is rarely pulled out by sudden drainage (a process called siphoning). This ensures the sewer gas barrier remains intact.
Easy Cleaning: The curved section is usually accessible, making it easier to clean out dropped items (like jewelry) or clear minor clogs.

What is an S-Trap? (The Outdated Hazard)

The S-Trap is named because its shape resembles the letter "S." It was common in older homes (pre-1930s or earlier) but is now banned by modern plumbing codes in most regions.

The issue with the S-Trap is that it connects directly from the fixture, vertically down into the main drainpipe in the floor, creating a deep, uninterrupted "S" curve.

Why the S-Trap is a Problem:

Self-Siphoning: When a large volume of water flows through an S-Trap (like after flushing a toilet on the same line or even simply draining a full sink), the momentum of the water often creates a strong vacuum. This vacuum siphons the standing water out of the trap, completely breaking the liquid seal.
Sewer Gas Entry: Once the water seal is lost, sewer gases have a direct, open path into your home, leading to bad smells and potential health concerns.
Code Violation: If you find an S-Trap in your older home, it should be replaced with a correctly vented P-Trap to meet current safety standards.
‍

__wf_reserved_inherit — Before Drain that was place

Actionable DIY Tip: Keeping Your Trap Working

The greatest enemy of your drain trap is lack of use. If you have a sink or shower that isn't used often (like in a guest bathroom), the water in the trap can evaporate over time, breaking the seal.

Simple Fix: Once every few weeks, simply run water in all unused sinks for about 30 seconds. This action replenishes the water barrier and ensures your trap is functioning as designed.

‍

Need Help Upgrading Your Home's Plumbing?

If you suspect you have an outdated S-Trap or are dealing with persistent drain odors, the Haslett Handyman team can professionally assess your plumbing system. We ensure all your drains are correctly vented and equipped with modern, code-compliant P-Traps for lasting safety and comfort.

Contact us today to schedule a plumbing inspection!

‍

Plumbing repairs — traps, drains and fixture work
Slow or clogged shower drain — if it is slow as well as smelly
Leaking toilet base? How to fix the wax seal — the other common source of a sewer smell