What is an S-Trap? Why is it a problem?

Sunday, October 14, 2018

An S-trap looks like the photo above: kind of a sideways S-loop configuration before it heads directly downward. This type of drain has been banned by the building codes for decades, because it is considered an “unvented” drain. Venting is necessary for a sink to drain properly. In essence, sufficient air has to get into the pipe in order to displace the water and allow it to exit easily.    A simple experiment can demonstrate the problem with an unvented drain. Place your thumb over a straw that is partially immersed in a glass of water. As you lift the straw out of the glass, the water level in straw stays intact, rising about the surrounding water in the glass. When you release your thumb, the water in the straw drains to the level of the rest of the water in the glass.

   Although an S-trap is able to gulp some air for displacing the draining water, so it’s not exactly like the straw experiment, the air is not sufficient for good drain flow. Plus, S-traps tend to suck out the water seal in the trap (necessary to keep sewer gas from rising up into through the sink into the home) as they finish draining.

   A properly installed P-trap, like in the diagram above, will always keeps it’s water seal. If you have S-trap drain, and notice sewer-type odors in the room, you can run the water slowly down the drain for a few seconds to replenish the trap-seal as a temporary fix. But, of course, the best solution is having a licensed plumber bring the drain piping up to modern standards. 

Here’s an example of an S-trap that might not be obvious at first glance. Both sinks have a pipe configuration that creates an S-trap, with the potential for siphoning. 

   Another requirement of an efficient plumbing drain system is at least one vent pipe that extends above the roof, to allow sewer gas that rises into the drain piping behind the sink traps to exit into the atmosphere. The vent pipe also provides atmospheric pressure to the drains. 

   Occasionally, we see vent pipes that terminate in the attic or on an exterior wall near a window. Both of these installations allow the possibility of sewer gas migrating back into the home.

   Although TV home-improvement shows sometimes make installing your own plumbing look easy, doing it right requires both training and years of experience. Plumbing repairs should always be performed by a licensed plumber. The plumbing inside your home is so critical that every building department in the United States requires plumbers to be licensed.

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Here’s links to a collection of our other blog posts about PLUMBING PIPES:

How can I protect my pipes to keep them from bursting during a hard winter freeze in North Florida?

Can galvanized steel pipe still be used for new water lines in a house? 

How can I tell if I have cast iron pipes in my house? 

Why can't a sanitary tee be used for a horizontal-to-horizontal drain pipe connection? 

What is the difference between green and white sewer drain pipes?

Is a washing machine drain hose required to be secured at the standpipe?

What are the abandoned pipes sticking out of the wall in my house?  

What are the code requirements for plumbing vent terminations?

What are the code requirements for layout of drain piping under sinks?

What causes a gurgling sound when a bathtub or sink drains? 

What is a "combination waste and vent" in a plumbing system? 

What is a building trap?  

What is a galvanized nipple?

What are the pipes sticking out near my water valves?

How do you accurately find a broken water pipe leak under the floor slab?

What is the difference between water pipe and sewage (waste) pipe? 

Are plastic pipes (PVC, CPVC, and PEX) safe for drinking water? 

Is a hot water faucet handle required to be on the left? 

What is a dielectric union? 

What's that powdery crust on the pipe connections at the water heater? 

If all the plumbing drains have water in them and you can still smell sewer gas, what's causing the problem?  

How can I tell what type of plumbing pipe I have?

Why is there a flexible accordion pipe under the sink? 

What is the difference between PVC and ABS plumbing pipe?

What is the difference between water service pipe and water supply pipe? 

What are the pipes on my roof? 

• How can I find out what type of water pipe runs underground from the water meter to the house (service pipe)?

What is a P-trap?

Why is old galvanized steel water pipe a problem for homebuyers?

What does polybutylene pipe look like? Why is it a problem? 

• Which water pipes are an insurance problem and possibly uninsurable?

• Can you connect CPVC pipe directly to a gas water heater?  

     Visit our PLUMBING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles. 

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