An S-trap, like in the photo above, 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. It’s called an S-trap because of the sideways S-loop it makes before it heads directly downward.

   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. 

    See our blog post What plumbing traps are illegal by code? for a list of other prohibited trap designs.  

• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

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

• 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.