Just about every time we call out an unvented “S-trap” in a home inspection, like the one shown above, someone says “Is that some new code thing? It probably wasn’t required when this house was built.” But actually it's been around for quite awhile.

   The original 1948 edition of the Uniform Plumbing Code (UPC) required venting for all drains, and all the codes we know of that address plumbing standards require venting. Properly designed venting is necessary because unvented systems can have all of these problems:

• The fixture may drain slowly.
• It may make a gurgling sound while draining because it is trying to gulp air and drain liquid out of the same drain opening.
• An unvented S-trap also sometimes sucks all the water out of the trap as it drains. The trap water seals the drain against sewer gas rising up, so a stinky sewer smell wafts through the house. The methane gas can also be dangerous. No fart jokes here, this is serious.

    And, of course, another reason is that the building codes—all of them that include plumbing standards—require venting. The traditional way to vent plumbing fixtures is with pipes through the roof. A home is required to have at least one, and it is often encased in a lead sleeve “roofing boot.” But it is also possible to install an air admittance valve near the plumbing fixture, which opens as necessary to provide air in the pipes for drainage, and then closes afterwards.

    To learn more, see our blog posts What is an auto vent, air admittance valve, or check vent? and What is a P-trap? and What is an "S-Trap" under my sink? Why is it a problem? and What is a "combination waste and vent" in a plumbing system?

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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? 

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

• 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? 

• 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. 

 Illustration - Code Check