A horizontal tee is not a problem for pressurized water supply piping, but will not function properly for horizontal drain pipes, which only flow by gravity and need a directional nudge at corners. The International Plumbing Code at 706.3 requires that “fittings shall be installed to guide sewage and waste in the direction of flow.” Also, horizontal tee or sanitary tee connections are specifically banned by the code at Table 706.3, FITTINGS FOR CHANGE OF DIRECTION, except for when the branch of the tee is used for venting. 

    The photo above shows an example of the wrong way to tie two sink drains into a single waste arm. The effluent from one trap is likely to surge over into the other trap, and then splash back, before heading in the desired direction.

    One of our plumber friends advised us that the technical term for this defect is “shittyus maximus,” which he says is Latin for “installed by homeowner.” The 90º double elbow fitting (also called a double quarter-bend elbow) shown below would be acceptable. A 45-degree wye fitting, double combo, combination wye and one-eighth bend branches, or other approved equivalent configuration is also correct.

    Although a horizontal-to-vertical use of a sanitary tee under a kitchen sink (as shown below) is a less likely to cause a drain problem, it is also not a rated application. A 90º double tee is also the correct fitting here.

And, just for giggles, here’s an example of the wrong use of a sanitary tee, connected to an unvented S-trap, exiting through an unrated accordion-type tailpiece pipe. Very ingenious, Rube-Goldberg-style handyman work on a kitchen remodeling that is totally wrong.

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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 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? 

• 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.