Why can't a sanitary tee be used for a horizontal-to-horizontal drain pipe connection?
Saturday, June 16, 2018
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 sanitary tee connections are specifically banned by the code at Table 706.3, FITTINGS FOR CHANGE OF DIRECTION, in the code.
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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