Homes built before 1965 may use a double sanitary tee to accommodate drainage from two toilets that are installed back-to-back.
The double tee may not be effective for installations using bulk performance flush toilets (Pressure Lite®,
Class Five®, and Class Six®).
Bulk performance flush toilets forcefully flush the water out of the bowl.
Forceful flushing can cause some of the water to cross through the double tee and travel into the waste line of the toilet installed directly behind the toilet being flushed.
New construction uses a 45° double wye fitting for back-to-back toilet installations.
The "Y" connector directs the water downward, eliminating direct flow into the opposite waste line.