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From the 2021 UPC Illustrated Training Manual, Chapter 7, SANITARY DRAINAGE

701.3.3 Type. Fittings used for drainage shall be of the drainage type, have a smooth interior water-way, and be constructed to allow 1/4 inch per foot (20.8 mm/m) grade.

The drainage system relies on gravity, not on any pressure system, to move the effluent through the system. In order for the waste to travel down the system the installation must be uniformly smooth without sharp turns or snags. To accomplish this, pipe and fittings are designed and manufactured with smooth bores and transitions between pipe and fittings to provide the best pathway of least resistance (see Figure 701.3.3). Solids are entrained in the effluent and must be transported to their destination without plugging the system. Research and design of pipe and fittings are accomplished for a reason; therefore, the system must be installed correctly or it will not accomplish its designated task. The pipe must be cut, deburred and joined with fittings correctly.

The 2021 Uniform Plumbing Code Illustrated Training Manual is available for purchase here.

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