Shower drains rely on gravity to function, so drain pipes must slope downward away from the shower drain and toward the waste line resulting in the septic tank or tank. At a rough-in, the drain line is laid out from the location of the shower enclosure to the waste line leading to the septic tank or tank.
No Set Method
There’s no specific method for setting the thickness of a shower drain rough-in, but pipes codes typically need that shower drains drop by at least a quarter-inch each foot of run to ensure adequate drainage. That would mean a 12-foot-long shower drain line must drop by at least 3 inches between the shower drain and also the point where it connects the main waste line. Plumbing codes also want a water-filled P-trap in the shower drain line to avoid backup of sewer or septic gasoline to the shower enclosure, so an installation must allow room for the snare.