I don't know where you came up with your info, but the building codes in my area are specific,
Waste line slope-¼ inch to 3 inches per foot
Pipes must slope slightly downhill to drain properly. The standard slope is anywhere from ¼ inch to 3 inches per foot depending on the use. An accurate calculation is essential for properly functioning plumbing
It's not just about water in the lines, it's about keeping "solid waste" moving. Faster slope causes water to drain too fast leaving solids behind, and flatter slope doesn't have enough force to push solids down line.
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