I have been directly (first hand) involved in the design, inspection and construction of a few million feet of sanitary sewers. All of our 6 inch laterals were designed at 1% minimum slope (12 inches per 100 feet)(we don’t use 4 inch laterals but they can have the same slope). Greater slopes up to 1/4+/- inch per foot (2.5 %) are fine. All of our designs are reviewed and approved by state government authorities prior to construction, by law. All designs must comply with “Ten States Standards” and state law.
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