Oil built-ups within the walls of the pipe are the number one reason for drain blockages. As we all know, oil and water don't mix. Naturally, the "drain line carry" removes all solids that go into a drain by the use of gravity. Using a 2% grade works well when using gravity as the source. Anything less than 2% won't allow water to flow correctly and thus allow solids to built up faster within pipes. Anything higher than 2% will make all solids within the pipe flow faster than water and cause blockages further down the pipe. That is why 2% of the grade will allow solids and water to flow better, accordingly creating a self-scouring effect.
All plumbing systems need to be up to current plumbing codes, but even then, drain pipes still get clogged after time if you don't have them routinely cleaned. It's like atherosclerosis.
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