Does anyone know what the minimun allowed sag in a sewer pipe is?

I guess you are talking about the slope which should be 1/4" per ft.

While 1/4" to the foot is the best the absolute minmun fall allowed on a sewer line is 1/8 inch to the foot.

While 1/4" to the foot is the best the absolute minmun fall allowed on a sewer line is 1/8 inch to the foot.

1/8" grade is only allowed if you increase the pipe by one full pipe size, right?

We just "crack the bubble" on all our 4" installations. No complaints over the years as yet. Cheers, Tom