: : : : Minimum 1/4" per foot for 3" and smaller.

: : :
: : : May I add that one does not want to exceed a slope of 1/2" per foot on these same size pipes.
: : : Otherwise, a situation could develop where the liquid waste flows past or first down the drain, potentially allowing the solid waste to get stuck and accumulate, eventually causing a blockage.

: : : If you slope at 1/4" per foot the wastes will travel together and help to prevent drain blockages.

: : : : : what is the recommended slope for drain line, main & branches

: A fast waste will allow the water to outrun and leave the solids. This testing is performed by organizations like IAPMO and ANSI in addition to the toilet manufacturers. hj's calling this code standard a old wives tale is wrong. What is the research you refer to hj? LonnythePlumber

I work in the mountains and septic tanks are frequently set so that I have very sever grades to the tanks. I talked to my inspectors and the health dept guys (govern septic locations) about this since I had always heard the "too much grade leaves the solids behind" story, too. They said that it really was based on older types of drainage pipes that had rough uneven internal walls. The products of today have much smoother internal walls and that the steeper grades did not cause solids to remain...
Deb
The Pipewench