: : It is an "old wive's" tale. Research has found that it does not happen.

: : : : 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