Rate of slope on stool line to septic tank Home › Forums › Public Forums › Drainage & Sewerage › Rate of slope on stool line to septic tank This topic has 1 reply, 2 voices, and was last updated 24 years, 7 months ago by hj. Viewing 1 reply thread Author Posts 19 Apr 2000 at 1:17 am #278520 MasterPlumbersKeymaster We are trying to install a new sewer line (approx. 18 ft.) from the stool to the septic tank…we have a 1.6 gal.stool and need to know the slope for the new line. 19 Apr 2000 at 3:50 am #298680 hjParticipant If the pipe is 3″ then 1/4″ is the minimum. If 4″ then you can reduce it to 1/8″ per foot. Author Posts Viewing 1 reply thread You must be logged in to reply to this topic. Log In Username: Password: Keep me signed in Log In