One source specifically states that horizontal 3"+ drains require 1/8" slope (< 3" require 1/4" slope). Another source says 3" drains require 1/4" slope. I am installing a 3" drain under slab that will accomadate a 1/2 bath in a shop. The main 3" drain will run approximately 26' before it enters the septic tank. Do I go with 1/8" or 1/4" slope?

My design is to start the drain with a 3" closet bend. Down stream 2.5' from the bend I will install a 2" to 3" waste T with the 2" entrance at the top. From the waiste T the main 3" drain continues straight for 23' to the septic tank. At the waste T I will use a 2" long sweep 90 degree elbow that I will run horizontally from the waste T for approximately 2.5' before using another 90 degree long sweep elbow which will send this 2" wet vent/drain pipe vertical in order to drain the sink and vent the system. Comments or suggestions would be appreciated.

Thank you,

Kubota_Kid - Shelton, WA