i am in the process of building an addition that will have a full bath in it. i am close to doing the ground work for the bathroom (slab on grade addition) and have conflicting advise on the size of the waste line to the septic. i have been told to use 4" line by some and 3" line from others. i will be tying into the existing line to the septic tank with a vertical drop hook up. i thought 4" would leave too much open pipe and therefore increase the risk of liquid out running solids and that the 3" would drain better with less risks??