A fall is the amount of slope or angle which a drain line must have in draining away sewage from a house. Sewage is not powered by any suction or air pressure. It moves completely on its own by water pressure, but mostly by gravity.
It is very important that the fall or slope of the piping is paid particular attention to. If the fall isn’t sufficient, the homeowner can find him or herself having problems with the overall drainage of the home.
Amount of Fall
The sewage lines coming from your house need to be higher than the spot where they join into the municipal sewer lines. The average percentage of fall needed is two or three percent, or else the piping will need a pump. The average fall for a drainage system is ¼ inch per linear foot.
Whatever fall you finally set the pipes at, remember that slightly more is better than slightly less in drainage, and to prevent back up or slow drainage, use the recommended fall and have a contractor or engineer find exactly what level the municipal drainage sewer pipe is at.