The building drain; also referred to by some as the "Main Drain", is the lowest horizontal section of drain pipe that serves the most distant water closet(toilet) and terminates to a building sewer. Typically the building drain in most homes would be under the basement floor and begin where the farthest stack or branch that serves a water closet, connects into it, and continue until one meter(1m) outside the foundation wall, where it becomes a building sewer.
Sizing and rules pertaining to building drains
Building drains like all drains are sized by the amount of waste they may receive which is calculated in fixture units. The minimum size of every building drain is 3 inch but must be increased to 4 inch before leaving the building and have a main cleanout close to where it exits the foundation. The size of the building drain is also dependent on the grade of the pipe, as a shallower grade on the pipe will allow it to carry less fixture units, for example if the pipe is 3" it must have a minimum grade of 1/4" per foot (1:50), but a 4" building drain may be graded at 1/8" per foot (1:100).
In some areas where it may be possible for a public sewer to back up and cause flood damage, a building drain can be protected with a backwater valve. Commonly referred to by the manufacture's name, a "Mainline" backwater valve must be open under normal operation and only close when the building drain is subject to flooding, by means of a float type mechanism.