A building sewer(sanitary) is a continuation of a building drain that is 1m outside of the building's exterior wall and the building sewer will either lead to a private disposal field or a public sewage system. The reason the name of the pipe changes from a building drain to a building sewer after it leaves the building is because different rules apply to it; such as:
- Minimum size of a building sewer is 4"
- Change of direction and grade is limited without additional cleanouts
- Different joining and piping materials may be permitted
Size and slope of a building sewer
The sizing and minimum slope for a building sewer is dependent on the number of fixture units draining into it, but as a rule of thumb, since every building sewer has to be at least 4" any home will be more than served by that size. The grade of a building sewer at 4" is allowed to be 1/8" per foot(1cm per meter) and even less if the size of the pipe is larger. Consult your local code for acceptable practices in your area.