A branch in a drainage system is a soil or waste pipe that serves at least two fixtures located on a single story and will connect at it's downstream end(in the direction of flow) to a stack, building drain, another branch or possibly a sump. The upstream end of a branch will connect two or more fixture outlets and may receive discharge from a stack or terminate in a vent, which oddly enough would not be a branch vent.
Areas that may be subject to flooding will often protect the drainage by placing backwater valves on branches that are located below grade; although now, mainline backwater valves are often placed directly on the building drain.
Sizing a Branch and other rules
A branch is sized by the number of drainage fixture units being served along the branch and the size will increase as the fixture units increase. Branches that are acting as a wet vent or circuit vent will need to follow the rules pertaining to those.
- Branches must be run at a minimum grade of 1:50 (1/4" per foot)
- Branches serving water closets must increase to 4" downstream of the third WC.