A horizontal drain pipe collects the discharge from two lavatories, a bathtub and a water closet and carries it over to the soil stack. What is the correct name for that pipe?
- A horizontal branch drain
- BA fixture drain
A fixture drain covers only the run from a single trap to the first junction with other drainage. Once two fixtures combine, the pipe downstream is no longer a fixture drain.
- CThe building drain
The building drain is the lowest horizontal piping that receives everything in the structure. A branch on an upper floor is well upstream of it.
- DA stack offset
An offset describes a stack that jogs sideways and returns to vertical. It is not a pipe that fixtures connect into on a floor.
Why A is correct
Piping that runs horizontally and gathers fixture discharges on its way to a stack or the building drain is a horizontal branch. The name carries its own sizing table and its own fixture unit limits, separate from stacks.
What this question is testing
Checks precise use of drainage terminology, specifically the boundary between a fixture drain and a horizontal branch, which is where most terminology errors happen on plan review.
On the job
The naming controls which column of the sizing table you read. Horizontal branches and stacks share a table but have separate columns and separate limits, and the building drain uses a table that also depends on slope. Say the wrong name on a plan review and the reviewer will size the run from the wrong column, which usually means you get asked for a bigger pipe than the job needs.
Memory technique
One trap makes a fixture drain, two make a branch.
Exam tip
A fixture drain stops at the first junction with any other drain pipe. After that it is a branch.
Where to look it up
IPC Chapter 2 definitions, plus the horizontal branch and stack sizing table in Chapter 7. UPC uses the same concepts with its own tables.