What's the difference between horizontal branch & building drain? (2024)

Builder Bob said:

Basically one is inside and the other is outside the building.

Click to expand...

See Also

Double Combination Tee-Wye (Horizontal)What Is A Wye In PlumbingHow to Hook Up 2 Dishwashers to One Drain: An Easy Guide - HomeGearGeekSanitary drainage pipe system basics

How is "inside" and "outside" defined? For a commercial building, would a service corridor leading to the loading dock be considered inside or outside? What about a shopping mall with an open-air atrium that can be closed in the winter? Is it "inside" when the roof is closed and "outside" when the roof is open?

Builder Bob said:

Use your common sense ----- the building drain will be the last piece that connects the building sewer to a point 30" outside the exterior wall of the building. Look at the definitions in the plumbing code

Click to expand...

Unfortunately, one cannot quote "common sense" to a building inspector. I understand the gist of what you're saying, but I need a more rigorous understanding of the difference between these two definitions. If the definitions were clear to me, I would not have asked this question in the first place.

I understand that the Building Drain is essentially the trunk of the drain pipes that collects and carries waste to 30" beyond the exterior wall of the building. And a Horizontal Branch Drain makes sense as a section of piping that moves wastewater horizontally from multiple fixture drains to a stack.

But the definition for a Horizontal Branch Drain is circular and thus confusing:

Horizontal Branch Drain: A drainage branch pipe extending laterally from a soil or waste stack or building drain, with or without vertical sections or branches, that receives the discharge from two or more fixture drains or branches and conducts the discharge to the soil or waste stack or to the building drain.

If a Building Drain is what conducts the wastewater to the exterior of the building, then how can a Horizontal Branch Drain conduct wastewater from a Building Drain to the Building Drain? And if there are no stacks in the drainage system, then where does the Horizontal Branch Drain end and the Building Drain begin?

I am not trying to be pedantic; the distinction between the Building Drain and Horizontal Branch Drain is important because Table 915.2.2 lists different maximum DFUs between the two types of piping.

Per the IPC:

BUILDING DRAIN. That part of the lowest piping of a drainage system that receives the discharge from soil, waste and other drainage pipes inside and that extends 30 inches (762 mm) in developed length of pipe beyond the exterior walls of the building and conveys the drainage to the building sewer.

HORIZONTAL BRANCH DRAIN. A drainage branch pipe extending laterally from a soil or waste stack or building drain, with or without vertical sections or branches, that receives the discharge from two or more fixture drains or branches and conducts the discharge to the soil or waste stack or to the building drain.

Apply the language from the definitions from the IPC = The building drain does not receive anymore flow into it from the last branch connection -and then leads to 30" outside the building - on the purple 4" line sanitary line, the last section or bit of piping is at the lowest point and extends 30 inches outside of building is the building drain..........Next to grease trap.

Does this help?