# What's the difference between horizontal branch & building drain?



## Michael.L (May 8, 2019)

I'm looking at IPC Table 915.2.2 and there's a significant difference between the maximum DFUs allowed for a horizontal branch versus for a building drain. I realize that a building drain is the lowest point of a building's waste piping and that a horizontal branch can exist on elevated levels. But what if the building is single story and fixture drains are connected to horizontal piping that eventually slopes to the building drain without dropping down a stack? In other words, where does the "horizontal branch" end and where does the "building drain" begin in that scenario?


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## Builder Bob (May 8, 2019)

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


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## Michael.L (May 8, 2019)

Builder Bob said:


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


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?


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## Builder Bob (May 8, 2019)

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 ------


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## Michael.L (May 8, 2019)

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


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.


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## jar546 (May 8, 2019)

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.


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## Builder Bob (May 9, 2019)

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?


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## north star (May 9, 2019)

*@ * @ * @*

The Building Drain is THE main drainage piping in
the structure and at the lowest point in the structure
( the Primary ).........It receives wastes from other
Horizontal Drain Piping ( the Secondary ), and other
soil and waste piping......All wastes inside the structure
are directed to the Building Drain, which then carries
it to a point 30" outside the exterior wall, at which point
the Building Drain Piping is now defined as the
Building Sewer Pipe.

Does this help ?

*@ * @ * @*


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## Paul Sweet (May 10, 2019)

I think the real question is why somebody pulled different numbers out of the air when they wrote the plumbing codes a century or so ago.


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## ADAguy (May 27, 2019)

Has it been that way for that long?


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## HForester (May 27, 2019)

Sometimes there just needs to be a number to establish a rule that everyone can follow. Doing work to the code is just about following rules; not questioning why the rule exists or the reasoning behind the rules.  Everybody else has to follow the rules so there is an even playing field (and also to make the installation safe).  People who don't follow the rules (trying to cut corners/save money) are eventually found out and either forced to follow the rules or end up going away from the enforcement jurisdiction ("gee, those guys are tough to please!")


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