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

[Michael.L]

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?

 

[Builder Bob]

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

 

[Michael.L]

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?

 

[Builder Bob]

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 ------

 

[Michael.L]

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.

 

[jar546]

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.

 

[Builder Bob]

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?

 

[north star]

@ * @ * @

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 ?

@ * @ * @

 

[Paul Sweet]

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.

 

[ADAguy]

Has it been that way for that long?

 

[HForester]

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!")

 

[AesclepianDream]

Running into this same problem in 2025. "The definition of horizontal branch seems circular in it reference to building drain. What's the difference?"

Like me, OP (#5) is also getting hung up on the 'from' and 'to' literalism in the definitions of horizontal branch, regarding a building drain.

I think the definition is doubly referrential, but not necessarily circular:

A horizontal branch extends laterally *from* the building drain -- is only a reference to its final termination as a defined entity; and

it conveys waste or soil *to* the building drain -- is a defintion of which way the fluid is expected to flow.

This, plus the fact that it's lateral and collects from multiple other drains is all that defines a horizontal branch. Everyone else (and esp. #8) has already given the defining characteristics of a building drain.

I hope this helps.

 

[jar546]

Running into this same problem in 2025. "The definition of horizontal branch seems circular in it reference to building drain. What's the difference?"

Like me, OP (#5) is also getting hung up on the 'from' and 'to' literalism in the definitions of horizontal branch, regarding a building drain.

I think the definition is doubly referrential, but not necessarily circular:

A horizontal branch extends laterally *from* the building drain -- is only a reference to its final termination as a defined entity; and

it conveys waste or soil *to* the building drain -- is a defintion of which way the fluid is expected to flow.

This, plus the fact that it's lateral and collects from multiple other drains is all that defines a horizontal branch. Everyone else (and esp. #8) has already given the defining characteristics of a building drain.

I hope this helps.

That’s not quite right. The horizontal branch drain and the building drain aren’t the same thing, even if they’re both horizontal and carry combined flow from multiple fixtures. The 2024 IPC defines them as two distinct components with different roles in the drainage system.

A horizontal branch drain is a lateral pipe that picks up from two or more fixtures and connects to either a stack or the building drain. It’s upstream — part of the collection process.

The building drain is the lowest pipe inside the building that all the soil and waste pipes flow into. It starts where those branches end and runs to 30 inches beyond the exterior wall before becoming the building sewer.

So while they’re connected and part of the same system, they’re not interchangeable. One’s a contributor, the other’s the collector. Definitions make that pretty clear.

 

[wwhitney]

Suppose we're plumbing the DWV on a single story house on a slab, and under the slab the horizontal drain pipes are all pitched at 1/4" per foot with no other vertical offsets of any kind. Then just considering those horizontal under slab drains, how do we determine the dividing line between the building drain and the horizontal branches (if any)?

The definition of "building drain" uses the limiter "lowest". That can't be taken literally, since all horizontal pipes are sloped, so the literally lowest portion would just be a little sliver of pipe 30" in developed length outside the footprint of the building. So it needs to be taken as "lowest tier". In other words, in the example above, all of the horizontal drain piping under the slab could be considered "lowest" and thus meets that portion of the definition.

I don't see any other limiters in the definition of "building drain" that would apply to the example above, so to me the simplest answer is that all of those horizontal under slab drains are in fact the building drain. Curious what other people think.

One obvious way to subdivide those horizontal under slab drains would be to limit the building drain to the portion that is the same size as the drain exiting the building. But the definition has no such verbiage.

Cheers, Wayne

 

[jar546]

Suppose we're plumbing the DWV on a single story house on a slab, and under the slab the horizontal drain pipes are all pitched at 1/4" per foot with no other vertical offsets of any kind. Then just considering those horizontal under slab drains, how do we determine the dividing line between the building drain and the horizontal branches (if any)?

The definition of "building drain" uses the limiter "lowest". That can't be taken literally, since all horizontal pipes are sloped, so the literally lowest portion would just be a little sliver of pipe 30" in developed length outside the footprint of the building. So it needs to be taken as "lowest tier". In other words, in the example above, all of the horizontal drain piping under the slab could be considered "lowest" and thus meets that portion of the definition.

I don't see any other limiters in the definition of "building drain" that would apply to the example above, so to me the simplest answer is that all of those horizontal under slab drains are in fact the building drain. Curious what other people think.

One obvious way to subdivide those horizontal under slab drains would be to limit the building drain to the portion that is the same size as the drain exiting the building. But the definition has no such verbiage.

Cheers, Wayne

Wayne, I get where you're coming from, but this feels like a classic case of reading too much into the definitions instead of just looking at how the system functions. The IPC doesn't just toss out terms like horizontal branch drain and building drain without purpose. They're defined separately for a reason.

A horizontal branch drain is any lateral pipe that picks up flow from two or more fixtures and ties into a stack or the building drain. That makes the building drain a receiving pipe, not just one of many running under the slab. Even if all those pipes are sloped the same and share a trench, only one of them is carrying the full load out toward the sewer. That’s the building drain.

The definition also says the building drain receives the discharge from soil, waste, and other drainage pipes inside. So it’s not just about being horizontal or being under the slab; it’s about location and function in the system. The key is that once no other branches are connecting and everything is flowing in one direction toward the sewer, you're in building drain territory.

I get that the code uses words like “lowest,” but you don’t need to interpret that as a literal elevation reading. It’s describing position in the flow hierarchy. In the real world, plumbers don't label every under-slab pipe the same just because of pitch. They know where the building drain starts because that’s where all the waste is headed.

This isn’t about creative interpretation; it’s about understanding how plumbing systems are laid out and what each part is doing.

 

[wwhitney]

Wayne, I get where you're coming from, but this feels like a classic case of reading too much into the definitions instead of just looking at how the system functions.

I understand how the system functions, but the question of building drain vs horizontal branch is a terminological one. That's what definitions are for. The definitions need to express the dividing line between the two terms.

The rest of your answer seems to be based on the idea that the building drain is the portion of the piping system through which all the waste exiting the building flows. [That is, all the waste through a particular pipe exiting the building, as the building may have multiple separate connections to the building sewer.]

But there is nothing in the definition that says anything like that. Indeed, Table 710.1(1) is titled in part ". . . including branches of the building drain." So the IPC expects that the building drain can be branched, which would mean that part of it does not carry all the flow through a particular exit.

I guess it would be helpful to understand why the IPC makes a distinction between the building drain and a horizontal branch. Why is Table 710.1(1) different from 710.1(2)?

Say (pure speculation) the reason is "clogs in the lowest pipe in the system will cause multiple/all fixture not to work properly, so we want reduced DFUs in such pipes to reduce the risk of clogs." Then the definition should be something like "that part of the lowest portion of the piping system that receives the discharge of multiple horizontal branches or stacks." The building drain would extend from the exit upstream to include all junctions of two horizontal branches or a horizontal branch and a stack.

Cheers, Wayne

 

[steveray]

Suppose we're plumbing the DWV on a single story house on a slab, and under the slab the horizontal drain pipes are all pitched at 1/4" per foot with no other vertical offsets of any kind. Then just considering those horizontal under slab drains, how do we determine the dividing line between the building drain and the horizontal branches (if any)?

The definition of "building drain" uses the limiter "lowest". That can't be taken literally, since all horizontal pipes are sloped, so the literally lowest portion would just be a little sliver of pipe 30" in developed length outside the footprint of the building. So it needs to be taken as "lowest tier". In other words, in the example above, all of the horizontal drain piping under the slab could be considered "lowest" and thus meets that portion of the definition.

I don't see any other limiters in the definition of "building drain" that would apply to the example above, so to me the simplest answer is that all of those horizontal under slab drains are in fact the building drain. Curious what other people think.

One obvious way to subdivide those horizontal under slab drains would be to limit the building drain to the portion that is the same size as the drain exiting the building. But the definition has no such verbiage.

Cheers, Wayne

I sort of agree, but JAR is right….no one is running a bunch of 1.5-2” pipes to the terminus where it leaves the building. They are going to take them to a 4” building drain somewhere inside the building and then out, tying more in along the way…

 

[wwhitney]

I sort of agree, but JAR is right….no one is running a bunch of 1.5-2” pipes to the terminus where it leaves the building. They are going to take them to a 4” building drain somewhere inside the building and then out, tying more in along the way…

Sure, but where does anything in the IPC say that either the building drain is unbranched, or that it's all one size?

Cheer, Wayne

 

[AesclepianDream]

It seems to me that the DFU allotment for building drains is lower so that a bigger pipe is sized for the same flow rate, in order to prevent a full-bore flow when the system is at maximum capacity (713.2, 713.3) - which would be more permissible in the horizontal branches, as they are closer in developed length to the vent stacks than BD's. This argument requiring air flow substantiates the sizing requirements for offsets in Sections 710 and 711. Building drains therefore seem designed to have a vent inbuilt, but I think it's difficult to prescribe a specific developed length between vent stacks and building drains, as each dwv system would have to balance the fluid dynamics for their specific use case.

So, to Jar's point, a waste pipe defined as a building drain couldn't be sized as a horizontal drain, and in the example that Wayne gave, each of the under slab horizontal drains, if they extend beyond 30" of developed length beyond the exterior wall, would have to then be sized as building drains, rather than as horizontal branches. That number seems to me, the lowest common proportional denomenator between building drains and vent stacks.

But also to Wayne's point, while it may (generally) be more efficient to have only one building drain, that decision is made on a case by case basis, depending on the system's requirements. The code doesn't designate lawful method - or exactly how the system must be plumbed - only the requirements to provide for many different use cases and system demands.

I'd love to hear your take on this proposition. -AesclepianDreamer

 

[wwhitney]

It seems to me that the DFU allotment for building drains is lower so that a bigger pipe is sized for the same flow rate, in order to prevent a full-bore flow when the system is at maximum capacity (713.2, 713.3) - which would be more permissible in the horizontal branches, as they are closer in developed length to the vent stacks than BD's. This argument requiring air flow substantiates the sizing requirements for offsets in Sections 710 and 711.

That sounds quite plausible.

But also to Wayne's point, while it may (generally) be more efficient to have only one building drain, that decision is made on a case by case basis, depending on the system's requirements.

I guess my explanation of the example wasn't clear--the under slab horizontal drains in my example would all combine to a single drain before exiting the building. My point is that nothing in the definition says that the building drain can't be branched and can't consist of different sizes. Maybe I'll post an example with a diagram for greater clarity.

Cheers, Wayne

 

[wwhitney]

Maybe I'll post an example with a diagram for greater clarity.

Below is a semi-randomly selected diagram for under-slab DWV that I found in an image search. If all the black lines shown are horizontal, under the slab and sloped at 1/4" per foot without any vertical offsets, it seems to me that everything shown falls in the category of "building drain" per the definition.

If you disagree, please indicate what specific part of the definition or other IPC requirement for the building drain would exclude any of the drains in the diagram from being part of the building drain.

Cheers, Wayne

P.S. Not sure why the washer standpipe appears to rise out of the slab under the washer--so that little diagonal jog on the bottom right should be omitted for this example. Likewise for the vertical jog drawn under the primary vanity--that would be the trap arm, so not under the slab. Also, not clear on what the fixture in the bottom right of the righthand bathroom would be.

 

[AesclepianDream]

I should have said 'credit' rather than 'point,' because I was saying the ways in which I agreed with the both of you.

As for the diagram you shared, if we view the IPC through the comprehension of the nature of building drains I outlined, then the distinguishing factor between horiz. branches and build. drains is sizing in terms of DFU allotment for the latter (B.D.'s) to have a cross-sectional area which permits air flow as well as waste/soil tansmission. In which case, provided there is proper venting, none of the drains from any of these fixtures needs to be designated (and therefore over sized) as a building drain except where the drains depart the building's footprint on the north side, near the clean out - as the code states, from 30" to the exterior wall, at which point it becomes the building sewer, because it is self-venting all the way to the public sewer. They *can be*, but then they would be oversized for the application.

Does that make sense? I'm saying in the spirit of the code rather than the letter, that I think the job of the codes for venting are to get enough airflow down to where the DW becomes building drain, so that it can self-regulate from there, provided the DFU rate is to code for the fixtures serviced.

Humble regards, AsclepianDreamer

 

[wwhitney]

Does that make sense? I'm saying in the spirit of the code rather than the letter

Well, if your proposed explanation says that (almost) none of the drains in my diagram are the building drain, and the letter of the code says they all are, then your proposed explanation doesn't match the letter of the code. That doesn't necessarily mean your proposed explanation is wrong, it just means that if it is correct and complete, the code writers did a terrible job of writing the definition of "building drain."

For your proposed explanation, I'd write the definition of "building drain" as something like "That portion of the drainage system that is either outside the footprint of the building but within 30" of developed length from the footprint of the building, or at the lowest level of piping within the footprint of the building and more than X feet of developed length from any connected vent." For some value of X.

But so far in this discussion I'm at the stage of the analysis of "do we agree on what the letter of the definition says?"

Cheers, Wayne

 

[steveray]

Well, if your proposed explanation says that (almost) none of the drains in my diagram are the building drain, and the letter of the code says they all are, then your proposed explanation doesn't match the letter of the code. That doesn't necessarily mean your proposed explanation is wrong, it just means that if it is correct and complete, the code writers did a terrible job of writing the definition of "building drain."

For your proposed explanation, I'd write the definition of "building drain" as something like "That portion of the drainage system that is either outside the footprint of the building but within 30" of developed length from the footprint of the building, or at the lowest level of piping within the footprint of the building and more than X feet of developed length from any connected vent." For some value of X.

But so far in this discussion I'm at the stage of the analysis of "do we agree on what the letter of the definition says?"

Cheers, Wayne

Looking forward to your code change submittal and public input…