Building Code Glossary
Horizontal Branch Drain: Building Code Definition
How the IPC 2009 define horizontal branch drain, where the wording differs, and which chapters depend on the term.
On this page
Last updated September 7, 2026 · Definitions quoted from the 2009 International Codes as transcribed in this site's building-codes collection, with the source chapter linked for each quotation. Usage counts are computed over the chapters hosted here, one copy per code. The [ICC code library](https://codes.iccsafe.org/codes) publishes the current model text and the [ICC adoption maps](https://www.iccsafe.org/advocacy/code-adoption-maps/) show which edition a jurisdiction has adopted; those sources control.
Direct answer
Horizontal branch drain is a defined term in the International Plumbing Code (IPC) 2009, in Definitions. The code's own words are: “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.” No other I-Code in this corpus defines it, so the IPC 2009 definition is the one to work from. The term is used in 3 chapters of this corpus, most often in Vents (IPC 2009), Vents (IRC 2009), Definitions (IPC 2009). These pages transcribe the 2009 model text; verify the definition against the edition and amendments your authority having jurisdiction has adopted.
Content and cited inputs reviewed by Scott Barrington, Harth founder and architectural technology specialist · Published 2026-09-07 · Updated 2026-09-07.
What does this term mean?
Horizontal Branch Drain — IPC 2009, Definitions:
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.
What must be verified before using this guidance?
Horizontal Branch Drain is defined once in this corpus, in the Definitions chapter of the International Plumbing Code (IPC) 2009:
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.
Where horizontal branch drain is used in the code
The term appears in 3 chapters of the 2009 I-Codes hosted here. The chapters that lean on it hardest are the ones to read next.
| Chapter | Code | Mentions |
|---|---|---|
| Vents | IPC 2009 | 20 |
| Vents | IRC 2009 | 11 |
| Definitions | IPC 2009 | 1 |
The term in use
How the code actually writes about horizontal branch drain, quoted from the 2009 text hosted here:
The wet vent shall be considered the vent for the fixtures and shall extend from the connection of the dry vent along the direction of the flow in the drain pipe to the most downstream fixture drain connection to the horizontal branch drain.
Only the fixtures within the bathroom groups shall connect to the wet-vented horizontal branch drain.
The terms horizontal branch drain is built out of
The definition above leans on 5 other defined terms. Each carries its own code definition, and a local amendment to any of them changes what horizontal branch drain covers:
- Building Drain — IRC 2009: The lowest piping that collects the discharge from all other drainage piping inside the house and extends 30 inches (762 mm) in developed length of pipe, beyond the exterior walls and conveys the drainage to the building sewer.
- Building — IBC 2009: Any structure used or intended for supporting or sheltering any use or occupancy.
- Branch — IRC 2009: Any part of the piping system other than a riser, main or stack.
- Waste — IRC 2009: Liquid-borne waste that is free of fecal matter.
- Drain — IRC 2009: Any pipe that carries soil and water-borne wastes in a building drainage system.
Horizontal Branch Drain: what to open, and in what order
Every row is a document this term depends on. The ICC code library and the ICC adoption maps cover the middle rows.
| Check | Where | Confirmed |
|---|---|---|
| Read the definition in the adopted IPC | Definitions | |
| Re-check the requirement that uses it | Vents (IPC 2009) | |
| Re-check the requirement that uses it | Vents (IRC 2009) | |
| Re-check the requirement that uses it | Definitions (IPC 2009) | |
| Confirm the dependent definition | Building Drain | |
| Confirm the dependent definition | Building | |
| Confirm the dependent definition | Branch | |
| Local amendment to any of the above | Jurisdiction's amendment list |
A research aid, not a code determination. The authority having jurisdiction decides which edition and which amendments apply.
Where can you explore related guidance?
- Explore all glossary guidance
- Building Drain: Building Code Definition
- Building: Building Code Definition
- Branch: Building Code Definition
- Waste: Building Code Definition
Sources and method
Definitions quoted from the 2009 International Codes as transcribed in this site's building-codes collection, with the source chapter linked for each quotation. Usage counts are computed over the chapters hosted here, one copy per code. The ICC code library publishes the current model text and the ICC adoption maps show which edition a jurisdiction has adopted; those sources control.
Source links
Frequently asked questions
What is the code definition of horizontal branch drain?
The International Plumbing Code (IPC) 2009 defines it in Definitions as: “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.” The 2009 model text is reproduced here; the definition that binds your project is the one in the edition your jurisdiction has adopted, as amended locally.
Which parts of the code depend on horizontal branch drain?
In this corpus the term is used across 3 chapters, led by Vents in the IPC 2009. Those are the sections whose scope moves if the definition is amended locally, so read them together with Chapter 2 rather than on their own.
What is a horizontal branch drain?
In the 2009 I-Codes hosted here it is a defined term rather than a description. The IPC 2009 states, in Definitions: “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.” It is defined in IPC 2009. Because it is a defined term, whatever the adopted edition says it means is what every section using it means — so check Chapter 2 of that edition and any local amendment before relying on this wording.
What is the definition of a building drain?
Building Drain is a separate defined term. The IRC 2009 defines it as: “The lowest piping that collects the discharge from all other drainage piping inside the house and extends 30 inches (762 mm) in developed length of pipe, beyond the exterior walls and conveys the drainage to the building sewer.” That is not the same as horizontal branch drain, which the IPC 2009 defines as “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 …” — read both before applying either, and confirm the wording in the edition your jurisdiction has adopted.
Is a 45° considered horizontal in plumbing?
Plumbing is a separate defined term. The IRC 2009 defines it as: “For the purpose of this code, plumbing refers to those installations, repairs, maintenance and alterations regulated by Chapters 25 through 33.” That is not the same as horizontal branch drain, which the IPC 2009 defines as “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 …” — read both before applying either, and confirm the wording in the edition your jurisdiction has adopted.
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