Building Code Glossary
Common Vent: Building Code Definition
How the IRC 2009, IPC 2009 define common vent, 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
Common vent is a defined term in the International Residential Code (IRC) 2009, in Definitions. The code's own words are: “A single pipe venting two trap arms within the same branch interval, either back-to-back or one above the other.” The IPC 2009 words it differently, in Definitions: “A vent connecting at the junction of two fixture drains or to a fixture branch and serving as a vent for both fixtures.” The IPC 2009 defines the same term with different wording, so the controlling text depends on which code governs the question in front of you — the comparison below sets the versions side by side. The term is used in 7 chapters of this corpus, most often in Fuel Gas (IRC 2009), Vents (IPC 2009), Vents (IRC 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?
Common Vent — IRC 2009, Definitions:
A single pipe venting two trap arms within the same branch interval, either back-to-back or one above the other.
What must be verified before using this guidance?
Two of the I-Codes in this corpus define Common Vent, and the wording is not identical — read the version that governs your question.
| Code | Where it is defined | Definition |
|---|---|---|
| IRC 2009 | Definitions | A single pipe venting two trap arms within the same branch interval, either back-to-back or one above the other. |
| IPC 2009 | Definitions | A vent connecting at the junction of two fixture drains or to a fixture branch and serving as a vent for both fixtures. |
Where common vent is used in the code
The term appears in 7 chapters of the 2009 I-Codes hosted here. The chapters that lean on it hardest are the ones to read next.
| Chapter | Code | Mentions |
|---|---|---|
| Fuel Gas | IRC 2009 | 71 |
| Vents | IPC 2009 | 15 |
| Vents | IRC 2009 | 14 |
| Chimneys and Vents | IMC 2009 | 3 |
| Chimneys and Vents | IRC 2009 | 2 |
| Definitions | IPC 2009 | 1 |
| Definitions | IRC 2009 | 1 |
The term in use
How the code actually writes about common vent, quoted from the 2009 text hosted here:
Where two or more appliances are connected to a common vent or chimney, each vent connector shall be sized in accordance with Section G2428 or other approved engineering methods.
Stack vents and vent stacks connected into a common vent header at the top of the stacks and extending to the open air at one point shall be sized in accordance with the requirements of Section 916.1.
An individual vent is permitted to vent two traps or trapped fixtures as a common vent.
The terms common vent is built out of
The definition above leans on 4 other defined terms. Each carries its own code definition, and a local amendment to any of them changes what common vent covers:
- Branch Interval — IRC 2009: A vertical measurement of distance, 8 feet (2438 mm) or more in developed length, between the connections of horizontal branches to a drainage stack.
- Fixture Branch — IPC 2009: A drain serving two or more fixtures that discharges to another drain or to a stack.
- Venting — IRC 2009: Removal of combustion products to the outdoors.
- Branch — IRC 2009: Any part of the piping system other than a riser, main or stack.
Common Vent: 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 IRC | Definitions | |
| Read the definition in the adopted IPC | Definitions | |
| Re-check the requirement that uses it | Fuel Gas (IRC 2009) | |
| 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 | Chimneys and Vents (IMC 2009) | |
| Confirm the dependent definition | Branch Interval | |
| Confirm the dependent definition | Fixture Branch | |
| Confirm the dependent definition | Venting | |
| 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
- Branch Interval: Building Code Definition
- Fixture Branch: Building Code Definition
- Venting: Building Code Definition
- Branch: 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 common vent?
The International Residential Code (IRC) 2009 defines it in Definitions as: “A single pipe venting two trap arms within the same branch interval, either back-to-back or one above the other.” 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 common vent?
In this corpus the term is used across 7 chapters, led by Fuel Gas in the IRC 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 type 3 venting?
Venting is a separate defined term. The IRC 2009 defines it as: “Removal of combustion products to the outdoors.” That is not the same as common vent, which the IRC 2009 defines as “A single pipe venting two trap arms within the same branch interval, either back-to-back or one above the other.” — read both before applying either, and confirm the wording in the edition your jurisdiction has adopted.
What is a common vent system?
In the 2009 I-Codes hosted here it is a defined term rather than a description. The IRC 2009 states, in Definitions: “A single pipe venting two trap arms within the same branch interval, either back-to-back or one above the other.” It is defined in IRC 2009, IPC 2009. It is not the same as Vent System, which the IRC 2009 defines separately as “Piping installed to equalize pneumatic pressure in a drainage system to prevent trap seal loss or blow-back due to siphonage or back pressure.” 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 a common vent in plumbing?
In the 2009 I-Codes hosted here it is a defined term rather than a description. The IRC 2009 states, in Definitions: “A single pipe venting two trap arms within the same branch interval, either back-to-back or one above the other.” It is defined in IRC 2009, IPC 2009. It is not the same as Plumbing, which the IRC 2009 defines separately as “For the purpose of this code, plumbing refers to those installations, repairs, maintenance and alterations regulated by Chapters 25 through 33.” 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.
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