Building Code Glossary
Alternating Tread Device: Building Code Definition
How the IBC 2009, IFC 2009 define alternating tread device, 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
Alternating tread device is a defined term in the International Building Code (IBC) 2009, in Means of Egress. The code's own words are: “A device that has a series of steps between 50 and 70 degrees (0.87 and 1.22 rad) from horizontal, usually attached to a center support rail in an alternating manner so that the user does not have both feet on the same level at the same time.” The IFC 2009 carries the same definition word for word, so the term means the same thing in each. The term is used in 6 chapters of this corpus, most often in Means of Egress (IBC 2009), Means of Egress (IFC 2009), Special Detailed Requirements Based on Use and Occupancy (IBC 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?
Alternating Tread Device — IBC 2009, Means of Egress:
A device that has a series of steps between 50 and 70 degrees (0.87 and 1.22 rad) from horizontal, usually attached to a center support rail in an alternating manner so that the user does not have both feet on the same level at the same time.
What must be verified before using this guidance?
Two of the I-Codes in this corpus define Alternating Tread Device, in identical words.
| Code | Where it is defined | Definition |
|---|---|---|
| IBC 2009 | Means of Egress | A device that has a series of steps between 50 and 70 degrees (0.87 and 1.22 rad) from horizontal, usually attached to a center support rail in an alternating manner so that the user does not have both feet on the same level at the same time. |
| IFC 2009 | Means of Egress | A device that has a series of steps between 50 and 70 degrees (0.87 and 1.22 rad) from horizontal, usually attached to a center support rail in an alternating manner so that the user does not have both feet on the same level at the same time. |
Where alternating tread device is used in the code
The term appears in 6 chapters of the 2009 I-Codes hosted here. The chapters that lean on it hardest are the ones to read next.
| Chapter | Code | Mentions |
|---|---|---|
| Means of Egress | IBC 2009 | 46 |
| Means of Egress | IFC 2009 | 39 |
| Special Detailed Requirements Based on Use and Occupancy | IBC 2009 | 4 |
| General Building Heights and Areas | IBC 2009 | 2 |
| Definitions | IFC 2009 | 2 |
| Definitions | IBC 2009 | 1 |
The term in use
How the code actually writes about alternating tread device, quoted from the 2009 text hosted here:
The combination of riser height and projected tread depth provided shall result in an alternating tread device angle that complies with Section 1002.
In buildings without an occupied roof, access to the roof from the top story shall be permitted to be by an alternating tread device.
A steel ladder, alternating tread device or spiral stairway is permitted to be provided from the gridiron to a scuttle in the stage roof.
The terms alternating tread device is built out of
The definition above leans on 1 other defined term. Each carries its own code definition, and a local amendment to any of them changes what alternating tread device covers:
- Device — IRC 2009: Aunit of an electrical system that carries or controls electrical energy as it principal function.
Alternating Tread Device: 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 IBC | Means of Egress | |
| Read the definition in the adopted IFC | Means of Egress | |
| Re-check the requirement that uses it | Means of Egress (IBC 2009) | |
| Re-check the requirement that uses it | Means of Egress (IFC 2009) | |
| Re-check the requirement that uses it | Special Detailed Requirements Based on Use and Occupancy (IBC 2009) | |
| Re-check the requirement that uses it | General Building Heights and Areas (IBC 2009) | |
| Confirm the dependent definition | Device | |
| 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
- Device: Building Code Definition
- Public Way: Building Code Definition
- Stairway: Building Code Definition
- Guard: 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 alternating tread device?
The International Building Code (IBC) 2009 defines it in Means of Egress as: “A device that has a series of steps between 50 and 70 degrees (0.87 and 1.22 rad) from horizontal, usually attached to a center support rail in an alternating manner so that the user does not have both feet on the same level at the same time.” 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 alternating tread device?
In this corpus the term is used across 6 chapters, led by Means of Egress in the IBC 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 an alternating tread device?
In the 2009 I-Codes hosted here it is a defined term rather than a description. The IBC 2009 states, in Means of Egress: “A device that has a series of steps between 50 and 70 degrees (0.87 and 1.22 rad) from horizontal, usually attached to a center support rail in an alternating manner so that the user does not have both feet on the same level at the same time.” It is defined in IBC 2009, IFC 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 an alternating stair pattern?
Stair is a separate defined term. The IBC 2009 defines it as: “A change in elevation, consisting of one or more risers.” That is not the same as alternating tread device, which the IBC 2009 defines as “A device that has a series of steps between 50 and 70 degrees (0.87 and 1.22 rad) from horizontal, usually attached to a center support rail in an alternating manner so that the user does not have both feet on …” — read both before applying either, and confirm the wording in the edition your jurisdiction has adopted.
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