Building Code Glossary
Flame Spread Index: Building Code Definition
How the IBC 2009, IRC 2009, IFC 2009, IMC 2009 define flame spread index, 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
Flame spread index is a defined term in the International Building Code (IBC) 2009, in Interior Finishes. The code's own words are: “A comparative measure, expressed as a dimensionless number, derived from visual measurements of the spread of flame versus time for a material tested in accordance with ASTM E 84 or UL 723.” The IRC 2009, IFC 2009, IMC 2009 define 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 20 chapters of this corpus, most often in Plastic (IBC 2009), Building Planning (IRC 2009), Interior Finish, Decorative Materials and Furnishings (IFC 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?
Flame Spread Index — IBC 2009, Interior Finishes:
A comparative measure, expressed as a dimensionless number, derived from visual measurements of the spread of flame versus time for a material tested in accordance with ASTM E 84 or UL 723.
What must be verified before using this guidance?
Four of the I-Codes in this corpus define Flame Spread Index, and the wording is not identical — read the version that governs your question.
| Code | Where it is defined | Definition |
|---|---|---|
| IBC 2009 | Interior Finishes | A comparative measure, expressed as a dimensionless number, derived from visual measurements of the spread of flame versus time for a material tested in accordance with ASTM E 84 or UL 723. |
| IRC 2009 | Definitions | A comparative measure, expressed as a dimensionless number, derived from visual measurements of the spread of flame versus time for a material tested in accordance with ASTM E 84. |
| IFC 2009 | Interior Finish, Decorative Materials and Furnishings | A comparative measure, expressed as a dimensionless number, derived from visual measurements of the spread of flame versus time for a material tested in accordance with ASTM E 84 or UL 723. |
| IMC 2009 | Definitions | The numerical value assigned to a material tested in accordance with ASTM E 84 or UL 723. |
Where flame spread index is used in the code
The term appears in 20 chapters of the 2009 I-Codes hosted here. The chapters that lean on it hardest are the ones to read next.
| Chapter | Code | Mentions |
|---|---|---|
| Plastic | IBC 2009 | 46 |
| Building Planning | IRC 2009 | 27 |
| Interior Finish, Decorative Materials and Furnishings | IFC 2009 | 21 |
| Interior Finishes | IBC 2009 | 13 |
| Roof-ceiling Construction | IRC 2009 | 8 |
| Fire and Smoke Protection Features | IBC 2009 | 7 |
| Special Detailed Requirements Based on Use and Occupancy | IBC 2009 | 6 |
| Duct Systems | IMC 2009 | 6 |
| Duct Systems | IRC 2009 | 6 |
| Types of Construction | IBC 2009 | 3 |
| Wood | IBC 2009 | 3 |
| Hydronic Piping | IMC 2009 | 3 |
8 further chapters mention the term.
The term in use
How the code actually writes about flame spread index, quoted from the 2009 text hosted here:
Unless otherwise indicated in this section, foam plastic insulation and foam plastic cores of manufactured assemblies shall have a flame spread index of not more than 75 and a smoke-developed index of not more than 450 where tested in the maximum thickness intended for use in accordance with ASTM E 84 or UL 723.
Flame spread index and smoke-developed index for wall and ceiling finishes.
Interior wall and ceiling finish shall have a flame spread index not greater than that specified in Table 803.3 for the group and location designated.
Flame Spread Index compared with Smoke-developed Index
These two are the closest pair in the 2009 I-Codes: about 74 per cent of their defining words are the same. The code states both and compares neither, so the difference is worth naming.
| Wording unique to it | |
|---|---|
| Flame Spread Index (IBC 2009) | visual, spread, flame, ul, 723 |
| Smoke-developed Index (IBC 2009) | smoke, obscuration |
Everything else in the two definitions is shared. If a project turns on which of the two applies, those words are the whole of the test — and a local amendment that edits one of them without editing the other moves the boundary between flame spread index and smoke-developed index.
Flame Spread Index: 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 | Interior Finishes | |
| Read the definition in the adopted IRC | Definitions | |
| Read the definition in the adopted IFC | Interior Finish, Decorative Materials and Furnishings | |
| Read the definition in the adopted IMC | Definitions | |
| Re-check the requirement that uses it | Plastic (IBC 2009) | |
| Re-check the requirement that uses it | Building Planning (IRC 2009) | |
| Re-check the requirement that uses it | Interior Finish, Decorative Materials and Furnishings (IFC 2009) | |
| Re-check the requirement that uses it | Interior Finishes (IBC 2009) | |
| 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
- Trim: Building Code Definition
- Smoke-developed Index: Building Code Definition
- Interior Finish: Building Code Definition
- Interior Wall and Ceiling Finish: 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 flame spread index?
The International Building Code (IBC) 2009 defines it in Interior Finishes as: “A comparative measure, expressed as a dimensionless number, derived from visual measurements of the spread of flame versus time for a material tested in accordance with ASTM E 84 or UL 723.” 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.
Do the I-Codes define flame spread index the same way?
No. IBC 2009, IRC 2009, IFC 2009, IMC 2009 all define the term, but the wording is not identical, and a difference in wording can change which requirements are triggered. The comparison table above sets the versions side by side so you can read the one that governs your question.
Which parts of the code depend on flame spread index?
In this corpus the term is used across 20 chapters, led by Plastic 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 the definition of flame spread index?
In the 2009 I-Codes hosted here it is a defined term rather than a description. The IBC 2009 states, in Interior Finishes: “A comparative measure, expressed as a dimensionless number, derived from visual measurements of the spread of flame versus time for a material tested in accordance with ASTM E 84 or UL 723.” It is defined in IBC 2009, IRC 2009, IFC 2009, IMC 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.
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