Building Code Glossary
Unit Skylight: Building Code Definition
How the IRC 2009 define unit skylight, 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
Unit skylight is a defined term in the International Residential Code (IRC) 2009, in Building Planning. The code's own words are: “A factory assembled, glazed fenestration unit, containing one panel of glazing material, that allows for natural daylighting through an opening in the roof assembly while preserving the weather-resistant barrier of the roof.” No other I-Code in this corpus defines it, so the IRC 2009 definition is the one to work from. The term is used in 5 chapters of this corpus, most often in Glass and Glazing (IBC 2009), Building Planning (IRC 2009), Plastic (IBC 2009). These pages transcribe the 2009 model text; verify the definition against the edition and amendments your authority having jurisdiction has adopted. Record the jurisdiction, the adopted edition, the application date and any local amendment that rewrites the definition before you use it on a live project, because a term redefined by amendment changes every requirement that depends on it.
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?
Unit Skylight — IRC 2009, Building Planning:
A factory assembled, glazed fenestration unit, containing one panel of glazing material, that allows for natural daylighting through an opening in the roof assembly while preserving the weather-resistant barrier of the roof.
What must be verified before using this guidance?
Unit Skylight is defined once in this corpus, in the Building Planning chapter of the International Residential Code (IRC) 2009:
A factory assembled, glazed fenestration unit, containing one panel of glazing material, that allows for natural daylighting through an opening in the roof assembly while preserving the weather-resistant barrier of the roof.
Where unit skylight is used in the code
The term appears in 5 chapters of the 2009 I-Codes hosted here. The chapters that lean on it hardest are the ones to read next.
| Chapter | Code | Mentions |
|---|---|---|
| Glass and Glazing | IBC 2009 | 14 |
| Building Planning | IRC 2009 | 8 |
| Plastic | IBC 2009 | 2 |
| Definitions | IBC 2009 | 1 |
| Definitions | IECC 2009 | 1 |
Unit Skylight compared with Skylight, Unit
These two are the closest pair in the 2009 I-Codes: about 93 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 | |
|---|---|
| Unit Skylight (IRC 2009) | daylighting |
| Skylight, Unit (IBC 2009) | lighting |
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 unit skylight and skylight, unit.
The terms unit skylight is built out of
The definition above leans on 2 other defined terms. Each carries its own code definition, and a local amendment to any of them changes what unit skylight covers:
- Roof Assembly — IBC 2009: A system designed to provide weather protection and resistance to design loads.
- Fenestration — IRC 2009: Skylights, roof windows, vertical windows (whether fixed or moveable); opaque doors; glazed doors; glass block; and combination opaque/glazed doors.
Unit Skylight: 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 | Building Planning | |
| Re-check the requirement that uses it | Glass and Glazing (IBC 2009) | |
| Re-check the requirement that uses it | Building Planning (IRC 2009) | |
| Re-check the requirement that uses it | Plastic (IBC 2009) | |
| Re-check the requirement that uses it | Definitions (IBC 2009) | |
| Confirm the dependent definition | Roof Assembly | |
| Confirm the dependent definition | Fenestration | |
| 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
- Roof Assembly: Building Code Definition
- Fenestration: 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 unit skylight?
The International Residential Code (IRC) 2009 defines it in Building Planning as: “A factory assembled, glazed fenestration unit, containing one panel of glazing material, that allows for natural daylighting through an opening in the roof assembly while preserving the weather-resistant barrier of the roof.” 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 unit skylight?
In this corpus the term is used across 5 chapters, led by Glass and Glazing 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 a skylight in a building?
Building is a separate defined term. The IBC 2009 defines it as: “Any structure used or intended for supporting or sheltering any use or occupancy.” That is not the same as unit skylight, which the IRC 2009 defines as “A factory assembled, glazed fenestration unit, containing one panel of glazing material, that allows for natural daylighting through an opening in the roof assembly while preserving the weather-resistant barrier of the roof.” — read both before applying either, and confirm the wording in the edition your jurisdiction has adopted.
Related pages
- Underlayment: Building Code Definition
Underlayment — IBC 2009 definition: One or more layers of felt, sheathing paper, nonbituminous saturated felt or other approved material over …
- Ungrounded: Building Code Definition
Ungrounded — IRC 2009 definition: Not connected to ground or to a conductive body that extends the ground connection.
- Unit Heater: Building Code Definition
Unit Heater — IMC 2009 definition: A self-contained appliance of the fan type, designed for the delivery of warm air directly into …
- Unstable (Reactive) Material: Building Code Definition
Unstable (Reactive) Material — IBC 2009 definition: A material, other than an explosive, which in the pure state or as commercially produced, will …
- Unvented Room Heater: Building Code Definition
Unvented Room Heater — IRC 2009 definition: An unvented heating appliance designed for stationary installation and utilized to provide comfort heating.
- Unwanted Fire: Building Code Definition
Unwanted Fire — IFC 2009 definition: A fire not used for cooking, heating or recreational purposes or one not incidental to the …