# Underlayment: Building Code Definition

> How the IBC 2009, IRC 2009 define underlayment, where the wording differs, and which chapters depend on the term.

- Canonical: https://harth.build/resources/glossary/underlayment/
- Last updated: 2026-09-07
- Data source: 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.

## Key facts

| Fact | Value |
| --- | --- |
| Defined in | IBC 2009, IRC 2009 (2009 editions transcribed in this corpus) |
| Primary source | IBC 2009 Roof Assemblies and Rooftop Structures (model text, not a jurisdiction's adopted text) |
| Chapters using the term | 11 (counted across the codes hosted here) |
| Wording identical across codes | No (compare before citing one code's wording under another) |

## Direct answer

Underlayment is a defined term in the International Building Code (IBC) 2009, in Roof Assemblies and Rooftop Structures. The code's own words are: “One or more layers of felt, sheathing paper, nonbituminous saturated felt or other approved material over which a steep-slope roof covering is applied.” The IRC 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 11 chapters of this corpus, most often in Roof Assemblies and Rooftop Structures (IBC 2009), Roof Assemblies (IRC 2009), Floors (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](https://nz.linkedin.com/in/barrington), Harth founder and architectural technology specialist · Published 2026-09-07 · Updated 2026-09-07.*

## What does this term mean?

**Underlayment** — IBC 2009, Roof Assemblies and Rooftop Structures:

> One or more layers of felt, sheathing paper, nonbituminous saturated felt or other approved material over which a steep-slope roof covering is applied.

![Labeled code-term definition diagram: Underlayment: Building Code Definition](https://efwwel0vyt9vy5so.public.blob.vercel-storage.com/images/glossary/underlayment/diagram-1344.webp)

*How the IBC 2009, IRC 2009 define underlayment, where the wording differs, and which chapters depend on the term.*

## What must be verified before using this guidance?

Two of the I-Codes in this corpus define **Underlayment**, and the wording is not identical — read the version that governs your question.

| Code | Where it is defined | Definition |
| --- | --- | --- |
| IBC 2009 | [Roof Assemblies and Rooftop Structures](https://harth.build/resources/building-codes/us-states/maryland/ibc-2009/roof-assemblies-and-rooftop-structures.md) | One or more layers of felt, sheathing paper, nonbituminous saturated felt or other approved material over which a steep-slope roof covering is applied. |
| IRC 2009 | [Definitions](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/definitions.md) | One or more layers of felt, sheathing paper, nonbituminous saturated felt, or other approved material over which a roof covering, with a slope of 2 to 12 (17-percent slope) or greater, is applied. |

## Where underlayment is used in the code

The term appears in 11 chapters of the 2009 I-Codes hosted here. The chapters that lean on it hardest are the ones to read next.

| Chapter | Code | Mentions |
| --- | --- | --- |
| [Roof Assemblies and Rooftop Structures](https://harth.build/resources/building-codes/us-states/maryland/ibc-2009/roof-assemblies-and-rooftop-structures.md) | IBC 2009 | 140 |
| [Roof Assemblies](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/roof-assemblies.md) | IRC 2009 | 134 |
| [Floors](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/floors.md) | IRC 2009 | 21 |
| [Wood](https://harth.build/resources/building-codes/us-states/maryland/ibc-2009/wood.md) | IBC 2009 | 11 |
| [Structural Design](https://harth.build/resources/building-codes/us-states/maryland/ibc-2009/structural-design.md) | IBC 2009 | 2 |
| [Definitions](https://harth.build/resources/building-codes/us-states/maryland/ibc-2009/definitions.md) | IBC 2009 | 1 |
| [Interior Finishes](https://harth.build/resources/building-codes/us-states/maryland/ibc-2009/interior-finishes.md) | IBC 2009 | 1 |
| [Definitions](https://harth.build/resources/building-codes/us-states/maryland/iecc-2009/definitions.md) | IECC 2009 | 1 |
| [Definitions](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/definitions.md) | IRC 2009 | 1 |
| [Roof-ceiling Construction](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/roof-ceiling-construction.md) | IRC 2009 | 1 |
| [Wall Construction](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/wall-construction.md) | IRC 2009 | 1 |

### The term in use

How the code actually writes about underlayment, quoted from the 2009 text hosted here:

> In addition, an underlayment of sis-inch (15.9 mm) Type X water-resistant gypsum backing board or gypsum sheathing shall be placed under minimum nominal l/z-inch-thick (12.7 mm) wood structural panel solid sheathing or I-inch (25 mm) nominal spaced sheathing.

— [IBC 2009 §1505.7](https://harth.build/resources/building-codes/us-states/maryland/ibc-2009/roof-assemblies-and-rooftop-structures.md)

> For roof slopes from two units vertical in 12 units horizontal (2: 12) up to four units vertical in 12 units horizontal (4: 12), double underlayment application is required in accordance with Section R905.2.7.

— [IRC 2009 §R905.2.2](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/roof-assemblies.md)

> Where used as subflooring or combination subfloor underlayment, wood structural panels shall be of one of the grades specified in Table R503.2.1.1 (1).

— [IRC 2009 §R503.2.1.1](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/floors.md)


## The terms underlayment is built out of

The definition above leans on 3 other defined terms. Each carries its own code definition, and a local amendment to any of them changes what underlayment covers:

- **[Roof Covering](https://harth.build/resources/glossary/roof-covering.md)** — IBC 2009: The covering applied to the roof deck for weather resistance, fire classification or appearance.
- **[Approved](https://harth.build/resources/glossary/approved.md)** — IBC 2009: Acceptable to the code official or authority having jurisdiction.
- **[Slope](https://harth.build/resources/glossary/slope.md)** — IRC 2009: The fall (pitch) of a line of pipe in reference to a horizontal plane.

## Underlayment: what to open, and in what order

Every row is a document this term depends on. The [ICC code library](https://codes.iccsafe.org/codes) and the [ICC adoption maps](https://www.iccsafe.org/advocacy/code-adoption-maps/) cover the middle rows.

| Check | Where | Confirmed |
| --- | --- | --- |
| Read the definition in the adopted IBC | Roof Assemblies and Rooftop Structures | |
| Read the definition in the adopted IRC | Definitions | |
| Re-check the requirement that uses it | Roof Assemblies and Rooftop Structures (IBC 2009) | |
| Re-check the requirement that uses it | Roof Assemblies (IRC 2009) | |
| Re-check the requirement that uses it | Floors (IRC 2009) | |
| Re-check the requirement that uses it | Wood (IBC 2009) | |
| Confirm the dependent definition | Roof Covering | |
| Confirm the dependent definition | Approved | |
| Confirm the dependent definition | Slope | |
| 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](https://harth.build/resources/glossary.md)
- [Roof Covering: Building Code Definition](https://harth.build/resources/glossary/roof-covering.md)
- [Approved: Building Code Definition](https://harth.build/resources/glossary/approved.md)
- [Slope: Building Code Definition](https://harth.build/resources/glossary/slope.md)

## 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](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.

### Source links

- [codes.iccsafe.org](https://codes.iccsafe.org/codes)

- [iccsafe.org](https://www.iccsafe.org/advocacy/code-adoption-maps/)

## Frequently asked questions

### What is the code definition of underlayment?

The International Building Code (IBC) 2009 defines it in Roof Assemblies and Rooftop Structures as: “One or more layers of felt, sheathing paper, nonbituminous saturated felt or other approved material over which a steep-slope roof covering is applied.” 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 underlayment?

In this corpus the term is used across 11 chapters, led by Roof Assemblies and Rooftop Structures 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 underlayment?

In the 2009 I-Codes hosted here it is a defined term rather than a description. The IBC 2009 states, in Roof Assemblies and Rooftop Structures: “One or more layers of felt, sheathing paper, nonbituminous saturated felt or other approved material over which a steep-slope roof covering is applied.” It is defined in IBC 2009, IRC 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.

---

- Continue in Harth (open this topic in the Harth agent): https://app.harth.build/agent?context=%2Fresources%2Fglossary%2Funderlayment%2F
- View this page in a browser: https://harth.build/resources/glossary/underlayment/
- All resources for agents: https://harth.build/llms.txt