Underlayment: Building Code Definition

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, 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.

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.

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.

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

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

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

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 — IBC 2009: The covering applied to the roof deck for weather resistance, fire classification or appearance.
• Approved — IBC 2009: Acceptable to the code official or authority having jurisdiction.
• Slope — 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 and the ICC adoption maps cover the middle rows.

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 Covering: Building Code Definition
• Approved: Building Code Definition
• Slope: 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

• codes.iccsafe.org

• iccsafe.org