Fire-Rated Millwork Specifications: IBC, FRTW, and FR Assembly Documentation

Fire-rated millwork requirements come up on virtually every commercial project in a Type I or II building — high-rises, hospitals, schools, government buildings. Yet they're one of the most misunderstood requirements in the millwork trade. The confusion usually centers on two questions: what exactly is required (combustible wood millwork is typically allowed in non-combustible buildings — with conditions), and how does that requirement get documented in the shop drawings.

Getting fire-rated millwork documentation wrong doesn't just result in a submittal rejection — it can result in a building department stop-work order if fire-rated materials aren't verified before installation. Our millwork shop drawing services on Type I and II commercial projects always include fire-rating documentation as a standard part of the submittal package.

IBC Construction Types and Why They Matter for Millwork

The International Building Code classifies buildings by construction type (I through V), with Types I and II being non-combustible construction — steel, concrete, masonry structures. The basic rule: combustible materials (wood) are not permitted as structural elements in Type I and II buildings.

The exception that makes wood millwork possible in these buildings is IBC Section 603: Combustible Materials in Types I and II Construction. Section 603 lists specific categories of combustible material that are permitted in otherwise non-combustible construction. The relevant items for millwork:

• Interior wall and ceiling finish materials — including wood paneling, wainscoting, and trim — are permitted when they meet the required interior finish flame spread classification (Class B or C depending on occupancy and location per IBC Table 803.11)
• Wood trim, blocking, and millwork — specifically permitted in Section 603.1, items 3 and 25, subject to area limitations and flame spread requirements
• Blocking, furring, and nailers — permitted in any thickness when not more than a specific percentage of the assembly

The critical takeaway: wood millwork is permitted in Type I and II buildings — but with material finish requirements that standard untreated wood millwork may not meet. This is where FRTW and flame spread testing come in.

ASTM E84: The Flame Spread Test

ASTM E84, also known as the Steiner Tunnel Test, is the primary test method for interior finish materials. It measures:

• Flame Spread Index (FSI): how quickly flame travels across the material surface. Benchmarked to 0 (inorganic cement board) and 100 (red oak). Lower is better.
• Smoke Development Index (SDI): how much smoke the material produces. Maximum 450 for most interior finish applications.

IBC classifies interior finishes based on FSI:

• Class A: FSI 0–25 — required in the most restrictive locations (exit enclosures, exit passageways, corridors in high-rise and healthcare)
• Class B: FSI 26–75 — required in corridors, rooms, and spaces in most commercial occupancies
• Class C: FSI 76–200 — permitted in some non-critical spaces in certain occupancies

Standard untreated wood typically has an FSI of 70–200 depending on species — which means it may not meet Class A or B requirements without treatment. Untreated MDF has an FSI of approximately 100–200. High-pressure laminate (HPL) typically tests at FSI 10–25 depending on manufacturer and substrate — most HPL products have published ASTM E84 test reports.

Fire-Retardant Treated Wood (FRTW)

FRTW is wood that has been pressure-impregnated with inorganic salts or other fire-retardant chemicals that reduce ignition and flame spread. To qualify as fire-retardant treated, wood must achieve:

• FSI of 25 or less (Class A) after ASTM E84/UL 723 testing
• No evidence of significant progressive combustion when the 30-minute test is continued for an additional 20 minutes (UBC requirement incorporated by some jurisdictions)

FRTW types defined by the American Wood Council (AWC):

• Interior Type A: for dry applications (interior millwork, cabinet cores, blocking in conditioned spaces)
• Interior Type B: for interior applications where moisture is possible (not typical for millwork)
• Exterior type: for exterior applications; typically not required for building interior millwork

The shop drawing must specify FRTW explicitly when required: "All wood substrates to be Fire-Retardant Treated Wood, Interior Type A, per AWC guidelines. Provide ICC evaluation report with submittal."

Non-Combustible Substrate Alternatives

In areas where even FRTW doesn't meet the code requirement — or where the project spec requires non-combustible substrates throughout — alternative casework construction is used:

• Steel cabinet boxes with wood veneer or laminate faces: The structural box is 16 or 18 gauge steel; the visible surfaces are wood veneer or HPL (which must still meet the required ASTM E84 classification). Common in healthcare and Type I commercial.
• Aluminum framing with panel infill: Aluminum extrusion frames with fiber cement, calcium silicate, or gypsum panels. Used for wall paneling systems where the IBC requires non-combustible construction throughout.
• Gypsum board substrate with applied finish: For wall panels and wainscoting — the structural substrate is gypsum (non-combustible), with an applied HPL, ceramic tile, or phenolic resin face. The face finish must meet the Class A or B requirement.

The shop drawing must specify the substrate type and the ASTM E84 classification of each finish material. "HPL finish — Class A per ASTM E84 test report [report number]" is the level of specificity required.

Documenting Fire-Rated Specifications in Shop Drawings

The fire-rating documentation in the shop drawing package must include:

• Materials note on each sheet: state the substrate (FRTW Interior Type A, or steel, or fiber cement) and the face finish with its ASTM E84 classification
• General notes sheet: list all materials used on the project with their ASTM E84 test classification and report numbers; reference to ICC evaluation reports for FRTW products
• Reference to IBC section: note the applicable IBC section permitting the material (e.g., "Wood trim per IBC Section 603.1, Item 3")
• Submittal attachments: ASTM E84 test reports for surface finish materials, ICC-ESR evaluation report for FRTW products, letter of conformance from the fabricator

Some AHJs require that material samples accompany the shop drawing submittal for fire-rated projects, particularly for custom finishes not covered by a published ASTM E84 report. Confirm this requirement with the architect before submitting.

For more on the submittal process and what architects and GCs review, see our article on the millwork submittal process. For AWI grade standards that interact with fire-rated specifications, see our AWI millwork standards guide.

Common Fire-Rated Millwork Specification Errors

• Specifying "fire-rated" without a standard. "Fire-rated wood substrate" is not a specification — the drawing must state the test method (ASTM E84), the required FSI classification, and the AWC FRTW type designation
• Treating FSI as a property of wood species. FSI is a property of the treated assembly — FRTW is chemically treated wood, not a species selection. Red oak and pine treated with the same fire retardant will both meet Class A; untreated, neither will in most cases
• Forgetting surface finishes. Even if the substrate is FRTW or non-combustible, the applied face finish (veneer, HPL, paint) must also meet the required ASTM E84 classification — the test applies to the finished surface, not just the substrate
• Not confirming local amendments. Some jurisdictions adopt the IBC with local amendments that are stricter than the base code. Always confirm the applicable code edition and local amendments with the AHJ at the start of the project

Check our millwork drawing rates for commercial projects with fire-rating documentation requirements, which involve additional note preparation and submittal package assembly time.

Frequently Asked Questions