Roof Assemblies and Fire Classification: Class A, B and C Roofs, Slopes and Underlayment

Direct answer

Fire classification describes how a roof covering resists fire arriving from outside the building. ASTM E108 and UL 790 expose the covering and deck to spread-of-flame, intermittent-flame and burning-brand tests; Class A is effective against severe exposure, Class B moderate and Class C light. Under IBC 2021 Section 1505.1 and Table 1505.1, the minimum roof covering is Class B for Types IA, IB, IIA, IIIA, IV and VA and Class C for Types IIB, IIIB and VB. Group R-3 and U buildings may use nonclassified roofing with at least 6 feet of fire separation distance from the roof edge. For houses, IRC R902.1 requires Class A, B or C roofing where the law designates it or where the roof edge is less than 3 feet from a lot line. Separately, each covering has a minimum slope: 2:12 for asphalt shingles, 2½:12 for clay or concrete tile, 3:12 for metal shingles, 4:12 for slate and 1/4:12 for membranes.

Content and cited inputs reviewed by Harth team · Published 2026-10-27 · Updated 2026-10-27.

What does this term mean?

Roof Assemblies and Fire Classification: Class A, B and C Roofs, Slopes and Underlayment: Roof coverings are fire-classified Class A, B or C by testing to ASTM E108 or UL 790; IBC Table 1505.1 sets the minimum class by construction type, IRC R902.1 requires a classified roof near lot lines, and each covering has its own minimum slope, underlayment and ice-barrier rules.

What are the key takeaways?

• Fire test standard: ASTM E108 or UL 790
• Class A, B, C: Severe, moderate, light fire-test exposure
• Minimum class by type: Class B for IA, IB, IIA, IIIA, IV, VA; Class C for IIB, IIIB, VB
• IRC trigger: Roof edge less than 3 ft from a lot line

What do Class A, B and C roof ratings mean?

A roof fire classification describes how a roof covering, installed on its deck, resists fire that arrives from outside the building: wind-blown embers, burning debris or a neighbouring fire. It is not an hourly fire-resistance rating; the roof construction's hours come from Table 601 and the assembly listings, while the covering's class comes from a separate test.

IBC 2021 Section 1505.1 and IRC 2021 Section R902.1 require listed Class A, B and C coverings to be tested to ASTM E108 or UL 790. The standard exposes the covering and deck to a spread-of-flame test, an intermittent-flame test and a burning-brand test, at intensities that rise from Class C to Class A. The code defines the classes by exposure:

Many coverings reach Class A only as part of a specific listed assembly, so the listing (deck, underlayment or cap sheet, slope limits) must be matched on site, not just the product name.

Which coverings count as Class A without a listing?

IBC 1505.2 and IRC R902.1 deem these assemblies Class A by description:

• Brick, masonry or an exposed concrete roof deck.
• Ferrous or copper shingles or sheets, metal sheets and shingles, clay or concrete roof tile, or slate installed on noncombustible decks (the IBC also includes metal sheets on noncombustible framing without a deck).
• Copper sheets of at least 16 ounces per square foot over combustible decks.
• Slate over underlayment on combustible decks; the IBC specifies ASTM D226 Type II underlayment.

Fire-retardant-treated wood shakes and shingles must be pressure-treated to AWPA C1 and labeled with their tested class (IBC 1505.6, IRC R902.2).

Minimum roof class by construction type (IBC Table 1505.1)

The IBC sets the minimum covering class from the building's construction type. The more fire-resistive A subtypes and Type IV need at least Class B; the unprotected B subtypes need at least Class C.

Three footnotes change the answer. A building in a wildland-urban interface area or an Appendix D fire district may need more. Group R-3 and U buildings may use nonclassified roofing where there is at least 6 feet of fire separation distance from the leading edge of the roof. Small buildings in Types IIB, IIIB and VB (two stories or less, 6,000 sq ft of projected roof or less, 10 feet from lot lines) may use specified cedar or redwood shakes and shingles built per Section 1505.7. Rooftop photovoltaic systems must carry a UL 2703 fire classification that meets the same table (1505.9).

For one- and two-family dwellings and townhouses, IRC R902.1 does not tie class to construction type. It requires Class A, B or C roofing where the jurisdiction designates it by law or where the roof edge is less than 3 feet from a lot line.

Minimum slopes and underlayment by roof covering

Every covering in IBC Section 1507 and IRC Section R905 has a minimum slope, and several need double underlayment in a band of low slopes. Values are from the 2021 editions.

The IBC and IRC differ on wood shakes (4:12 versus 3:12), so a house built under the IRC can use shakes at a slope the IBC would reject.

How is underlayment applied on a shingle roof?

IRC Table R905.1.1(2) spells out the application for asphalt shingles. Where wind design is not required by Figure R301.2.1.1:

• From 2:12 up to 4:12: two layers. Start with a 19-inch strip parallel to the eave, then lay 36-inch-wide sheets starting at the eave, each overlapping the one below by 19 inches.
• 4:12 and steeper: one layer applied shingle fashion from the eave, lapped 2 inches.
• End laps: 4 inches, offset by 6 feet.

Where wind design is required, the two-layer method applies at every slope and Table R905.1.1(3) requires cap nails in a 12-inch grid with 6-inch spacing at laps. Self-adhering polymer-modified bitumen underlayment labeled to ASTM D1970 is an accepted alternative (R905.1.1, Exception 1).

For commercial shingle roofs, IBC 1507.2.8.3 requires a drip edge at eaves and rakes, lapped 2 inches, with a vertical leg of at least 1½ inches extending ¼ inch below the sheathing and 2 inches back onto the roof, fastened at 12 inches on center. Underlayment goes over the drip edge at eaves and under it at rakes. Shingle nails must be 12-gage with a ⅜-inch head and penetrate ¾ inch into the sheathing, with at least four per strip shingle (1507.2.5 and 1507.2.6).

Where is an ice barrier required?

IRC R905.1.2 requires an ice barrier where Table R301.2 records a history of ice forming at the eaves and backing up water. It applies under asphalt shingles, metal roof shingles, mineral-surfaced roll roofing, slate and slate-type shingles, wood shingles and wood shakes, but not tile. The barrier is two layers of underlayment cemented together or a self-adhering polymer-modified bitumen sheet, run from the lowest roof edge to at least 24 inches inside the exterior wall line. On roofs of 8:12 or steeper it must also extend at least 36 inches along the slope from the eave edge. Detached accessory structures without conditioned floor area are exempt. Commercial roofs have a parallel ice-barrier provision in IBC Section 1507.1.2; check its wording in the adopted edition.

What else affects a roof assembly?

• Wind resistance. Coverings must also meet the wind classification tables in IBC Section 1504 and IRC R905; fire class and wind class are separate listings.
• Chimney crickets. A chimney wider than 30 inches parallel to the ridge that does not intersect the ridge needs a cricket (IRC R1003.20).
• Above-deck insulation and radiant barriers. IBC 1508 and 1510 require them to pass NFPA 276, UL 1256 or FM 4450 as part of the roof assembly.
• Adopted edition. Values here are typical 2021 model-code values; verify them against the edition and amendments adopted by the authority having jurisdiction.

Worked roof specification checks for three buildings

Three example roofs taken through fire class, slope, underlayment and ice barrier using the 2021 IBC and IRC.

Ice barrier width calculation for the 6:12 house

IRC R905.1.2 measures the ice barrier to a point 24 inches inside the exterior wall line, so the sheet width needed along the slope depends on the overhang, wall thickness and pitch.

At 4:12 the same eave needs 48 × 1.054 = 50.6 inches along the slope, and at 12:12 it needs 48 × 1.414 = 67.9 inches, so a steeper roof or a deeper overhang always pushes the membrane further up the roof. Values are typical model-code values; verify against the edition adopted by the authority having jurisdiction.

Where can you explore related guidance?

• Explore all assemblies guidance
• IBC Construction Types Explained: Type I, II, III, IV & V
• IBC Allowable Building Height and Area: Tables 504.3, 504.4 and 506.2 Explained
• Typical Exterior Wall Assemblies (Layer by Layer)
• Chimney and Fireplace Code Requirements: Hearth Extensions, Termination and Clearances (IRC R1001–R1006)

Sources and method

Fire classification, Table 1505.1 and the commercial covering rules are from the 2021 International Building Code, Chapter 15, Roof Assemblies and Rooftop Structures. Residential classification, slopes, underlayment tables and ice barriers are from the 2021 International Residential Code, Chapter 9, Roof Assemblies. The test descriptions refer to ASTM E108 and UL 790, as summarized in the ICC-ES overview of ASTM E108. Tables are summarized rather than reproduced; the worked checks and ice-barrier calculation are Harth calculations. Treat all values as typical model-code values and verify them against the edition adopted by the authority having jurisdiction, the product listing and any state or local amendments.

Source links

• codes.iccsafe.org

• codes.iccsafe.org

• icc-es.org