Tech Today

Understanding fire ratings


Roof assemblies play important roles in buildings' overall abilities to resist fires. For this reason, building codes generally require buildings' roof assemblies to meet specific fire-resistance characteristics. Roof assemblies typically are required to resist the spread of fires originating from buildings' exteriors and interiors. The specific degree of fire resistance required primarily is based on a building's construction, occupancy and location.

Exterior exposure

Roof assemblies' abilities to resist fires originating on buildings' exteriors are determined by subjecting representative small-scale specimens of roof assemblies to laboratory testing using one of several recognized standard test methods: ASTM E108, "Standard Test Methods for Fire Tests of Roof Coverings"; UL 790, "Standard for Tests for Fire Resistance of Roof Covering Materials"; or FM 4470, "Approval for Class I Roof Covers." These test methods provide the basis for Class A, Class B and Class C fire ratings.

Class A-rated roof assemblies are effective against severe fire test exposure. A Class A fire rating is the most stringent rating available for exterior fire resistance. Class B-rated roof assemblies are effective against moderate fire test exposure. And Class C-rated roof assemblies are effective against light fire test exposure. A Class C fire rating is the least stringent rating available for exterior fire resistance. If a roof assembly cannot pass the Class C criteria, it typically is considered to be nonclassified.

Certain roof assembly types are exempt from fire testing based on their inherent ability to resist fires originating on a building's exterior. For example, inthe 2003 International Building Code (IBC), brick, masonry, slate, clay or concrete tile, exposed concrete, ferrous or copper shingles or sheet roof assemblies are recognized as being Class A-rated without being tested. Similarly, IBC recognizes other metal (aluminum) sheet or shingle roof assemblies as being Class B-rated without testing.

Because the specific exterior fire rating required for a building largely is based on a building's construction classification, IBC's Table 1505.1 provides a listing of the minimum exterior fire ratings required for specific construction types. Construction types are determined in IBC using Chapter 6—Type of Construction.

It is important to realize Class A-rated roof assemblies are required for few buildings. According to IBC, a Class A-rated roof assembly only may be necessary if it is part of interior fire-rating requirements, a building is located in a specific fire district, or the Urban-Wildland Interface Code also applies and specifically requires a Class A-rated construction.

Interior exposure

Roof assemblies' abilities to resist fires originating in buildings' interiors also are determined by subjecting representative small-scale specimens of roof assemblies to one of several laboratory tests. These tests are FM 4450, "Test Standard for Class I Insulated Steel Deck Roofs"; FM 4470; and UL 1256, "Standard for Fire Test of Roof Deck Constructions."

FM 4450 and FM 4470 provide the technical basis for Class I ratings, such as FM Global's I-60, I-90, I-120, etc., approval ratings. UL 1256 provides the technical basis for one-hour, one-and-a-half-hour, two-hour, etc., interior-fire ratings.

The specific interior-fire rating required for a building is determined by occupancy and type of construction. For example, using IBC, it is necessary to consider Chapter 3—Use and Occupancy Classification, Chapter 6—Type of Construction and Chapter 7—Fire-resistance-rated Construction to determine interior-fire classifications.

Learn the basics

I encourage you to become familiar with buildings' exterior and interior fire-rating requirements. In many instances, compliance with such requirements meets specific provisions in building codes. I also encourage roof system designers to closely consider buildings' fire-rating requirements. Any specific exterior or interior fire-rating requirements for roof assemblies should be specified clearly.

Detailed information regarding specific roof assemblies' exterior and interior fire-resistance ratings are provided in FM's Approval Guide and UL's Building Materials Directory and Fire Resistance-rated Construction Directory. UL's directory information also is available online at www.ul.com.

Mark S. Graham is NRCA's associate executive director of technical services.

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