What is the Flame Spread Rating?
Many factors influence fire spread within buildings, and one of the most important is the interior finish material.
Back in the late 1940’s and early 1950’s combustible acoustical tile used on ceilings was of serious consequence in loss of life in building fires. After several major incidents involving schools, nursing homes, hospitals and similar structures, revision of fire protection standards brought correction of this defect, so today the fire spread characteristics of such materials are regulated by code requirements.
So, what does something like a class a fire rating mean? Today there are several methods of evaluating surface burning characteristics of building materials to judge their suitability within an occupancy.
The most widely recognized laboratory test of such fire characteristics is defined in NFPA Standard No. 255 – Method of Test of Surface Burning Characteristics of Building Materials (also Standard E-84 of the American Society for Testing and Materials and No. 723 of Underwriters’ Laboratories, Inc.). This standard defines the equipment and test method that can be used to discern the flame spread rating of a material. It should be noted that this rating is a number, calculated from results of the test. The number indicates the relative rate at which flame will spread over the surface of a material, as compared with flame spread on asbestos-cement board (rated zero) and on red oak (rated 100). It should be emphasized that this rating number is not the rate at which the flame actually spreads along the surface and is not an indication of the fire resistance of the material.
The method and equipment used in this evaluation is commonly referred to as the Tunnel Test; the test equipment is referred to as the Steiner Tunnel named after its designer A.L. Steiner, formerly of Underwriters’ Laboratories, Inc., Chicago, Illinois. In the test procedure, the sample of material (18 inches wide, 25 feet long) is installed on the underside of the removable top panel (see sketch), A gas flame is applied at one end and a regulated constant draft is directed through the tunnel from the flame end. The progress of the flame front along the sample is observed through side windows and timed. From these observations a flame spread rating can be calculated. For example, if the flame travels 19-1/2 feet in less than 5-1/2 minutes (the time required for flame to spread on 19-1/2 feet of red oak), the rating is 100 times 5-1/2 divided by the time (minutes) in which flame spreads 19-1/2 feet on the sample.
The purpose of such testing is to provide architects and fire protection engineers with adequate information so that they can select appropriate material that will not contribute to the problem of life safety from fire within structures. Obviously, the speed of flame over surface interior material can affect the safety of people within a building if flame spread is faster than evacuation can be accomplished, or if fire spreads throughout an entire building before adequate fire protection measures can extinguish the blaze.
For the purpose of applying flame spread limits to interior finish material, NFPA Standard No. 101 – The Life Safety Code groups flame spread ratings into five classes:
- Class A flame spread rating 0-25
- Class B flame spread rating 26-75
- Class C flame spread rating 76-200
- Class D flame spread rating 201-500
- Class E flame spread rating over 500
The installation of a complete automatic sprinkler system in structures having these interior finish materials modifies the fire hazard risk.
The appendix of the Life Safety Code contains a tabulation of the interior finish requirements of the code, particularly with respect to the use of such material for exits, access to exits and other spaces. Fire officers and fire prevention bureau personnel should study these requirements and evaluate their own local regulation in this area of fire safety.