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What Is The Flame Spread?

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.

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:

Flame Spread Rating Chart

  • 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 regulations in this area of fire safety.


What does a Class A Fire Rating mean?

A Class A fire rating is a classification of a material‘s resistance to fire, based on its ability to withstand flames and prevent the spread of fire. A Class A rating is the highest rating that a material can receive which indicates that the material is highly resistant to fire. The rating is given to materials that are effective at preventing the spread of fire and that do not easily ignite.

To receive a Class A fire rating, a material must pass specific tests that measure its performance in a fire. These tests typically evaluate the material’s flame spread, smoke production, and ability to resist ignition. A material that receives a Class A rating has been deemed to have the highest level of fire resistance and is considered safe for use in environments where fire safety is a concern.

Some common materials that may receive a Class A fire rating include fire-resistant building materials such as:

  • Concrete
  • Brick and stone
  • Fire-retardant treated wood
  • Fire-resistant insulation
  • Fire-rated doors and windows

What are Class B through E Fire Ratings Typically used for?

Class B through E fire ratings are classifications of materials’ resistance to fire, based on their ability to withstand flames and prevent the spread of fire. These ratings are typically used for different types of construction materials and products in various settings.

Here’s a breakdown of what each class of fire rating typically means and where they may be used:

  • Class B fire rating: Materials with a Class B fire rating are moderately resistant to fire. These materials have a slow rate of flame spread and can help to slow down the spread of fire in a building. Class B fire ratings are typically used for materials such as:
      • Lower-grade fire-retardant wood
      • Some types of foam insulation
      • Certain types of roofing materials
  • Class C fire rating: Materials with a Class C fire rating are less fire-resistant than Class B materials. They have a moderate rate of flame spread and can help to slow down the spread of fire. Class C fire ratings are typically used for materials such as:
      • Untreated wood, like plywood, fiberboard, and certain siding materials.
      • Some types of interior finishes
      • Nowadays, with today’s fire codes, fewer types of roofing materials on certain commercial buildings
  • Class D and E fire rating: Materials with a Class D fire rating are typically used for materials such as: certain types of metals and chemicals that can pose a significant fire hazard. These materials are often used in industrial settings and require specialized fire suppression systems to prevent fires from spreading. Materials with a Class E fire rating are typically used for electrical components such as wiring, circuit breakers, and outlets. These materials are designed to resist fire and prevent electrical fires from spreading. 

Overall, the specific fire rating requirements for different materials will vary depending on the building code regulations and safety standards in a particular area. It’s important to consult with a qualified professional to determine the appropriate fire rating for a specific material or product.