Flame Spread 101

December 13, 2023   -  

Understanding flame spread can unlock new innovations in building products, help select the best options for an assembly, and, ultimately, provide added safety and protection. 

With $16 billion lost due to fires in 2021, according to the U.S. Fire Administration, there’s good reason to choose materials that are less likely to propagate flames and contribute to fire growth.. 

In fact, the International Building Code (IBC) and the International Fire Code (IFC) limit the allowable fire performance of building assemblies, structural elements, interior finishes, and other materials based on occupancy classification. 

By gaining an understanding of flame spread, how it’s tested, and the classifications, you can work to ensure you’re meeting code requirements and making informed decisions.

What Is Flame Spread?

Flame spread is the propagation of flame across the surface of a material. It’s measured by how fast the material burns and how far the flame spreads once ignited. Flame spread can be both vertical, traveling up a wall to a ceiling, and horizontal, traveling across a floor assembly.

Flame spread is also commonly known as the surface burning characteristic rating of a material.$16 billion was lost due to fires in 2021

What Factors Influence Flame Spread?

As the building industry has advanced, there are many products available that are fire resistant, fire retardant, or offer some level of passive fire protection. Before deciding what materials to use in your building assemblies, it’s important to first understand the factors that influence flame spread:

  • Density: Materials with higher density, such as concrete, decrease the rate of flame spread.
  • Moisture content: Flame spread can be slowed using materials with high moisture content, such as gypsum board (or drywall). 
  • Surface emissivity: Choosing fiberglass insulation or brick can help slow burning.
  • Surface temperature at ignition: Reducing the use of high surface temperature materials, such as plastic, can reduce flame spread.
  • Thermal conductivity: Reducing thermal conductivity in materials, such as metals, can help to reduce flame spread.Factors that influence flame spread

Which Tests Measure and Assess Flame Spread?

Fire testing of building materials, structures, and components should be conducted in an accredited facility, under conditions specified in the appropriate ASTM, NFPA, UL, or UBC standard. 

The IBC stipulates acceptable testing methods based on construction type.

Steiner Tunnel Test

The Steiner tunnel is a 2-ft.-by-25-ft. long horizontal tunnel that is a closely controlled environment to ensure repeatable test results. The tunnel is named after the primary UL Engineer, Albert J. Steiner.

Tests conducted using the Steiner tunnel include:  

  • Surface Burning Characteristics of Building Materials (ASTM E84, NFPA 255, UL 723, UBC 8-1) 
  • Extended Duration Surface Burning Characteristics of Building Materials (ASTM E2768)

ASTM E84 is commonly known as the “tunnel test” or the “Steiner tunnel test” and is used to evaluate the surface burning characteristics (flame spread and smoke development) of various materials and components for many different products, including building materials, coatings, insulation, finishes, fabrics, and other surfaces requiring classification.ASTM E84

ASTM E2768, often called a 30-minute Steiner tunnel test, is used to measure surface flame spread and smoke density under specific fire exposure conditions. 

It is most often used to assess fire-retardant-treated wood but is also used to test ignition-resistant materials. A full-scale (25 ft.) Steiner tunnel furnace is used to evaluate the ability of a product to limit the surface spread of flame when evaluated for 30 minutes.

Fire Test for Assemblies

NFPA 285, Evaluation of Fire Propagation Characteristics of Exterior Wall Assemblies Containing Combustible Components, assesses the vertical and lateral flame spread from a window opening along an exterior wall. It requires a full-scale, two-story fire test structure. 

During the test, the flame spread cannot reach 10 feet above the top of the window opening and five feet from the centerline of the window opening in either direction, with no flame propagation to the interior of the second-story room. NFPA 285 compliance is required by the IBC for certain types of construction.

Room-Corner Tests

Room corner tests use a full-scale room structure with an ignition source located in the corner of the room. Tests using this assembly include: 

  • Room Fire Test Standard for Interior of Foam Plastic Systems (UBC 26-3)
  • Fire Test of Interior Finish Materials (UL 1715)

These tests assess a product’s ability to reduce or eliminate fuel contribution, flame spread, and prevention of flashover. UBC 26-3 is specifically designed to evaluate foam products in building structures. UL 1715 is designed to test the effectiveness of barrier materials as protection for other combustible materials.

What Are the Different Tunnel Test Flame Spread Ratings?

Each tunnel test will produce a numerical flame spread index (rating), usually 200 or less. The ratings are grouped into classes, labeled from A through C, with Class A being the best with an index of 0-25. The building code dictates what level of flame spread is permitted. 

For example, if a flame-spread rating of 150, or Class C, is permitted, then the majority of wood products may be used as interior finishes without special requirements for fire-retardant treatments or coatings.Common materials with rating

Flame Spread Rating Chart


Flame Spread Index

Smoke Developed Index

Class A



Class B



Class C



Using the Flame Spread Index to Your Advantage

The flame spread index offers a simple way to ensure materials and assemblies meet all necessary building and fire codes. But, your rating is only as good as the testing facility you choose.

When selecting a fire testing provider, ensure IAS accreditation and compliance with the appropriate ASTM, UL, NFPA, or UBC standards. A facility like ours that offers high-capacity cranes, multiple moveable test frames, Steiner tunnel testing, and both vertical and horizontal furnaces will be able to handle all your fire testing needs.

Find out how NGC Testing Services can help with your fire testing.


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