Materials used in building construction can burn if they are organic, containing carbon that can ignite in air. Various wood products, plastics and many fibers are combustible, which means they will burn if ignited with a flame. A test used to determine the combustibility of various materials is the flame spread rating.
When a combustible product is ignited with a flame, it burns at a rate that depends on several conditions, including the amount of airflow and the materials used to make the product. Wood fiberboard can be made with various glues that can accelerate the flame burn rate. These factors need to be compared in a standard test, and the flame spread rating allows direct comparison of different materials.
A common flame spread rating test is the tunnel test. The material to be tested is attached to the top of a tunnel-shaped room, with openings only at each end. Windows placed along the length of the tunnel room allow the flame spread to be observed. The sample is at least 18 inches (46 centimeters) wide and 25 feet (7.6 meters) long. A timer starts when a flame is applied to one end of the sample, and stops when the flame has burned a specific length of the sample.
The rate at which flame moves across the sample is dependent on the amount of airflow. Air movement in the tunnel is carefully controlled so the flame spread rating can be directly compared for different building materials. One end of the tunnel contains an air control damper for incoming air, and any smoke from the test is removed at the other end with a fan.
In order to compare flame spread values for different materials, standards are needed. Standards are materials that burn consistently in different tests, giving a similar value for all tests. Zero flame spread rating is obtained by attempting to ignite a board made from asbestos fiber and cement, which will not burn. A value of 100 is given to red oak wood, which gives a range of combustibility to use for all other materials.
Building code authorities use flame spread rating information to create standards for new building construction. Low rating materials may be required for hospitals, in areas where fires are more common, such as residential garages, or in areas of the world where fires are a seasonal problem. Class I materials are typically considered the most fire-resistant, and Class V are the most combustible.
Material cost may not be the only factor when determining the cost of flame-proofing a building. Concrete-based board may not be more expensive than a wood board on a per-board basis, despite having a very low fire rating. Cost of construction may be more dependent on the structural weight of non-combustible materials. Cement or concrete materials can be quite heavy, and can cost more because of stronger structures needed to support them, and increased labor to install them.