Insurance underwriters analyze four factors, including construction, occupancy, protection and external loss exposures, when determining eligibility and premiums for commercial and residential property insurance. The Insurance Services Office, which supplies predictive risk scoring tools to the insurance industry, developed the construction classification system to rate fire-resistive qualities of different construction types. These classes, defined by type and percentage of materials used, determine how long a supporting structure can withstand exposure to fire without collapse.
Construction Class 1--Frame
Buildings in Class 1 consist of ceilings, floors and weight-bearing supports made from combustible materials, such as wood. A building's exterior, whether wood, masonry or metal, has no impact on the "Frame" classification since these materials do not support the structure in any way. Class 1 structures present the highest risk of loss during a fire.
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Construction Class 2--Joisted Masonry
Class 2 buildings consist of combustible floors and ceilings; however, the load-bearing exterior walls are constructed of noncombustible materials, such as concrete, hollow masonry blocks, stone, brick, heavy timber or any other material with a one hour or higher fire-resistance rating. While safer than Class 1 structures, joisted masonry construction is still at high risk of collapse and damage.
Construction Class 3--Noncombustible
Buildings in Class 3 utilize noncombustible materials, including gypsum and metal, in the ceiling, floors, load-bearing supports and exterior bearing walls. Noncombustible materials are not necessarily fire resistive, however. While the noncombustible elements may not catch fire, a building filled with combustible items can create enough heat to weaken the noncombustible structure.
Construction Class 4--Masonry Noncombustible
Class 4 structures have exterior, supporting and nonsupporting walls of masonry, with a thickness of four inches or more and a one hour or higher fire-resistance rating. The roof and floors are comprised of noncombustible or slow-burning materials. This type of construction offers more protection against total collapse; however, the noncombustible floor and ceiling materials are still subject to heavy damage depending on the severity of the fire.
Construction Class 5--Modified Fire Resistive
Buildings with a Class 5 rating consist of load-bearing walls, roofs and floors made from fire-resistive materials with a one to two hour fire-resistance rating and a thickness of no less than four inches. Class 5 structures also include additional structural steel protection in the form of coatings, such as clay tile, concrete, gypsum or other noncombustible materials. While this type of construction provides a higher level of safety and a lower degree of risk, it is often much more costly than other construction methods.
Construction Class 6--Fire Resistive
Class 6 is the highest rated construction type from a risk and safety standpoint. All walls consist of solid masonry with a thickness of four inches or more, hollow masonry 12 inches or more in thickness or 8- to 12-inch thick hollow masonry with a fire-resistance rating of two hours or more. Floors and roofs consist of reinforced concrete, no less than four inches thick with a two hour fire resistance rating. Fire-resistive structures must also include protected metal support structures with no less than a two hour fire resistance rating. While this is the safest type of construction class, it is also the most expensive to construct.