Categories of building construction established by Insurance Services Office, Inc. (ISO), in its Commercial Lines Manual (CLM) for purposes of developing rates for insuring commercial property, based on susceptibility to damage by fire. The six ISO CLM building construction categories and the associated ISO construction codes, from the least fire-resistive category to the most fire-resistive category, are as follows.

• Frame—Exterior walls of wood, brick veneer, stone veneer, wood ironclad, or stucco on wood. (Construction Code 1)
• Joisted Masonry—Exterior walls of masonry material (adobe, brick, concrete, gypsum block, hollow concrete block, stone, tile, or similar materials) with combustible floor and roof. (Construction Code 2)
• Noncombustible—Exterior walls, floor, and supports made of metal, asbestos, gypsum, or other noncombustible materials. (Construction Code 3)
• Masonry Noncombustible—Same as joisted masonry except that the floors and roof are of metal or other noncombustible materials. (Construction Code 4)
• Modified Fire Resistive—Exterior walls, floors, and roof of masonry or fire-resistive material with a fire resistance rating of at least 1 hour but less than 2 hours. (Construction Code 5)
• Fire Resistive—Exterior walls, floors, and roof of masonry or fire-resistive materials with a fire resistance rating of at least 2 hours. (Construction Code 6)

The American Association of Insurance Services (AAIS) uses nearly identical building construction categories in its materials addressing the development of rates for insuring commercial property in several of its programs.