• Fire Resistant Construction (ISO) is one of six categories of building structures established by the Insurance Services Administration, Inc. (ISO) in the Commercial Lines Manual (CLM) for the purposes of developing commercial real estate insurance rates based on exposure to fire damage. . The CLM fire design description followed by the appropriate ISO building code includes exterior walls, floors, and roofs made of masonry or fire retardant materials with a fire rating of at least 2 hours (building code 6). The Building Code ranks this category of building structures in six categories, where 1 is the least fire resistant and 6 is the most fire resistant.