• Frame Construction (ISO) is one of six building structure categories established by the Insurance Services Administration, Inc. (ISO) in the Commercial Lines Manual (CLM) for the purposes of developing commercial property insurance rates based on exposure to fire damage. The framing description in CLM, followed by the appropriate ISO building code, is exterior walls of wood, brick cladding, stone cladding, iron-clad wood, or stucco on wood (building code 1). 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.