The Insurance Services Office (ISO) standardizes property risk by classifying commercial buildings into six distinct construction classes. These classes run from Class 1 (most combustible) to Class 6 (most fire-resistant). Understanding the technical details of these classifications is essential for accurate risk pricing and capacity allocation.
Class 1: Frame (Combustible)
Class 1 properties have load-bearing walls, floors, and roofs constructed of wood or other combustible materials. Underwriters view these risks with caution due to rapid fire spread. They command higher rates and lower capacity, particularly in areas exposed to natural catastrophes (like wildfires or hurricanes).
Class 2: Joisted Masonry
In Class 2 buildings, the exterior load-bearing walls are made of masonry (like brick, concrete blocks, or stone), but the floors and roof are made of wood or combustible materials. While the masonry walls offer some fire resistance, the interior combustible frame means fire can still cause complete structural collapse.
Class 3: Non-Combustible
Class 3 buildings feature load-bearing walls, floors, and roofs constructed of non-combustible materials (typically unprotected structural steel). While steel does not burn, structural steel loses its load-bearing strength at around 1,100°F (593°C), leading to rapid buckling and roof collapse if not protected by a sprinkler system.
Class 4: Masonry Non-Combustible
This class combines masonry exterior walls with a non-combustible roof and floor system (steel purlins, metal decks, or concrete slabs). It is highly favored for commercial warehouses and retail plazas due to its resilience against exterior fire exposures and moderate construction cost.
Class 5: Modified Fire Resistive
Class 5 buildings feature structural steel frames that have been protected with fire-resistant coating (concrete encasement, plaster, or spray-applied fireproofing) or load-bearing concrete walls. The structure has a fire resistance rating of 1 to 2 hours, giving firefighters ample time to control the blaze.
Class 6: Fire Resistive
Class 6 represents the gold standard of property risk. These structures are made of reinforced concrete or protected structural steel with a fire resistance rating of 2 hours or more. High-rise office buildings and hospitals are typically Class 6. They command the lowest rates and the highest underwriting capacity.
Underwriting Strategy
When assessing a commercial submission, knowing the exact ISO Construction Class is critical. It determines the base rate and dictates the maximum PML (Probable Maximum Loss) model. Ensuring this data is extracted verbatim from broker surveys prevents pricing errors and speeds up treaty clearances.