An overview of ISO construction types

When looking into construction, whether for office buildings or residential properties, it’s crucial to understand the different Insurance Services Office (ISO) construction types. These types classify buildings based on their structural materials and fire resistance. And, help determine their insurance ratings. Let’s understand what these ISO construction types mean and why they are important.

What are ISO construction types?

ISO construction types are classifications that the insurance industry uses to assess the risk associated with a building’s structure. And, this risk influences insurance premiums. For example, building types that offer better fire protection than others will have lower risk. These types are defined by factors such as the materials of walls, floors, and roofs, including whether those materials are combustible.

Needless to say, ISO construction classifications are important for building owners, builders, and insurance underwriters. These classifications deal with the combustibility of a building, or how well the building will do in case of a fire. 

What factors impact ISO construction classification?

There are two main factors used to define the ISO classification. 

Construction materials: This includes the building construction materials used in the construction of the structural frame, exterior and interior load-bearing as well as non-bearing walls and partitions, floor and ceiling construction, including joists and supporting beams.

Fire rating: This rating is measured for the construction industry based on the amount of time a passive fire protection system can withstand a fire resistance test. Not all construction classifications have fire-resistance ratings as part of them.

Why does ISO construction matter?

Understanding ISO construction types is crucial. Especially, if you’re thinking about insuring a property. Buildings made from fire-resistive materials are much safer and cheaper to insure than those made with combustible materials. 

The classification also affects the choice of materials for roofs and walls. For example, metal roofing is popular for its durability and resistance to fire, while brick veneer can offer excellent exterior protection but still requires a fire-resistant interior structure.

What are the different ISO construction types?

Here’s a closer look at the most common ISO construction types – from the strongest fire resistance to the least.

ISO 1: Fire-resistant construction

This is the gold standard for fire resistance. Such buildings are built with fire-resistant materials such as concrete blocks, steel frames, or reinforced concrete. These materials are designed to withstand intense fires for a long time without collapsing. 

The typical fire rating of ISO 1 construction is over two hours. These buildings are designed to maintain structural integrity even under extreme fire conditions, making them ideal for properties where fire protection is a top priority.

For instance, the floor and roof systems are typically made from heavy steel or concrete. They provide added strength and protection. And they are less likely to suffer extensive damage from a fire.

ISO 2: Joisted masonry

This category includes buildings with non-combustible exterior walls, typically brick veneer or concrete block, but combustible interior framing, like wood joists. Think of older apartments or small office buildings — solid exterior, but wood inside.

ISO 2 joisted masonry provides moderate fire resistance. While the masonry exterior is durable, the joisted wood floors and roof structures make the building more vulnerable during a fire.

ISO 3: Non-combustible

The walls, floors, and roofs are built with non-combustible materials such as metal roofing and steel frames, but the fire resistance isn’t as high as ISO 1. Also, a step up in fire protection from ISO 3, ISO 4 masonry construction includes concrete block or brick exterior walls paired with non-combustible floors and roofing, often metal roofing and steel decking.

The materials won’t catch fire easily, but they don’t necessarily prevent structural damage during a fire. Think of low-rise commercial or industrial buildings where the materials are sturdy but not highly fire-resistant.

ISO 4: Masonry non-combustible

The exterior walls are made using concrete block or brick veneer, and the roof and floor are typically metal roofing and steel decking. You’ll find this type of construction in many office buildings and commercial structures. It provides good fire protection, as the main structure won’t easily collapse in a fire.

ISO 5: Modified fire-resistive

This is similar to ISO 1 but with slightly less fire resistance. ISO 5 uses materials such as concrete blocks or heavy steel but has a lower fire resistance rating, typically around one to two hours. The walls, floors, and roof are made from non-combustible materials. However, they won’t hold up as long under fire conditions as ISO 1 buildings.

ISO 6: Semi-fire-resistive

In this construction, the building materials are a mix. For example, you might find concrete blocks or brick exterior walls with a metal roof. However, the overall fire resistance is limited. These buildings are common in areas where some fire protection is necessary, but the cost of higher fire resistance isn’t justified.

Last thoughts

Understanding ISO construction types is more than an academic exercise — it’s a practical necessity for property owners, builders, real estate professionals, and insurance underwriters. Knowing whether a building is ISO 1 frame, ISO 2 joisted masonry, or ISO 4 masonry can directly affect construction decisions, insurance costs, and overall building safety.

With fire safety and risk evaluation becoming more critical than ever, the right fire rating classification helps protect not just property, but also lives and investments. Whether you’re insuring a building, planning a remodel, or evaluating new construction, ISO construction types are a foundation worth understanding.

Written by Narayan Shrouthy. September 19, 2021

Narayan is a content writer for Kukun. With experience in both content writing and editing, Narayan mixes his writing and love for reading with his passion for music. As an active musician, He aims to be a key player in Kukun's content experience.