A guide to fire rating classifications

Fire is a staple of life on earth. We use it to provide light, heat, and fuel for cooking food. It can also be incredibly destructive when we don’t respect it.

The world has seen many great fires throughout history, and we’ve gone to great lengths to reduce the amount of damage they cause. One of the ways we’ve sought to mitigate fire damage is through fire rating classifications. Modern building materials are labeled with fire rating classifications, which are numerical indicators of how the materials will respond when exposed to fire.

Different classifications refer to different levels of flammability and smoke production. Understanding the fire-resistance rating of materials gives you the knowledge you need to make effective choices.

What are the 5 different classes of fire?

Fire burns differently depending on the fuel source, so they are classified based on what that fuel is. Each type is diverse enough that we use different products and techniques in fighting them.

We also utilize other building materials in areas where hotter, tougher to suppress fires may erupt. According to UCLA, these are the five classes:

  • Class A – Class A fires involve solid materials like wood, trash, and textiles. Ordinary combustibles. These fires are most familiar to us and are the most easily extinguished.
  • Class B – Class B fires involve flammable liquids like gasoline, alcohol, diesel, or oil (not cooking oils). The volatility of the fuel source makes them more difficult.
  • Class C – Class C fires involve electrical components and equipment as fuel sources. This includes fires started by faulty wiring in walls, circuit breakers, and appliances.
  • Class D – Class D fires involve metals. In rare instances, metal can ignite. This is primarily a problem in lab environments, as well as certain production and other industrial processes.
  • Class K – Class K fires involve cooking oils and fats. Sometimes they are lumped in with Class B, but they are their own beast. These fires are typically started when someone leaves a pan unattended.

What are fire rating classifications?

Fire class ratings are a way of classifying materials by their ability to support and propagate fire and also indicate the smoke produced by the material. This is determined by a flame spread index.

Flame spread index is a numerical value that is typically obtained by examining how the material responds during a ten-minute tunnel test. The flame spread index value is expressed as an arbitrary numerical value, where the asbestos-cement board has a rating of zero, and red oak has a value of one hundred.

Each of the fire rating classifications has a flame spread index range assigned to it. Having a basic understanding of the fire-resistance rating of structural elements is invaluable in protecting property against fire. These are the classes recognized by the National Fire Protection Association Life Safety Code, NFPA No. 101.


What is a Class A or Class 1 fire rating?

A class 1 fire rating is the best fire rating of materials that can be achieved. Class A fire ratings indicate a flame spread rating somewhere between zero and 25.

Materials that fall into Class A or Class 1 include things like brick, gypsum wallboard, and fiber cement exterior materials. These materials do not burn well and are very unlikely to contribute fuel to a fire.

What is a Class B or Class 2 fire rating?

A Class B or Class 2 fire rating is the next best rating on the list. The flame spread rating of Class B would fall between 26 and 75. This rating is typical for slower-burning whole wood materials.

A whole wood material would be wood planks that are in the same form as they were when they were cut from the tree. They burn more quickly than Class A materials and more slowly than Class C materials.

What is a Class C or Class 3 fire rating?


A Class C or Class 3 fire rating has a flame spread rating between 76 and 200. This rating incorporates building materials like plywood, fiberboard, and hardboard siding panels. It also includes any of the faster burning whole woods.

Ratings between 201 and 500 would be considered a Class D material, and Class E materials include anything with a flame spread rating above 500. Classes D and E are not considered effective against any form of fire exposure. 


What is the difference between Class 1 and Class 0 fire rating?


You may have seen Class 1 and Class 0 fire ratings being used somewhat interchangeably. They are very similar, with Class 0 essentially being an improvement on a Class 1 material.

To comply with Class 0, the material must first conform to Class 1, but must also contain a coating that doesn’t add fuel to fires. Materials typically need at least one coat of an intumescent paint or varnish that swells or chars when exposed to heat and at least one coat of a flame paint finish.


What is a Class A fire retardant?

Now that you’ve had fire ratings explained, you can apply what you’ve learned to fire retardants. Just as a Class A material has a flame spread rating between zero and twenty-five, a fire retardant has a flame spread rating between zero and twenty-five. The same would hold true for any of the five fire rating classifications.

If the rating of your paint or other coating is of a higher class than the material being coated, it will improve the flame spread rating of the material.

Why Does It Matter?

Not everyone has experienced a fire in their lives. Anyone who has knows the value of a good fire protection plan. If you live in an area prone to wildfires, or you live in very close proximity to other houses or units, it’s a good idea to protect your home from spreading fires.

Knowing how your building materials will hold up against fire is a great first step toward effective protection. Choosing materials with higher fire rating classifications may be the barrier that saves your home from a fire.