Flame Spread Classification for Building Materials

Yet these technical labels have very practical consequences. They tell you how quickly fire is likely to travel across the surface of a material and how much that surface will contribute to fire growth. In other words, they are part of the answer to a fundamental question: what is flame spread, and how can we control it in real buildings?

This guide walks through the basics of flame spread classification, including how different standards approach the problem, what the flame spread index and flame spread rating numbers actually mean, and how to apply them sensibly when selecting products like fire doors, wall systems, glazing, and smoke vents.

What is flame spread, in plain language?

Before diving into any formal flame spread classification, it helps to get clear on the concept itself: what is flame spread?

In simple terms, flame spread describes how quickly and how far fire moves across the surface of a material once it has ignited. We are not talking about the entire structure collapsing, but about the visible flames creeping along a wall, ceiling, door leaf, or cladding panel.

If a material has a low tendency for surface burning, flames will either not travel very far or will die out altogether. If it has a high tendency for surface burning, flames can race along the surface, carrying fire into adjoining rooms or upwards towards higher storeys.

Flame spread classification aims to put numbers or classes on this behaviour so that designers, specifiers, and regulators can compare materials and set limits in building regulations and standards.

Why flame spread classification matters for real buildings

There are three main reasons why flame spread classification is so important for fire resistant solutions:

1. Control of fire growth on internal linings: Interior walls, ceilings, and soffits can either slow fire down or accelerate it. Limiting the flame spread rating of linings helps keep escape routes usable for longer and restricts flashover.
   
   
2. Limiting external fire spread: For cladding and external walls, fast-moving flames can spread fire to higher floors or adjacent buildings. Appropriate flame spread classification for external surfaces supports compartmentation and rescue operations.
   
   
3. Supporting overall fire strategy: Surface burning is just one variable alongside fire resistance, smoke production, and heat release, but it strongly influences fire growth and the time available for evacuation and intervention.
   

The key point: a poor flame spread rating on a large surface can undermine the benefits of otherwise robust fire resistant doors, walls, or smoke control systems.

Different systems of flame spread classification

Several test methods and classification systems are used around the world. They are based on similar ideas but are not interchangeable. When you read test reports or certificates, you will often see one or more of the following.

UK and legacy British Standard – BS 476 Part 7

Historically, many UK projects have used BS 476-7, which measures the “surface spread of flame” along a vertically mounted specimen. The result is expressed as one of four classes, based on how far the flame front travels across the surface within defined time periods:

Class 1: little or no surface spread of flame

Class 2: limited spread

Class 3: medium spread

Class 4: rapid spread

This is a form of flame spread classification, but it does not use a flame spread index in the same way as North American systems.

European “Euroclass” system – EN 13501-1

Across Europe, including the UK for many product types, flame spread classification has largely moved to EN 13501-1. This standard groups construction products into Euroclasses A1, A2, B, C, D, E or F, based on reaction-to-fire tests that measure ignitability, heat release, and surface flame spread, with additional suffixes for smoke production (s1–s3) and flaming droplets (d0–d2).

For example:

A1 and A2: essentially non-combustible materials

B and C: limited contribution to fire

D and E: higher contribution to fire

F: no performance determined

While EN 13501-1 is broader than a simple flame spread rating, surface flame spread is one of the key parameters driving the final class.

ASTM E84 and the flame spread index

In North America, the most widely recognised test is ASTM E84, often simply called the “Steiner tunnel” test. Materials are mounted in a tunnel, exposed to a standard flame, and the speed and extent of flame travel are measured. The outcome is expressed as:

• A numerical flame spread index (FSI)
  
• A smoke developed index (SDI)

The lower the flame spread index, the slower the flame progresses compared with reference materials. Based on the FSI, building codes typically define three classes of flame spread classification:

Class A (or Class 1): flame spread index 0–25 – the most restrictive class

Class B (or Class 2): flame spread index 26–75

Class C (or Class 3): flame spread index 76–200

When specifiers ask for a “Class A flame spread rating”, they are usually referring to this ASTM E84 scale.

Canadian CAN/ULC S102 flame spread rating

In Canada, the closely related CAN/ULC S102 standard is used to evaluate surface burning characteristics. As in ASTM E84, the test produces numerical values for flame spread and smoke, which are combined into a flame spread rating and a smoke developed classification. These values are referenced in the National Building Code of Canada when setting limits for interior finishes and other building components.

Again, this is a type of flame spread classification, but the exact acceptance limits differ from those in US model codes.

How flame spread is actually tested

Although test rigs vary, most flame spread classification methods share some common principles:

1. Standard ignition source: A gas burner with a defined heat output is placed near the edge or underside of the specimen.
   
2. Controlled airflow: The test tunnel or chamber controls ventilation so results are comparable between labs.
   
3. Measurement of flame front: Testers record how far and how fast the visible flame front travels over the surface within a set time.
   
4. Observation of other effects: Heat release, smoke production, and flaming droplets may also be recorded, depending on the standard.

From these observations, the laboratory calculates either a flame spread index or assigns a class in the relevant flame spread classification scheme.

It is important to note that these are comparative tests under controlled conditions. They do not simulate every possible fire scenario, but they give a consistent basis for comparing materials and limiting the worst performers.

Understanding flame spread index and flame spread rating

In practice, you are likely to encounter two main types of numeric descriptor:

Flame spread index (FSI)

• A relative scale derived from tests like ASTM E84, where two reference materials (typically fibre cement board and red oak) define the boundaries of the scale.
  
• An FSI near 0 indicates very limited flame spread; higher numbers indicate faster surface burning.
  
• Building regulations then convert ranges of this index into classes as part of flame spread classification.

Flame spread rating (FSR)

• Used in Canadian standards such as CAN/ULC S102. Here, repeated tests are averaged to produce a flame spread rating and smoke developed classification that regulators use directly in code limits.
  
• Lower flame spread rating values indicate better performance.

When reading a test report, pay attention to:

• The test method (ASTM E84, EN 13501-1, BS 476-7, CAN/ULC S102, etc.)
  
• Whether the figure is a flame spread index or a flame spread rating
  
• Which class or code requirement that figure is claimed to satisfy

Without this context, a single number can be misleading.

Flame spread classification versus fire resistance

Flame spread classification is sometimes confused with fire resistance ratings, but they answer different questions:

• Flame spread classification focuses on surface burning behaviour – how flames move across the exposed face of a product.
  
• Fire resistance focuses on structural performance over time – how long a wall, floor, door, or other element can resist collapse, excessive temperature rise, or passage of fire and hot gases.

For example:

• A fire door leaf may have an FD60 fire resistance rating, meaning it can hold back fire for 60 minutes in a standard test. At the same time, its decorative facing or laminate may have a separate flame spread classification under EN 13501-1 or ASTM E84.
  
• A fire-resistant wall system may combine boards with a good flame spread rating on the room side and robust structural layers behind them to provide the required fire resistance period.

Both metrics are important, but they are not interchangeable. A product can have an excellent flame spread index yet poor fire resistance, or vice versa, depending on its construction.

Practical tips for using flame spread classification in projects

When you specify or review fire resistant solutions, consider the following points:

1. Check the right standard for your jurisdiction
   
   • In the UK and much of Europe, look for EN 13501-1 Euroclass results, and in some cases legacy BS 476-7 classification.
     
   • In North America, verify flame spread index and flame spread rating against the applicable building code.
     
     
2. Confirm scope and orientation
   Flame spread behaviour can vary with installation. Make sure that the test arrangement (e.g. wall or ceiling mounting, substrate, ventilation) reflects how the product will be used.
   
   
3. Beware of partial information
   A headline “Class A flame spread rating” or “Euroclass B-s1,d0” is useful, but you still need to check the complete test report, including coatings, adhesives, fixings, and any backing materials.
   
   
4. Consider the whole system
   The flame spread classification of a single panel is only part of the picture. In a fire door, for example, the leaf facing, glazing, intumescent seals, frame, and hardware all contribute to overall performance. For walls and ceilings, joints, penetrations, and edge details matter just as much as the main boards.
   
   
5. Integrate with your fire strategy
   Use flame spread index and flame spread rating limits to support the wider goals of your fire strategy: keeping escape routes clear, limiting external fire spread, and controlling property damage.

Common misconceptions about flame spread classification

A few myths come up repeatedly when talking about flame spread classification:

1. “If the flame spread index is low, the product is fireproof.”
   Not necessarily. Flame spread tests focus on surface behaviour, not long-term structural stability, load-bearing capacity, or performance of fixings and joints.
   
   
2. “Different standards give identical results.”
   They do not. A product with a good Euroclass result will often also perform well in ASTM E84, but the classifications cannot simply be translated number-for-number. Always refer to the standard actually required by your project’s codes or specifications.
   
   
3. “Any reference to a test is enough.”
   A certificate mentioning ASTM E84 or BS 476 without clear results, classes, or a flame spread index or flame spread rating should be treated with caution. Proper documentation should state the test method, sample description, results, and date.
   
   
4. “Surface spread does not matter for small areas.”
   Even relatively small high-risk areas – such as escape corridors or stair enclosures lined with combustible finishes – can significantly increase the risk to occupants if flame spread is rapid.
   

Bringing it all together

Flame spread classification may look like a dense technical topic, but its underlying purpose is straightforward: to compare how different materials support or resist the surface spread of fire, and to use that information to build safer environments.

If you remember only a few points, let them be these:

• When you ask “what is flame spread?”, you are really asking how a material behaves once fire reaches its surface.
  
• Flame spread classification systems – whether based on a flame spread index or a flame spread rating – give structured ways to compare that behaviour.
  
• Different regions rely on different standards, but all of them aim to limit fast-spreading surfaces, especially along escape routes and critical fire compartments.
  
• Surface performance is only one part of fire safety, but ignoring it can undermine otherwise strong fire resistance measures.

By understanding the basics of flame spread classification and knowing how to read flame spread index and flame spread rating data in test reports, you can make more informed choices about fire doors, wall linings, glazing, and other elements of your projects. That, in turn, supports safer, more resilient buildings where materials are chosen not only for appearance and cost, but also for the way they behave when it matters most.