Understanding BS EN 13501-1: A guide to reaction to fire classifications
- May 12
- 3 min read
Whether you are a manufacturer preparing a product for market, an architect specifying materials, or simply someone trying to make sense of a label on a building product — this guide explains what a reaction to fire classification is, how it is structured, and what each part of it means.
When a construction product undergoes reaction to fire testing, the outcome is expressed as a structured classification rather than a simple pass or fail result. This classification is defined by the European standard BS EN 13501-1, and it communicates three distinct pieces of information about how a material behaves when exposed to fire: its combustibility, its smoke output, and whether it produces flaming droplets.
Understanding this classification system is an important part of knowing what fire test data actually means and how to interpret the information on a product's technical documentation.
How the classification is structured
A full BS EN 13501-1 classification is made up of three components written together. For example: B-s1,d0. Each part independently describes a different aspect of the material's fire behaviour. Let's look at each component in turn.
Part 1: The primary classification (A1 to F)
The first element describes the material's overall reaction to fire — specifically, how much it contributes to the development and spread of a fire. The scale runs from A1 at the highest level of performance down to F, which indicates no determined classification.
A1
NON-COMBUSTIBLE
Does not contribute to fire under any test conditions.
A2
LIMITED COMBUSTIBILITY
Does not contribute to the development of a fire.
B
COMBUSTIBILE
Limited contribution to fire
C
COMBUSTIBLE
Minor contribution to fire.
D
COMBUSTIBLE
Contributes to fire.
E
COMBUSTIBLE
Resists a small flame for only a few seconds.
F
NO PERFORMANCE DETERMINED
Not classified or did not meet class E requirements.
Worth knowing 💡
A1 and A2 are distinct from the classes below them. Materials in these two classes are considered to pose no meaningful contribution to fire growth — an important distinction when understanding where higher-performing products are required under building regulations.
Part 2: Smoke production (s1, s2, s3)
The second part of the classification describes how much smoke a material generates when burning. Smoke production is classified independently of combustibility, because a material can have a strong primary classification while still generating a significant volume of smoke.
s1
LOWEST
Low smoke production
s2
MODERATE
Moderate smoke production
s3
HIGHEST
High smoke production
Part 3: Flaming droplets and particles (d0, d1, d2)
The third component describes whether a material, when burning, produces flaming droplets or particles that detach and fall. This matters because falling burning material can ignite other surfaces or present a hazard to people below.
d0
LOWEST
No flaming droplets or particles
d1
MODERATE
Limited flaming droplets
d2
HIGHEST
High quantity of flaming droplets
Reading a complete classification
Once you understand the three components, reading a full classification becomes straightforward. Here is how to interpret an example:
B-s1,d0
B
COMBUSTIBLE
s1
LOW SMOKE
d0
NO FLAMING DROPLETS
This classification tells you that the product has a limited contribution to fire, produces minimal smoke during burning, and does not shed flaming particles under the test conditions. Each component is determined through a specific series of test methods that form part of the BS EN 13501-1 test programme.
Why classifications matter for product manufacturers
For manufacturers, the classification is more than a label; it is the documented evidence of a product's fire performance. Construction products placed on the market are required to carry a classification so that those specifying or installing them can verify that the product meets the fire performance requirements of a particular application or regulation.
It is worth understanding that a classification is always specific to the product as tested including its substrate, thickness, fixing method, and end-use application. Changes to any of these conditions may affect whether the classification remains valid for a given use case. Understanding the scope of a classification is just as important as knowing the classification itself.
If you are unsure whether an existing classification applies to your product in its intended application, the appropriate next step is to consult the classification report itself or speak with a testing laboratory who can help you understand what has and has not been established through testing.
Have questions about the testing process?
At UKTC, we are UKAS accredited to carry out reaction to fire testing to BS EN 13501-1 and can help you understand what testing is required for your product. We are happy to discuss the process, explain what the test programme involves, and help you plan a testing scope that reflects your product's intended use.
Click here to contact us.
