BS EN 13501-1 Classifications A1, A2-s1,d0: Definition, Test Criteria and Application

EN 13501-1:2018 is the European standard that classifies construction products according to their reaction-to-fire performance. If you are specifying fire-retardant timber, installing lining panels or selecting façade materials, the classifications A1, A2-s1,d0 and B-s1,d0 will appear in technical datasheets, permit requirements and building control submissions. This article explains what each classification means, which test parameters are measured, and how to check whether a product's certification scope covers your project conditions.

What EN 13501-1 Measures – and What It Does Not

EN 13501-1:2018 applies to products used in walls, ceilings, façades, linings and other building applications. It does not replace fire-resistance classifications such as R, REI or load-bearing fire design. The standard defines seven reaction-to-fire classes: A1, A2, B, C, D, E and F. Class A1 covers non-combustible materials such as mineral wool and concrete.

For combustible materials – including timber and engineered wood products – class B is the highest Euroclass commonly achievable when exposed. Fire-retardant treatments can bring typical softwoods into class B, provided the treatment and installation conditions are covered by valid certification documents. For typical untreated softwoods used in cladding and lining applications, class B is generally not achieved without fire-retardant treatment.

Breaking Down the Classification Code: B-s1,d0

The classification code consists of three components. The letter (B) describes how the material behaves when directly exposed to flame and radiant heat. The suffix s1 defines smoke production. The suffix d0 refers to flaming droplets and particles. Each component is measured in separate test procedures and must meet defined thresholds.

B – Reaction to Fire

The primary test method is EN 13823, known as the Single Burning Item (SBI) test. A corner configuration of two panels is exposed to a 30 kW burner at the base for 21 minutes. The key measured parameters are FIGRA (Fire Growth Rate index), THR600s (Total Heat Release in 600 seconds) and LFS (lateral flame spread).

A secondary test, EN ISO 11925-2, applies direct flame ignition for 30 seconds to confirm no sustained flame spread.

s1 – Smoke Production

The suffix s1 is the strictest smoke class. SMOGRA must be below 30 m²/s² and TSP600s must be below 50 m². Class s2 allows up to SMOGRA 180 m²/s² – six times more smoke growth. Dense smoke reduces visibility and causes inhalation injuries well before flames reach an area, making the s1 classification critical in escape routes and enclosed stairwells.

d0 – Flaming Droplets and Particles

The suffix d0 means no flaming droplets or particles were produced during the test within 600 seconds. Flaming droplets are a secondary ignition hazard – a single droplet landing on a combustible surface can start a new fire front. Materials that produce flaming droplets receive a d1 or d2 classification.

A1 and A2-s1,d0: Non-Combustible and Near-Non-Combustible Classes

Class A1 is reserved for materials that contribute negligible heat or flame to a fire. No organic content is permitted. Typical A1 materials include mineral wool, concrete, calcium silicate masonry and ceramic products.

Class A2-s1,d0 is assigned to materials with very limited combustibility. These materials may contain small amounts of organic matter but must not sustain flame spread or contribute significant heat. The A2 classification requires testing under EN ISO 1182 (non-combustibility test) and EN ISO 1716 (calorific value). Materials must have a PCS (gross calorific value) below 3.0 MJ/kg and meet strict temperature rise and mass loss limits.

The A2-s1,d0 classification is often specified for thermal insulation in fire-sensitive applications such as escape routes, high-rise façades and buildings with heightened fire safety requirements.

How Test Results Compare: SPFR100 as a Worked Example

SPFR100, a fire-retardant coating for timber, was tested on solid spruce panels (18 mm and 50 mm thickness) treated with two coats at a total application rate of 240 g/m². Tests were carried out by the Forest and Wood Products Research and Development Institute, Latvia, accredited under LVS EN ISO/IEC 17025 (T-316), Notified Body NB 2040 under EU CPR 305/2011. Classification report number K16/2024.

The FIGRA value of 56.1 W/s is less than half the class B threshold of 120 W/s. The SMOGRA of 2.8 m²/s² is less than one tenth of the s1 limit of 30 m²/s². These are not borderline results.

When to Specify B-s1,d0, A2-s1,d0 or A1

Building regulations in many European jurisdictions require specific reaction-to-fire classes based on building height, occupancy type and fire load. High-rise residential buildings, hospitals, schools and buildings with limited evacuation capacity often mandate A2-s1,d0 or better for external wall cladding and insulation. Internal linings in escape routes may require B-s1,d0 or A2-s1,d0 depending on local fire codes.

Before specifying a product, confirm that the certification scope matches your substrate, thickness, coating system and end-use condition. A classification report such as K16/2024 defines the field of application – including minimum thickness, application rate and substrate type. If your project falls outside that scope, the classification does not apply.

Practical Checklist for Site Use

When fire-retardant products arrive on site, check the following:

Fire safety is not achieved by choosing a product with the right classification code. It is achieved by ensuring the entire system – substrate, treatment, application method and maintenance regime – matches the tested and certified configuration. For further guidance on fire-protective systems and sustainable material choices, consult the relevant national building codes and notified-body documentation.