What B-s1,d0 means for fire-retardant wood

EN 13501-1 test results and certified scope explained for fire-retardant wood.
By Priit Jänes, Products and Innovation, Solid Protect OÜ · Updated April 2026

If you are specifying fire-retardant wood for a building project, B-s1,d0 will appear in technical documents, permit requirements, and product datasheets. This article explains what the classification means, what the EN 13501-1 test numbers show, and how to check whether the classification applies to your product and project conditions.

What is EN 13501-1 and why does it matter?

EN 13501-1:2018 is the European standard for classifying construction products according to their reaction-to-fire performance. It 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. Class B is the highest Euroclass commonly achievable for exposed fire-retardant-treated wood, below the A1 and A2 classes used for non-combustible or near-non-combustible materials. For typical untreated softwoods used in cladding and lining applications, class B is generally not achieved without fire-retardant treatment. Untreated solid timber typically achieves class D or E. A properly applied and tested fire-retardant system can bring typical softwoods into class B, provided the treatment and installation conditions are covered by valid certification documents.

Breaking down B-s1,d0

B - reaction to fire

The letter B describes how the material behaves when directly exposed to flame and radiant heat. The primary test method is EN 13823 (the Single Burning Item test, or SBI), where a corner configuration of two panels is exposed to a 30 kW burner at the base for 21 minutes. Key measured parameters: FIGRA (Fire Growth Rate index, must be below 120 W/s for class B), THR600s (Total Heat Release in 600 seconds, must be below 7.5 MJ), and LFS (flame must not reach the specimen edges). A secondary test, EN ISO 11925-2 (direct flame ignition for 30 seconds), confirms 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.

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.

SPFR100 as an example: B-s1,d0 test results

SPFR100 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 SIA “Meža un koksnes produktu pētniecības un attīstības institūts” / 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. The values show SPFR100's performance in the tested and certified configuration, while project-specific use should always be matched with the relevant documentation. The certificate of conformity ET-0388-24 was first issued by Inspecta Estonia OÜ, part of Kiwa, on 14 February 2024 and remains valid until 13 February 2029, provided the technical description, manufacturing conditions, and factory production control are not significantly modified.

How B-s1,d0 compares to adjacent classes

The THR limit for class D is 50 MJ - more than eleven times the 4.4 MJ recorded for SPFR100-treated spruce. A larger heat release means a more intense fire, giving occupants less time to evacuate.

When B-s1,d0 is required by regulation

B-s1,d0 or better may be required for exterior façade cladding, escape routes, stairwells, corridors, public buildings, and certain mass timber applications. The exact requirement depends on the national building code, building height, use class, and occupancy.

Practical checklist for B-s1,d0 wood products

Before specifying or installing any B-s1,d0 classified wood product, confirm that the classification applies to the exact product and project conditions.

1. Check the required reaction-to-fire classification
   Confirm the required reaction-to-fire class under the relevant national building code and project fire strategy.
2. Verify the classification report or certificate
   Make sure the product has a valid EN 13501-1 classification report or certificate from an accredited body.
3. Check the field of application
   Review whether the classification covers the intended wood species, board thickness, substrate, air gap, orientation, fastening method, and end-use condition.
4. Match the tested system
   The classification applies only to the tested system. Changes in coating, treatment rate, substrate, timber type, or installation method may affect validity.
5. Follow the manufacturer’s application instructions
   Use the declared consumption rate, number of coats, drying time, curing conditions, and maintenance procedure.
6. Avoid untested coatings or modifications
   Do not apply additional paints, oils, lacquers, or topcoats unless they are included in the tested system or approved by the manufacturer.
7. Keep documentation on file
   Store certificates, classification reports, datasheets, and application records as part of the building’s fire safety documentation.

Summary

B-s1,d0 is the highest practical reaction-to-fire classification commonly achievable for exposed fire-retardant-treated wood while preserving the natural appearance of the wood. SPFR100 is designed for a wide range of absorbent wood and wood-based applications, and its published B-s1,d0 test values on spruce show comfortable margins against the EN 13501-1 class B, s1 and d0 limits. For any fire-retardant wood product, always confirm that the required classification, substrate, and installation conditions are covered by the relevant project documentation.

Priit Jänes is responsible for products and innovation at Solid Protect OÜ. Solid Protect develops and produces fire-retardant treatments for wood at its facility in Tänassilma, Estonia, and holds KIWA certification ET-0388-24 for SPFR100 under EN 13501-1:2018.

References: EN 13501-1:2018; EN 13823:2020+A1:2022; EN ISO 11925-2:2020, as used in classification report K16/2024; Classification report K16/2024 (SIA MKPPI, Latvia); KIWA certificate ET-0388-24 (Inspecta Estonia OÜ, 14.02.2024).

• Share: