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New Classes for Fire Retardant Treated Wood

A Nordtest method (NT Fire 054) to classify the durability of fire retardant treated wood was established in 2006 and a European specification (CEN / TS 15912) was adopted in 2013. They define classes for fire retardant treated wood for in- and outdoor use in buildings and thus constitutes a supplement to the standard fire classes. Classifications are a powerful tool for manufacturers to document fire retardant durability in it's products, and for users to be able to demand and products to live up to relevant requirements.


In accordance with the new system fire retardant treated wood is divided into classes mainly with regard to the fire retardants durability and effectiveness in humid environments, refer to table below.


The European classes are consistent with the Nordic ones but includes a new class for indoor use at lower levels moisture, INT 1. The Nordtest methods (NT Build 504 and NT Fire 053) are included as annexes in the European specification.


All types will of course comply with existing requirements for fire properties, according to the new European fire classification.

Classifications of fire retardant treated wood – Requirements in accordance with CEN / TS 15912



Fire Requirement Functionality Requirements for Different End Uses 

Area of Use

Fire Class, Initial

Moisture Properties

Fire Resistance Rating Following Exposure to Weather 


Short Duration

Relevant Fire Class




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Indoors, Dry

- " -

- Moisture content <20%

- Minimal amounts of visible salt on surface, not increasing

- No formation of liquid



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Outdoors, Humid

- " -



- Moisture content <28%

- Minimal amounts of visible salt on surface, not increasing

- No formation of liquid



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- " -

- " -

Retained Fire Class Following:

- Accelerated ageing

- Natural ageing

- Other documented ageing method 


Examples of Test Procedures 

Test for moisture sensitivity in indoor use (Class INT 1 och INT 2).


Testing of durability to weather in outdoor use (Class EXT). Accelerated ageing at the top and natural ageing at the bottom.