Fire retardant

WJ Timber Treatments has a dedicated facility, specifically for the purpose of impregnating fire retardant treatments to a variety of timber species. The BURNBLOCK fire retardant is applied in a controlled factory environment, using the latest high-pressure treatment technology. All substantiating documentation including fire classification reports are supplied with each treatment, providing our customers with the assurances they require.


BURNBLOCK is a non-toxic and highly effective fire retardant formula. The world-patented brand is a water-based treatment, composed of 100% natural ingredients. All of its components can be found in fruits and vegetables, or in nature in their elemental form, meaning BURNBLOCK is 100% biodegradable.

How it works

Fire requires three elements for ignition and continuance: fuel, oxygen and heat. BURNBLOCK effectively prevents oxygen from reaching the treated object, which in our case is the timber.  Without oxygen in the chain reaction, the risk of ignition is greatly reduced and the speed at which any flame spreads is vastly decreased.

There are four defined stages within a fire:

  • Ignition – the start.
  • Fire spread and growth – the speed at which flames and heat spread across materials.
  • Flash point – fire is out of control and all combustible materials are burning.
  • Decay – fire is put out.

BURNBLOCK treated timber effectively extends the length of time between ignition and flash point, providing more time for people to escape in the event of a fire and limits costs in relation to damage.

Treatment process

WJ Timber Treatments uses an impregnation process, through the use of an auto-clave, to apply the BURNBLOCK solution to various timber species. (See auto-clave below) The treatment is clear and invisible, leaving no staining or grease as a result.

Treatment process infographic

Kiln Drying

In many instances construction standards require timber post treatment, to hold between 10-18% moisture, depending on its use.  WJ Timber Treatments have installed their own kiln drying operation to return timber to an appropriate or specified moisture content level.  This careful process assists in binding the fire retardant agents in the wood and to reduce the risk of mould developing.  In addition, experienced operators ensure that there are no colour distortions and that any warping is prevented as a result of being exposed to high temperatures.


Burnblock treated timber has been tested in accordance with EN927-6, one of three test options contained within the new BS EN 16755:2017 standard.  The accelerated weathering process applies UV and the equivalent of more than 120 years of rainfall in a Scandinavian capital. The testing was performed on Thermo Pine and Western Red Cedar and both maintained a Euro Class B fire rating before and after the accelerating weathering, in accordance with test method ISO 5560-1: 2015. The results demonstrate that the effect of Burnblock diminishes at a rate equivalent to the degradation of the wood itself.  It is, therefore, a key consideration that the suitability of a timber species is appropriate for the specific application.

EN 16755 Hygroscopic testing documentation

Testing & Documentation

BURNBLOCK has been tested and certified to the highest European fire ratings by the most recognised institutes in Europe.  These include:

  • Danish Technological Institute – Denmark
  • RISE – Sweden
  • MPA – Germany
  • BRE – UK

Certification is verified by independent test institutes through the issue of a report, which defines the fire and smoke reactions against the European standard 13501-1:2007.  This is known as a classification report, which is issued with each fire impregnation treatment carried out by WJ Timber Treatments.  Please find the BURNBLOCK fire classification tables below which contain information about our treatable species and thicknesses, their applications, and reactions to both fire and smoke.

Solid Wood


  • Interior solutions
  • Exterior solutions
  • Walls
  • Ceilings
  • Floors
  • Scaffold Boards
  • Timber frames
Spruce 350-600 15 mm B-s1;d0 K1; K2;10/ B-s1;d0
Pine 450-600 20 mm B-s1;d0
Oak 500-750 20 mm B-s1;d0
Cedar 350-450 12.5 mm B-s2;d0
Siberian Larch 650-750 21.5 mm B-s1;d0

Classification:   EN 13501-1:2007+A1:2009

Fire Testing:     EN 13823 (SBI – Single Burn Item)

EN 14135:2004

Solid Wood Classification Reports: Spruce, Pine, Oak, Cedar, Siberian Larch

Thermo Wood


  • Interior solutions
  • Exterior solutions
  • Walls
  • Ceilings
  • Floors
  • Decking
Thermo Ash 590-650 21.5 mm B-s1;d0
Thermo Pine 450-500 19 mm B-s1;d0
Thermo Ayous 550-550 19 mm B-s1;d0
Accoya 400-700 19 mm B-s1;d0

Classification:   EN 13501-1:2007+A1:2009

Fire Testing:     EN 13823 (SBI – Single Burn Item)

EN 14135:2004

Thermo Wood Classification Reports: Thermo Ash, Thermo Pine, Thermo Ayous, Accoya


  • Interior solutions
  • Exterior solutions
  • Structural solutions
Birch Plywood 650-750 12 mm B-s1;d0 K1; K2;10/ B-s1;d0
Pine Plywood 450-600 12 mm B-s1;d0
Poplar Plywood 400-500 12 mm B-s1;d0

Classification:   EN 13501-1:2007+A1:2009

Fire Testing:     EN 13823 (SBI – Single Burn Item)

EN 14135:2004

Plywood Classification Reports: Birch, Pine, Poplar



  • Interior solutions
  • Exterior solutions
  • Sound insulation
  • Thermal insulation
  • Vibration insulation
  • Building material
Birch Plywood 650-750 12 mm R10; HL1/HL2/HL3 (flooring)
Birch Plywood 650-750 12 mm R1; HL1/HL2 (walls)
Birch Plywood 650-750 12 mm R7; HL1/HL2 (exterior walls)

Classification:   EN 45545-2:2013 – Fire behaviour of material and products used in trains.

Railway Classification Reports: Birch

Having achieved a smoke classification rating of s1, BURNBLOCK treated products meet the strict requirements for railway rolling stock in accordance with EN 45545-2:2013 and the London Underground.

CE Marketing

WJ Timber Treatments have been independently audited and certified under the Construction Product Regulation (CPR) in accordance with AVCP system 1.

This legal requirement audits the factory production control of the fire retardant facility and ensures the consistency of performance of the manufactured products.

Certificates of Constancy of Performance:

Solid Wood Panelling and Cladding – EN 14915:2013 + A1:2017

Wood Based Panels – EN 13986:2004 + A1:2015

WJ Timber Treatments provide a Declaration of Performance, in line with its CE Mark certification for each of its FR treatments.

Environmental Testing

BURNBLOCK has been tested to prove and substantiate its environmental credentials.  The solution holds a neutral ph value of 7.2 and none of its elements contain toxic substances.  As a result, nothing found in the BURNBLOCK solution is included on the list of Substances of Very High Concern. (SVHC)

BURNBLOCK biodegradable assesment documentBURNBLOCK health assessment document

Safe Handling & Waste Disposal

There is no requirement to use any special safety equipment or protection when handling BURNBLOCK as it is harmless to the skin and it is safe for people to use.  There may be some irritation, should the BURNBLOCK solution accidentally come into contact with the eye, however testing by the Danish Technological Institute confirms the irritation to be no more harmful than that of fruit juice.  Any waste containing the BURNBLOCK treatment can be considered non-hazardous and can be handled in the same way as non-treated timber.  For many of our customers, this results in considerable cost savings when comparing disposal charges associated with other fire retardant treated timber.

BURNBLOCK ph value documentNatural Substance Document

Structural Strength & Corrosively

There is no negative effect on the strength of treated timber after BURNBLOCK has been applied and it does not induce corrosion.  Therefore, there is no requirement to use stainless steel screws or brackets, which could lead to considerable cost savings on projects.

BURNBLOCK metal strength document


What is the difference between Euroclass B and Euroclass C.

Please see the table below which illustrates European classifications in relation to product combustibility.  Untreated wood is generally considered to have a Euroclass D rating however; it is possible to improve this considerably through the use of flame retardant treatments.

A Euroclass B fire rating is considered to be of very limited contribution to fire.

A Euroclass C fire rating is considered to be of limited contribution to fire.

Products are tested in accordance with the European Standard EN13501-1:2007, with treated materials having to undergo staged fire tests; single burn item test EN 13823:2002 and ignitability testing EN 11925-2:2010. The fire testing records data in relation to the spread of flame, heat release and smoke emission.  Through the use of FR treatments, timber products can achieve a fire classification rating to Euroclass B.

If a construction product achieves classification to Euroclass B it also conforms to Euroclass C as the testing criteria is more rigorous to attain a Euroclass B rating.

Increasing Severity BS476 Part 7 – British Standard EN 13501-1 – European Standard FR Build STA/WPA Standard Application
Class 0 Euroclass B High Pressure + Kiln Drying Process
Class 1 Euroclass C
Class 2 FR Build
Class 3
Euroclass D Untreated

What is meant by B, s1, d0?

The B relates to the Euroclass fire classification achieved as a result of undertaking a single burn item test (SBI).

s1 represents the level of smoke emitted during the burning process.  Smoke is measured and given one of the following ratings: s1, s2, s3.  An s1 rating is considered to emit little or no smoke, whilst an S3 classification produces large volumes of smoke.  A product with an s1 smoke rating significantly reduces smoke emission which is the primary cause of death in fires.

d0 describes the level of flaming droplets produced during combustion which can contribute to a fire spreading.  Timber in its natural form is given a d0 rating but other construction products are classified as one of the following: d0, d1, d2.

Classification according to EN13501-1:2007

Definition Construction Products
Non-combustible materials A1
A2 – s1 d0 A2 – s1 d1 A2 – s1 d2
A2 – s2 d0 A2 – s2 d1 A2 – s2 d2
A2 – s3 d0 A2 – s3 d1 A2 – s3 d2
Combustible materials: Very limited contribution to fire B – s1 d0 B – s1 d1 B – s1 d2
B – s2 d0 B – s2 d1 B – s2 d2
B – s3 d0 B – s3 d1 B – s3 d2
Combustible materials: Limited contribution to fire C – s1 d0 C – s1 d1 C – s1 d2
C – s2 d0 C – s2 d1 C – s2 d2
C – s3 d0 C – s3 d1 C – s3 d2
Combustible materials: Medium contribution to fire D – s1 d0 D – s1 d1 D – s1 d2
D – s2 d0 D – s2 d1 D – s2 d2
D – s3 d0 D – s3 d1 D – s3 d2
Combustible materials: Highly contribution to fire E E – d2
Combustible materials: Easily flammable F

What are the differences between Euroclass B and C and the British Standards Class 0 and Class 1?

Euroclass B and C have now replaced British Standards Class 0 and Class 1 as a result of the Construction Products Regulation (CPR) wanting to harmonise fire classification standards across Europe.  This move has subsequently made the British Standards redundant.  However, there are some instances where specifications continue to state that either Class 0 or Class 1 are required, which can be confusing.  Essentially, the Euroclass system is supported by newer and more modern test methods.  Below is a table highlighting where the Euroclass system exists in relation to other fire ratings and or terminology.

Increasing Severity BS476 Part 7 – British Standard EN 13501-1 – European Standard FR Build STA/WPA Standard Application
Class 0 Euroclass B High Pressure + Kiln Drying Process
Class 1 Euroclass C
Class 2 FR Build
Class 3
Euroclass D Untreated

What documentation do I need in order to provide evidence that my products have been treated to the specified requirements?

At the point of dispatch, WJ Timber Treatments supply a Treatment Certificate, an independently certified Classification Report and a Declaration of Performance (DoP) with each FR production.  The details provided on these documents should align with each other and with the details shown on your invoice.  Collectively, this documentation can be used to provide evidence of compliance to Building Control.

My specification requirement states I need to treat timber using an FR3 fire retardant?

These are commodity specifications devised by the Wood Protection Association (WPA) relating to the applied end use of FR treated timber.  They can be used as a guide to decide the most appropriate fire retardant treatment in a given environment. The list of specifications is contained in the table below:

FR1 Non load bearing for dry interior use
FR2 Load bearing dry interior use
FR3 Interior use with relative humidity
FR4 Weather protected exterior use
FR5 Weather exposed environment

Will the BURNBLOCK treatment alter the appearance of the timber?

In some cases, there may be a slight alteration to the colour of FR treated material.  This is species dependant and is due to the variations in the nature of the timber itself.  Please contact us to discuss any concerns in detail

How long does treatment take?

Lead times typically range between 4-6 weeks.  Within this time the timber is impregnated with the BURNBLOCK solution, in a factory-controlled process, and is kiln dried in order to reduce the moisture content to appropriate levels.  There are variations in the required drying cycles for each species of timber and therefore lead times can differ as a result.  Please contact us for further information.

For treatments to scaffold boards lead times are considerably shorter.

Can I profile timbers after treatment?

Burnblock is a maintenance-free treatment that protects the wood against the spread of fire. The wood is treated with a high-pressure treatment that delivers the natural and non-toxic fire retardant to the cellular structure of the wood. For most wood species the treatment penetrates to its core and no further treatment is required for where the wood has been cut, drilled or profiled.

Should I use a fire retardant impregnation treatment or a flame retardant surface application?

The impregnation process deeply penetrates the BURNBLOCK solution into all faces of the timber, providing permanent fire protection without the need for maintenance. Administering fire retardant treatments in a factory-controlled process grants the required assurance that results are measurable and consistent whilst providing full traceability. The impregnation process also prevents the need to apply multiple layers of surface coatings to achieve the required fire classification, resulting in considerable cost savings in relation to its application. The BURNBLOCK treatment provides consistent and assured protection, which may not be achieved through the use of brush applied flame retardant products.

Is it a legal requirement to buy from a treater with CE Mark approval under the CPR?

Yes. It is a requirement for anyone manufacturing FR treatments in a factory-controlled process to be audited by an independent notified body in accordance with the Construction Products Regulation (CPR). You can verify the legitimacy of a Constancy of Performance Certificate by getting in touch with the notified body who carried out the audit on your chosen supplier.

Do products maintain their structural strength after FR treatment?

he FR treatment process can downgrade the structural strength of graded products so we, therefore, recommend further testing is carried out, post-treatment, if structural integrity is critical.

What is a classification report and why is it important?

The independent institute elected to carry out the fire testing will issue a classification report along with all corresponding test analysis after the verification process is complete. The main function of the classification report is to declare what fire classification was achieved on the tested product in relation to the spread of flame, the volume and toxicity of smoke emitted and its susceptibility to produce flaming droplets.
Your chosen provider has a legal obligation to provide you with the Classification Report related to the product you are treating.

What to look for in a fire classification report