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.

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
Spruce350-60015 mmB-s1;d0K1; K2;10/ B-s1;d0
Pine450-60020 mmB-s1;d0
Oak500-75020 mmB-s1;d0
Cedar350-45012.5 mmB-s2;d0
Siberian Larch650-75021.5 mmB-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 Ash590-65021.5 mmB-s1;d0
Thermo Pine450-50019 mmB-s1;d0
Thermo Ayous550-55019 mmB-s1;d0
Accoya400-70019 mmB-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 Plywood650-75012 mmB-s1;d0K1; K2;10/ B-s1;d0
Pine Plywood450-60012 mmB-s1;d0
Poplar Plywood400-50012 mmB-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 Plywood650-75012 mmR10; HL1/HL2/HL3 (flooring)
Birch Plywood650-75012 mmR1; HL1/HL2 (walls)
Birch Plywood650-75012 mmR7; 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.

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 SeverityBS476 Part 7 – British StandardEN 13501-1 – European StandardFR Build STA/WPA StandardApplication
Class 0Euroclass BHigh Pressure + Kiln Drying Process
Class 1Euroclass C
Class 2FR Build
Class 3
Euroclass DUntreated

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

DefinitionConstruction Products
Non-combustible materialsA1
A2 – s1 d0A2 – s1 d1A2 – s1 d2
A2 – s2 d0A2 – s2 d1A2 – s2 d2
A2 – s3 d0A2 – s3 d1A2 – s3 d2
Combustible materials: Very limited contribution to fireB – s1 d0B – s1 d1B – s1 d2
B – s2 d0B – s2 d1B – s2 d2
B – s3 d0B – s3 d1B – s3 d2
Combustible materials: Limited contribution to fireC – s1 d0C – s1 d1C – s1 d2
C – s2 d0C – s2 d1C – s2 d2
C – s3 d0C – s3 d1C – s3 d2
Combustible materials: Medium contribution to fireD – s1 d0D – s1 d1D – s1 d2
D – s2 d0D – s2 d1D – s2 d2
D – s3 d0D – s3 d1D – s3 d2
Combustible materials: Highly contribution to fireEE – d2
Combustible materials: Easily flammableF

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 SeverityBS476 Part 7 – British StandardEN 13501-1 – European StandardFR Build STA/WPA StandardApplication
Class 0Euroclass BHigh Pressure + Kiln Drying Process
Class 1Euroclass C
Class 2FR Build
Class 3
Euroclass DUntreated

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 certificate and an independently certified classification report with each of its FR timber treatments.  These documents should be used to provide evidence of compliance to Building Control.  The classification report should be species-specific, include the fire rating achieved and should incorporate the minimum allowable thickness of treated timber.

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:

FR1Non load bearing for dry interior use
FR2Load bearing dry interior use
FR3Interior use with relative humidity
FR4Weather protected exterior use
FR5Weather 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?

Timber must be profiled or cut to application dimensions before treatment.  Any re-profiling post-treatment would mean the removal of fire retardant and is therefore likely to downgrade the products fire efficacy.

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.