When it comes to timber in construction, fire retardancy is all about creating time by minimising fire contribution, smoke development and creation of more burning material.
The fact is that timber is gaining popularity in construction. It is a global trend with increasingly large, tall and complex buildings using timber for facades, internal surface finishes and structural support.
Whilst timber’s aesthetic appeal, environmental credentials and engineering flexibility are driving this trend, designers must ensure that buildings must remain safe over the long term. Key to this is making sure that construction timber treatment is correctly specified to meet the regulatory demands which themselves have developed as sub-systems within an overall design that primarily protects lives and then minimises damage.
So how does society make sure that an increasingly popular, naturally combustible material achieves the regulatory thresholds for fire retardancy and sustained safe use in the fabric of a building?
In terms of construction materials’ fire retardancy, clear regulatory definitions exist making it straightforward to understand a material’s performance demands and, with those in mind, where it can be used. This matrix approach is how society manages a material’s influence within a building’s overall fire design and management systems. Of course, constructing the building is only the beginning – due care must be exercised to maintaining that system over the life of the building and consideration given to possible future change of use. Fire-retardant timber is as much a part of that whole-life design so the performance of that treatment must also be maintained.
Certification and assurance
Where does timber fit into this? Untreated timber is combustible and as such is defined as an E Grade material. Concrete, steel and similar materials are typically A1 Graded – no contribution to fire. Correctly chemically engineered and processed fire-retardant timber can achieve a B grading.
Fire-contribution performance is the first of three assessments. Two further measures are taken which relate to smoke development and burning material/droplet emissions.
Most building designers are familiar with these assessments, which are often referred to as ‘Euro Grading’. That said, the second and third measures, smoke and burning droplets, are not so readily understood and yet these are also critical performance parameters.
In terms of timber in construction, the highest possible grading is B, S1, d0 which looks like this:
Laboratories assess these criteria using standardised methods and certify that a specific material achieves the defined performance.
Of course, timber is available in many variants. Species such as pine, oak and so on. The wood might be sawn or fabricated – for example, plywood and cladding systems. This almost endless range of options means that each variant must be individually assessed for fire retardancy before it can be certified.
Typically, treatments are applied one of two ways: By hand (brush or spray) often on the building site with all the challenges these methods and environments present. The alternative is pressure impregnation where the treatment process takes place in a fully controlled, audited factory environment ensuring accurate, repeatable processing necessary to meet the certified performance for that timber species and variant – right to the heart of that material.
Factory processing of fire-rated materials requires the processor to achieve and then maintain a level of production control, repeatability and audit trails to assure the material’s users of its performance. This control and documentation must be applied to every batch and be retained for review before the treated material can be UKCA or EU CE marked.
In summary UKCA or EU CE marking requires three certification elements to be present before it can be applied. Individually these cover the material variant, the processing environment and the history of each specific batch. Without these in place it is hard to see how users can be assured that their timber is fire retardant to a known performance threshold.
Building sustainability
Once finished and in use the building must maintain its fire systems and the timber within that should retain its fire-retardant performance.
Where the treatment has been applied to the surface of the timber – for example by hand, on site during construction – then the timber should be assessed regularly to ensure all the applied treatment is undamaged and maintains the certified thickness. Where it is damaged or has thinned through weathering it should be recoated following the manufacturer’s instructions.
If the timber has been pressure impregnated then it may not require the same degree of ongoing monitoring and repair. Ideally the timber should be impregnated throughout, which means that accidental damage will not reveal untreated wood. Some impregnation treatments are also certified for weathering which in those cases means that the building’s users benefit from a treatment that lasts the lifetime of the timber. Arguably this is the most sustainable approach as once treated there’s no further work needed to maintain the timber’s fire-retardancy performance. It can also be much better value in terms of cost too.
Due attention should also be given to the chemicals used in the treatment. Traditionally fire-retardant treatments for timber used some pretty horrible chemicals – the sort of compounds that are lethal to all life, not just human. Modern fire-retardant timber treatments are the result of many years’ chemical engineering and laboratory testing – some are no more dangerous than lemon juice and yet with process rigor these treatments can deliver wood to the building site with the assurance of the highest possible levels of certified timber fire-retardancy performance. These treatments deliver the transformation of combustible timber into a fire-retardant material that is environmentally friendly, requires no special handling either in construction or recycling and vitally no repeat treatments.
Further information
Timber plays an important role in construction’s future and wood’s fire-retardant performance is critical to society’s safety. There are many factors in fire protection and management – where used timber must be correctly incorporated into the design by making sure that the appropriate use regulations are adhered to. With the assurance of certification we see timber facades being installed that not only look fantastic and are environmentally friendly but, most importantly, meet the highest possible levels of timber fire-retardancy performance.
For more information, go to www.wj-group.co.uk
About the Author
Business owner and Managing Director, Mark Eggleston formed WJ Group in 2007, initially constructing timber roof trusses for the leisure building sector. WJ went on to develop a leading presence in factory-controlled timber treatment – the first of three treatment plants opening in 2009.
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