Wood and Fire

 

Wood and Fire and a little food for thought!

Just at present*, although this is not a new phenomenon, there is an awful lot of disinformation (disingenuous to the point of malicious) being promulgated concerning wood and fire and most particularly in connection with wood in construction.  In the next few paragraphs are a number of facts about wood and how this natural, primordial, diverse, remarkable, beautiful, adaptable, infinitely renewable, paradoxical, carbon-capture-and-storage material behaves when subjected to fire.

Fact: wood burns. Indeed, it does, and that, fortuitously, was (still is, actually!) a very advantageous property as without it, mankind would not exist.  Even today half the world’s population (the poorer half) still relies on wood as their primary source of heat for warmth and cooking.

Before we get to the next Fact, a misconception: we need to consider the misuse of terminology by reference to its definition, and that term is ‘flammable’.

The definition of flammable is ‘easily set on fire’ or ‘easily ignited’

Fact: wood is not flammable.  Wood is ignitable and wood is combustible, cf. first Fact.  Petrol is highly flammable and yet its use is still permitted in motor cars and you are allowed to keep a can of it in your garage for the lawn mower; paper is flammable and yet its use is still permitted in children’s bedrooms and in schools and offices!  And how many other flammable consumer products do we live in close proximity with?

But neither is wood easily ignitable as anyone who has tried to start their log fire can attest.  As another example, take wood matches** which rely on the addition of a flammable accelerant to sustain the burn following initial ignition.

And for a practical demonstration of the ignitability of wood see The Saw Doctor experimenting with his blow torch!  

Fact: wood has an intrinsic and predictable ability, not possessed by all structural materials, to retain its integrity in fire which, together with its well understood combustibility and charring rate are used to advantage with ‘sacrificial charring’, the design principle by which timber structural elements can be designed to retain their structural integrity (i.e. remain standing) in fire events for a required duration, as determined by building purpose group.

 

After fire scene

That is primarily to enable safe evacuation of building occupants and it also gives fire services a level of confidence and security not always available with other structural materials.  That predictable performance in fire also highlights the paradoxical nature of wood in that it burns and yet it is still used for protection against fire in fire doors!

Fact: timber structures can be designed and constructed to considerably out-perform the statutory requirements for performance in fire.  Commonly that would be by using additional layers of plasterboard or by, for example, constructing floors with joists side-by-side and not at spaced centres.  (Check out nail-laminated timber). Those statutory requirements, as mentioned above, to enable safe evacuation of occupants, will never voluntarily be exceeded by house builders and developers because they don’t need to be and because it would add to the cost of construction reducing profitability, something that no house builder or developer will ever do willingly.  On the other hand, we are now seeing the gradual adoption by some of cross-laminated timber for housing!  

Fact: fire happens.  No matter what precautions are taken, because of human carelessness, deliberate action, accident or electrical fault, fires in buildings can never be entirely eliminated.  Because of that, precautions must be taken to prevent fire spread (e.g. fire doors; barriers in the building fabric; installation of sprinkler systems in multi-occupancy, multi-storey buildings) and the ignitability of wood and surface spread of flame can be retarded but wood can never be rendered completely non-combustible.  But again, it is the safe evacuation of building occupants rather than protection of the building which is the main purpose of the application of fire- and flame-retardant treatments.

Fact: wood never causes fires.  And wood will only burn whilst there is a source of heat to maintain the temperature necessary for combustion (cf. that log fire – without a bed of hot embers, log fires will not stay alight for long).  Once the heat source is exhausted wood will self-extinguish.  This is demonstrated most noticeably with cross-laminated timber (CLT), large-section timbers and glulam.

Smaller sections, however, are less resilient and will burn readily once fires have taken hold which is why it is essential that with timber buildings under construction, as indeed it is in any walk of life, particular care should be exercised where naked flames are involved.  (See below). For comprehensive advice refer to the Structural Timber Association

 

Fact: the reality that fire risk can never be eliminated and that carelessness is a common cause is easily demonstrated by the requirement for flammable materials in home furnishings to be treated with flame retardants/ignition inhibitors to reduce flammability; and the mandatory requirement for those products to carry permanent labels at the top of which must be ‘CARELESSNESS CAUSES FIRE’! See FIRA.co.uk for information. And it is the combustible and flammable contents of rooms which constitute the greatest fire loading in domestic fire incidents. (Just google ‘fire load’ or ‘fire loading’). 

**Hence, two similar warnings on that box of matches ‘KEEP AWAY FROM CHILDREN’ and ‘DANGER! FIRE KILLS CHILDREN.’

 

(When properly constructed, wood works!  For example – a number of years ago I had to do a site inspection following a domestic fire incident in a timber frame dwelling to determine what was recoverable.  My main recollections are that the roof trussed rafters were a complete write-off (spread of flame following flashover out through the window and up into the roof through eaves ventilation) and, most importantly, the fire had not spread to adjacent properties; it was mid-terrace. It was duly reinstated.  And the cause of the fire?  An aggrieved resident had set light to the back of a sofa!) 

As far as the use of wood in construction goes, whenever there is a building fire in the UK it has become common practice for the naysayers, generally from competing materials sectors, to condemn the use of wood, particularly in housing and even when wood has not been implicated or been a contributory factor, claiming that it is unsafe, even dangerous: disingenuous to say the least as their sole intention of this is to prevent wood in construction eating in to their market share!

And, unfortunately, the knock-on effect of this is that some insurers are more inclined to react to the misinformation than they are to employ common sense and are allegedly refusing to grant insurance on properties with even minimal amounts of exterior wood cladding as if it is now being expected to spontaneously combust!  Hopefully the forgoing will alleviate their concerns.

Wood-frame construction is, as a matter of fact, the most prevalent form of construction for housing in a number of regions worldwide including Australia, Canada, the U.S.A where, in a good year, they build in the region of 1.8 million new homes(!), New Zealand, Northern Europe, Japan and parts of South-East Asia and it is also particularly suited to use in earthquake regions for its resilience against seismic forces.

And as cross-laminated timber for medium-rise construction is increasingly finding favour around the world, additionally for the carbon sequestration potential as well as for its structural competence, in the UK the negative messaging mentioned above is having the intended adverse effect.  For Government it is the easy option, to prohibit the use of wood in the external walls of medium-rise construction rather than take the proactive approach and requiring sprinkler systems to be mandatory for all multi-occupancy buildings above a certain height; common practice in other countries.

In the new world order, wood has constantly had to jump through more hoops than any other (structural) material, more recently because it necessitates cutting down trees but no-one seems to make such a fuss over the mining of iron ore or limestone unless trees have to be cut down to get at them!   All forests are infinitely renewable when looked after properly and a forest with no commercial value will not remain forest for very long.  And it is also worth mentioning that wood is hugely undervalued, especially important when it is realised that third-party certification is not a charitable service particularly pertinent in light of the fact that of the trees being planted now, some are the legacy for at least three – and in tropical forests considerably more – generations hence.

*At time of writing, October 2019

 

Contributor:  John Park, FIMMM