Why Construction’s Hottest Material Burns Less Than You Might Think

NBT Updates

The crackle of wood in the hearth or on a campfire is one of those sounds that everyone loves, along with the scent of wood smoke and the warmth from the flames. But it would be a mistake to think that timber burns easily as part of a building’s structure; quite the opposite in fact.

Timber construction has undergone somewhat of a renaissance in recent years and its popularity is growing exponentially. Not only does wood provide varied design possibilities, it also offers a sustainable, renewable and recyclable material that provides carbon lock up for the lifespan of the building, helping to counter the effects of climate change. And that’s before we even touch on the aesthetic appeal of wood and its tactile comfort and familiarity.

Specifiers understand all of these benefits of timber construction, from timber-framed buildings to structures made entirely of wood and contemporary engineered building products fabricated from timber, such as CLT (cross-laminated timber). However, fire resistance has become a key driver for architectural due diligence and the UK Government has recently changed legislation to ban the use of any ‘combustible’ material at construction heights of 18m or above. Consequently, it’s important to understand the proven facts when it comes to timber building materials and combustibility.

If we consider the performance of metal versus wood during a fire, the fire resistance of wood becomes clear.  Steel for example will not begin to weaken until the heat rises to around 2800C but a building fire typically reaches temperatures between 7000C – 9000C; at which point steel only retains 10% of its strength! Conversely timber retains 70% of its strength in these temperatures, which can clearly be seen in the remarkable picture I’ve found to illustrate this blog, which shows metal roof trusses softened in a big roof fire being held up by a timber beam, which was subjected to the same blaze.

The wood is still intact in the image because wood chars, creating a protective layer of carbon around the timber component to protect the natural strength and structure of the material. Timber also has a large capacity to ‘soak up’ heat energy reducing the fire’s impact, particularly during the critical first 30 to 60 minutes of a fire. This is equally true of timber-based products, such as woodfibre insulation and CLT as independent test data gathered from tests commissioned by the BRE has shown. Indeed, woodfibre insulation offers considerably more resistance to fire than many ostensibly ‘non-combustible’ alternatives. Moreover, woodfibre also produces significantly lower levels of toxic fumes when exposed to fire than conventional alternatives.

Unfortunately, building more fire-safe homes is not as simple as banning combustible materials because many non-combustible materials perform poorly in a fire, even if they don’t burst into flames. For example, external cladding may be held in place using aluminium brackets secured through mineral wool insulation. Aluminium melts at 600-6600C but, as mentioned previously, a building fire is typically 700-9000C so structural failure of the brackets that hold the cladding in place can be affected affect. Consider too, the impact fire on an apartment building with PVC windows, which are not part of the over 18m ban, despite creating some of the most deadly gasses in a building fire.

So what is the answer? In my opinion, the response must be a more thorough approach to testing complete wall build-ups and accepting unpalatable results; even if it means finding alternatives to ubiquitous, commodity building envelope materials.

Safer electrics, greater public awareness and a reduction in the number of smokers are contributing to a reduced fire risk in UK homes but the work being done to minimise the spread of fire through improved design and specification of buildings is equally important, which is why it’s time to look again at the real benefits of timber construction.