Service life certainties

We continue to hear much of the ‘star’ qualities of timber –- wood stores carbon and can thus help in tackling the effects of climate change (800kg of CO₂ is locked up in 1m³ of timber). This is good news and will ensure timber markets grow and dominate construction products in years to come. If only it were that simple!

The good work nature does in sequestering carbon in wood is a given. As custodians of this renewable resource we have responsibility to make best use of it. If our manufacturing processes are too carbon intensive, or if our wood products at end of life can only be disposed of in landfill because of the materials inextricably combined with the wood, we are creating potentially unnecessary environmental impacts. Equally, if our wood products are taken out of service because they fail to meet the building client’s needs for service life expectations, or if they are poorly installed and have to be replaced before the planned service life interval, then the positive carbon story of our product unravels.

In the Industry Update I took the opportunity to consider the likely carbon benefits of wood in construction and some of the challenges that remain. I am passionate about the contribution that wood can make to the built environment as innovative products providing demand-led technical solutions. I also believe carbon is a major opportunity. At the centre of the issue, however, are durability and service life. My perspective on sustainable timber construction products is simple – service life is fundamental.

Prediction challenging

Some years ago in an open forum I was challenged, but did not accept, that wood was “by birthright” the low impact choice for construction. It has the best platform to build upon but work remains to be done. My concern at the time was echoed in the Construction Products Directive that demanded provision of six essential requirements for “a reasonable working life” for construction products. This was against a backdrop of other material technologies having established engineering-based models in routine use for service life planning. The in-service deterioration of materials such as concrete, steel and plastic is principally one-dimensional (corrosion, polymer degradation). It is multi-dimensional for wood. Service life prediction for wood products is challenging, not least because of this but also because of the diversity of products and the fragmented evidence base.

BS 8417 caused ripples in 2003 as the only published standard that linked wood preservation treatment quality with desired service life for construction products. Germany is now pushing to revise its standards to include service life. This is a challenge as a wood preservative manufacturer relies on another party to produce the treated wood product and the treated wood might then go on to be used in another product to meet service life criteria. Any such link requires target values tempered with populations of statistically valid data and performance-based evidence. The timber treatment quality scheme proposed by the Wood Protection Association will help tighten up on this objective.

Carbon store

Timber from a sustainable and legal source is a hugely credible carbon store and a capable construction product, be it as cross-laminated timber, a panel product, a box beam or a solid hardwood floor. But this low carbon platform of wood-based products runs the risk of being undone by weak service life data. For example, service life has a significant impact in the Green Guide for Specification impact ratings that underpin the Code for Sustainable Homes. It can be addressed – the British Woodworking Federation has done good work in compiling service life data for wood windows and has improved the evidence base for this product.

As an industry I think we need to be able to communicate to customers how long our products will last before repair, maintenance or replacement is needed. A first attempt has been made at a predictive model delivered by the pan-European R&D project “Woodexter”. It’s a simple tool that connects climate, moisture, building, design, sheltering, consequence of failure and resistance of the material to indicate whether a satisfactory service life will be reached. A guideline and tool are available for download (www.kstr.lth.se). It is work in progress, but progress it is – the first step towards performance-based engineering design for service life prediction of wood products.

The construction industry, both RMI and new build, needs products that have robust properties (dimensional stability, durability, light stabilisation, recyclability) that are multifunctional and designed and executed into buildings to a high standard.

There are heaps of exciting innovations

and developments in the pipeline for wood both near to market and more distant – new products, improved skills and better knowledge. It’s a healthy and vibrant sector that is not afraid to challenge itself and I am confident it will build upon R&D and take wood to the next level.

I look to the future when service life data will be collated and, more crucially, input to the ‘model’ that links design and performance. This will open to the construction professionals that are not wood enthusiasts the freedom of design with wood, whilst meeting customer requirements for maintenance and desired service life criteria in their buildings.