Timber trusses are greener than light gauge steel

“In comparison to the aviation industry, which contributes roughly 2-3% of global emissions, embodied carbon, specifically steel and concrete production, contributes about 8-10% of global emissions.” Zaid Alwan, Senior Lecturer, Department of Architecture and the Built Environment at Northumbria University.

In South Africa, like the rest of the world, the high environmental impact of buildings and the construction process is driving the development of green building rating tools. Putting the site preparation aside, the choice of building material is a significant contributor to greenhouse gas (GHG) emissions.

When it comes to roof trusses, light gauge steel (LGS) truss systems are slowly gaining popularity over the engineered timber roof trusses produced by the professional members of the Institute for Timber Construction South Africa (ITC-SA).

However, the urgency to mitigate climate change is refocusing the spotlight on timber because of the need to reduce the high carbon footprint of cement and steel production.

The CO2 captured during photosynthesis gives the wood a flying start over other construction materials. The captured CO2 needs monitoring and managing at end-of-life. End-of-life options include reuse, recycling, biomass energy extraction through combustion and anaerobic burial to fix most carbon. The presence of chemical adhesives, preservatives and coatings in the wood influences the end-of-life method.

Carbon pricing and trading policies are increasing, and all support CO2 abatement. These include CO2 compensation, forest carbon stock inventory tracking, and industry schemes incorporating life cycle assessment (LCA).

While LCA provides a holistic cradle-to-grave approach to describe and compare environmental impacts, construction-related LCAs are usually complex and challenging because of the site-specific effects, model complexity, scenario uncertainties and indoor environmental considerations.

“South Africa was the first country in Africa to implement a locally developed green building rating tool and has a growing number of rated green building projects,” explains Dr Philip Crafford, a wood scientist at the University of Stellenbosch.

By 2017 developers and builders were increasingly promoting materials and buildings as “green” and “environmentally sound,” without supporting evidence. Funded by the Hans Merensky Foundation, Crafford, Melanie Blumentritt, and Dr Brand Wessels quantified and compared the environmental impacts of timber and LGS roof truss systems in South Africa.

Using a simplified LCA approach, they compared SA structural pine, Biligom and LGS truss systems in low- and medium-income house designs. Biligom is a sawn timber product made from green finger-jointed E. grandis wood.

They referred to ISO 14041 to determine the bill of materials required to construct the roof truss systems of two houses with cement block walls, one of 42sqm and the other 168sqm.

Their calculations included all the truss material, bracing material, battens, purlins, nails and screws forming the roof structures. The tiles and insulation materials were excluded from the bill of materials but considered when determining the load-bearing capacity of the roofs.

Engineers from MiTek South Africa in Cape Town designed the roof structures with a 17.5° pitch and a service life of 50-years in the Western Cape. MiTek’s software produced detailed materials and cutting lists for the structural components per mass or volume.

Figure 1 shows the cradle-to-grave global warming potential (GWP) incline for three materials and two house sizes. The graph indicates that the two timber alternatives follow a similar near-flat GWP impact trend, whereas the LGS system shows a sharp increase between the small and larger house footprints. Compared to the timber alternatives, the increase is explained by the higher material mass ratio required to scale up the LGS systems from the 42m2 to the 168m2 house. The gradients in the graph show a trend and are not equitable because only two house footprints were analysed.

“Our results show that the two timber systems had the lowest environmental impact. Although the difference between the timber systems was small, LGS had a 40% higher normalised impact over all the assessed environmental impact categories,” they said.

The LCA research found that biogenic CO2 in timber plays a significant positive role in reducing the impact of global warming. Using the wood to generate energy at its end-of-life would reduce it further.

“This study demonstrates the potential advantage of using local timber products to reduce the environmental impact of the truss and building industry in South Africa.”

Source: Crafford PL, Blumentritt M, Wessels CB. The potential of South African timber products to reduce the environmental impact of buildings. S Afr J Sci. 2017;113(9/10), Art. #2016-0354, 8 pages. http://dx.doi.org/10.17159/ sajs.2017/20160354

Click here to read the document

Comments are closed.