Structural Timber

Timber has been used as a building material for centuries, in fact the oldest timber building is thought to be Horyuji Temple in Japan, estimated to have been constructed 1300 years ago. Although a lot of the original timber may have been replaced over time through restoration, it’s no small feat to have a building last for so long. Some archaeologists have found evidence of primitive constructions dating back to 5500-5000 BCE using wood to line wells.

Horyuji Temble in Nara, Japan

Source: Time Travel Turtle

Up until the 21st century, timber as a building material likely had an image of being suitable only for small residential constructions. Log cabins may have been the predecessors, but timber framed buildings rose in popularity for their flexibility, speed of construction and availability of materials especially in regions with forests. However, in more recent decades, advents in industrialisation have allowed this traditional and historic material to fast track itself into modern construction.

Engineered timber is a term for wood that has been cut and assembled in order to improve mechanical performance. For example, glue laminated timber (Glulam) is composed of multiple pieces of wood glued together so that natural warping can be countered, and the overall size of a single member can far exceed the natural form of trees. Similarly, cross laminated timber (CLT) uses layers of wood stacked on top of one another, where the grain directions are orientated perpendicular to one another, greatly improving the strength.

An example of CLT construction

Source: Arch Daily

These products still have most of the benefits of the rudimentary solid lumber: flexibility, fast construction and general good availability, and so are used on various projects around the world. There are also auxiliary benefits to these choices, for example construction sites tend to be less polluting both in particulates and noise due to not requiring heavy machinery. One other benefit that is now being drawn into focus is how sustainable the material can be in construction.

Common structural materials such as steel and concrete (and in fact most other materials) have associated greenhouse gas emissions, also known as embodied carbon. This accounts for both the chemically produced greenhouse gases from any production process, as well as from the production of energy required in the manufacturing processes. What makes these engineered timber products unique is that as the tree grows, carbon dioxide is sequestered into the wood. This means that from the growth of the tree, through to the production of the material, timber products have net negative GHG emissions.

Embodied carbon of 1 tonne of structural material

Sources: ArcelorMittal, Concrete Centre, Stora Enso & binderholz

The chart above shows how the carbon footprint of some timber products compares to some other construction materials. It’s important to note too, that a comparison by weight isn’t the most accurate, as structures designed with these materials will have different forms, volumes and weights, however whilst the magnitude of the difference may vary, the function of the timber performing as a carbon sink is clear. Furthermore, due to the limited amount of processing required in comparison to other materials, even when the sequestered carbon isn’t counted, the timber products still perform similarly or better than their more common counterparts.

Source: Ema Peter

This environmental benefit has started to resound in the construction industry, with a boom of timber-based structures. The combination of both CLT and Glulam can be used for the entirety of a structure above ground (typically foundations will be made out of concrete for durability), however hybrid structures are also common. These utilise the best of both worlds, by combining steel, concrete and timber in different ways to ensure that each material is used effectively. Importantly, the inclusion of timber ensures that the overall GHG emissions of a building can be dramatically cut helping to offset the emissions from other materials that don’t have these benefits.

Whilst timber-based buildings have a number of positives, it isn’t necessarily an easy win. Consideration must be given to safety, and of course, risk of fire. The Great Fire of London was a key shift in historic policy away from timber framed buildings to masonry and concrete through the London Building Act of 1667. Since then, regulations have since been updated to reintroduce timber to the built environment, designing efficiently and safely is still a priority. After all, a timber building that burns down not only risks the lives of its inhabitants, but also loses any potential environmental benefit by releasing the stored carbon back into the atmosphere. The end of life of timber products is also a contentious issue, as some scenarios result in GHG emissions greater than all the sequestered carbon originally stored (this will be covered in a later post in more detail).

Whilst the construction industry is still wary of these issues, there is an undeniable increase in the use of timber both in the UK and around the world.