Save this picture! Wood Innovation Design Centre / Michael Green Architecture. Image © Ema Peter

Concrete, an essential building material, has for decades offered us the possibility of shaping our cities quickly and effectively, allowing them to rapidly expand into urban peripheries and reach heights previously unimagined by mankind. Today, new timber technologies are beginning to deliver similar opportunities – and even superior ones – through materials like Cross-Laminated Timber (CLT).

To better understand the properties and benefits of CLT, we talked with Jorge Calderón, Industrial Designer and CRULAMM Manager. He discusses some of the promising opportunities that CLT could provide architecture in the future.

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Save this picture! via Flickr, User: U.S. Department of Agriculture. Licensed Under Public Domain

What is the difference between laminated timber and CLT?

Laminated timber is the result of joining boards to form a single structural unit. While they can be curved or straight, the grains are always aligned in one direction. With CLT, however, the stacking of boards in perpendicular layers allows the manufacture of plates or surfaces – or walls. It's a plywood made of boards that can reach enormous dimensions: between 2.40 m and 4.00 m high, and up to 12.00 meters long.

Due to the cross orientation of each of its longitudinal and transverse layers, the degrees of contraction and dilation of the timber at the level of the boards are reduced to a negligible amount, while the static load and shape stability are considerably improved. [1]

To transport CLT, the plates are cut into pieces and placed in containers or low-platform trucks.

What is the environmental impact of CLT?

CLT was first manufactured in Austria with the aim of reusing lower value timber. Today, the use of wood is again becoming a relevant factor in the construction industry because of environmental factors.

We usually design and build with concrete, but concrete's environmental footprint is enormous compared to that of wood. One ton of CO2 is emitted into the atmosphere for every cubic meter of concrete created. In contrast, CLT contains "sequestered carbon," or carbon naturally stored in wood during tree growth. Thus, despite all the energy used in the extraction and manufacturing processes, emissions from wood construction will never match the amount of carbon that is kept "sequestered" in the CLT.



Comparison of the energy consumption (GJ/m2) of various construction methods during production:

Save this picture! Holzbau, Rubner. Universidad de Trento, Italia. Image Cortesía de Jorge Calderón

Save this picture! Ryhove Gent / TRANS architectuur I stedenbouw. Image © Stijn Bollaert

How does CLT behave structurally, compared to other materials?

CLT has been called "the concrete of the future," and in a sense – it's true. It delivers at minimum the same structural strength as reinforced concrete, but it's a material with a high degree of flexibility that has to undergo great deformations to break and collapse – unlike concrete. Moreover, 1 m3 of concrete weighs approximately 2.7 tons, while 1 m3 of CLT weighs 400 kg and has the same resistance. The same goes for steel.

Physically, to achieve the same degree of insulation that a 100 mm thick CLT wall would afford, we would need to build a concrete wall 1.80 m thick (1/18 ratio).

Save this picture! MINIMOD Catuçaba / MAPA. Image © Leonardo Finotti

How does CLT behave against fire?

Fire in timber advances at a rate of 0.7 to 0.8 millimeters per minute. If a CLT wall is 100 mm, it would end up being consumed after more than 2 hours, even if it were untreated wood. This carbonization process is a natural phenomenon that allows trees to protect themselves.

Smoke, which is the main cause of death during a fire, moves from one room to the other through slits or open spaces that result from the convergence of different materials. Built correctly, CLT can be completely airtight. So when constructing with CLT, it's extremely important to select and manage all of the elements that make up the final structure, such as fittings, seals, joints, etc. It's estimated that 90% of the strength of CLT comes from its fittings and joints, while only 10% is attributable to the timber itself.

Save this picture! Capilla Sacromonte Landscape Hotel / MAPA Arquitetos. Image © Leonardo Finotti

How can we protect CLT from environmental conditions?

Moisture and weather are the most important enemies of wood. Exposed timber suffers, and since CLT is a structural component, we have to protect it to avoid its wear, corrosion, and collapse. While it's possible to add supplementary layers of coating to wood, such as fiber cement, brick, stone, or other materials, there are also ways to preserve exposed CLT.

Vegetable oils and mineral paints can meet these objectives if applied only once every 5 years, guaranteeing 25 years of protection without detachment or discoloration.

Vegetable oils are recommended for indoor use, while mineral paints work best outdoors, mainly on walls. These products, which are odorless and high performance, can be applied by anyone, following basic instructions and taking necessary precautions.

Save this picture! Antes y después de la aplicación de Osmo Wood Protect de Nuprotec. Image Cortesía de Jorge Calderón

CLT panels function as highly accurate components

When a project is started in CLT, everything is completely decided and predetermined at the factory, and it's not possible to make adjustments on site. So, more than builders, the people who work with CLT are assemblers, who must articulate virtually perfect pieces. CLT behaves with the precision of a piece of furniture, working with margins of error of 2 millimeters.

While the project stage can take a little longer, the assembly is of an amazing speed: in the case of a house of 200 m2, the assembly can take 5 days and occupy a minimum workforce (around 4 instructed people).

Regarding the regulations, there are regulations in the world that guide the design and construction with CLT, but they are the sum of different aspects present in the Standards commonly used in concrete and laminated timber. In 2017, the standard currently used in the United States was published, which is simply a summary of European Standards.

Save this picture! Proyecto de construcción de Jim Vlock 2018 / Yale School of Architecture. Image © Zelig Fok and Nicole Doan

Design and construction recommendations

As previously mentioned, it is essential to understand that the entire pre-construction process with CLT must be carefully developed. Design, planning, and permanent collaboration between the different actors are fundamental since the construction itself will be carried out exactly as defined in the previous stages.

During its manufacture, CLT must be made with structural wood –knowing the structural grade of each board– since the quality of the panel will be the result of the quality of the wood used. In addition, it's necessary to consider that the high precision of CLT must be able to be combined with the foundations that will receive it, avoiding, for example, concrete slabs that present imperfections. Even millimetric variations could generate huge headaches during installation.

Save this picture! Canovas Cassart House / Lacol + LaBoqueria. Image © Lacol / House Habitat

CLT panels are currently allowing the construction of buildings with up to 30 floors, in Canada, and up to 40 floors in Finland. The future is promising and we will remain attentive to your progress. Perhaps in some years, our cities will be transformed based on the warmth and texture of the wood, also changing the way in which the design and construction of our works are conceived.

[1] Exhibition by Jorge Calderón, delivered at the inaugural seminar of the technological dissemination program "Proyectar, manufacturar y construir de forma sostenible con sistema de madera contralaminada (CLT)" | 07.11.2017. Available on Youtube