There is always some hype around products coming onto the market when they offer some promise of savings, sustainability or environmental benefits. Cross Laminated Timber is no exception. First introduced in Europe in 1994, it gained support as an alternative building material and has grown in popularity in the US.

This post will introduce you to the benefits and drawbacks of Cross Laminated timber, particularly when it comes to sustainable building methods.

What Is Cross Laminated Timber?

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Cross laminated timber (CLT) is manufactured by joining sheets of solid wood in perpendicular layers, similarly to how plywood is made.

The boards or panels of cross laminated timber are made from layers of sawn timber that are arranged at ninety degree angles to each other. The boards consist of an uneven number of layers, starting at three layers for the thinner boards (usually five eighths of an inch) to two-inch boards, which consist of five layers of timber. The individual pieces are glued together and placed under pressure to set.

The orientation of the individual layers produces a board with significant tensile and compressive strength and superb structural rigidity.

The species of wood commonly used in manufacturing CLT are generally softwood species like spruce, pine, Douglas fir, and larch.

Initially hailed as the green replacement for concrete and steel, it didn’t take long for questions to be asked about the actual benefits of the timber.

Defenders of traditional concrete and steel construction methods were quick to raise issues around the amount of carbon released by the manufacturers of CLT and sawmills when they burn bark and other wood waste during the manufacturing process.

Forestry practices of leaving branches and residual tree sections in the field to either be burned or allowed to rot were also pointed out as releasing carbon into the atmosphere.

And then there is the issue of deforestation, but in this regard, the CLT team seems to have the upper hand.

High forest product demand creates the incentive to nurture and develop forest areas. Worldwide, countries with high levels of industrial timber usage are generally the countries with lower levels of deforestation, and the US and Canada are pretty good at sustainable wood production.

Trees absorb carbon dioxide throughout their lives and only release it when they are burned or die. The carbon is stored in the building by harvesting the timber and producing CLT boards.

Let’s begin with the features of the material that make it an excellent choice for building. At this point, I should share that I like timber, and that CLT is one of the nicest building materials I have come across. But, moving on without bias, here are the benefits of CLT.

Benefits of Cross Laminated Timber

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Environmental Sustainability

CLT consumes a large amount of timber, and on the surface, this would have a negative effect on sustainability, but there are some factors to consider.

As the timber sections required for CLT construction are dimensionally small, it allows for a more significant percentage of the harvested tree to be converted into useable lumber. 

Cross laminated boards utilize sections of lumber that would traditionally be discarded as being too small. This also makes it viable for smaller trees to be harvested for board manufacture.

The thinning out of densely packed smaller trees significantly reduces the threat of forest fires.

Tightly packed smaller trees act as kindling, so harvesting them not only fulfills a positive role in reducing the risk of forest fires but also utilizes a material that had no commercial value in traditional logging.

The controlled harvesting of lumber positively affects biodiversity and wildlife habitats while simultaneously reducing the risk of devastating wildfires. The removal of closely packed trees allows for a wider variety of vegetation to thrive and provides sources of food and shelter to wildlife.

Shorter Construction Time

I have arguments about this quality with my steel and concrete mates. They claim that the true construction time is from when the boards are laminated, formed and cut to size. While that may have some merit, the more important point is the time on the construction site, and in this regard, CLT is much faster.

Entirely how much faster depends on design parameters. Still, the sequence of pre-cut boards delivered in the correct sequence on the allocated date has an almost idealistic ring to it. The reality is that a crew of four to six craftspeople can install more than ten thousand square feet of building per day using CLT boards.

Less Waste on Site

Pre-cut panels delivered on-site result in far less wastage. Openings are pre-cut, and any waste produced can be fed back to the board manufacturers. The result is a cleaner site, less noise, fewer on-site workers and faster construction times.

With much of the technical expertise completed by the manufacturers, the construction site crew can utilize less skilled local workers. This can create a favorable job creation scenario in areas of high unemployment.

Board Sizes

Great strides have been made in the sizes of boards available, and the only restriction is the limits imposed by transportation. Boards start at five-eighths of an inch (16 mm) thick, but for general building projects, thicknesses of two inches up to as much as twelve inches are available.

Board widths can be as much as ten feet, with lengths up to fifty-two feet available.

The typical panel size is 7.87 ft to ten ft wide and 40 ft long (2.4m or 3m wide by 12.19m long).

Board Strength

Constructing CLT boards with alternating wood grain patterns produces a board with immense strength and stability. This makes it an ideal material for long spans while offering superior vertical strength.

CLT panels have an excellent record of surviving seismic events, making them the ideal building material for areas prone to seismic activity. Tests in Japan demonstrated that a seven-story CLT building could survive fourteen seismic events without substantial damage.

Fire Resistance        

It seems counter-intuitive, but CLT panels perform exceptionally well in retarding the spreading of fires. The outer layer behaves as a self-insulating material and creates a charring barrier so that the inside layers can maintain their strength.

The insulation qualities of the CLT boards result in almost zero transfer of heat from the charring side of the board to the opposite side.

Thermal Properties

The solid wood construction of CLT boards assists in preserving the internal temperature of the building. The more stringent tolerances required in CLT construction also ensure that the joints are airtight, or at least tight-fitting, and the larger boards ensure fewer joints.

The high tolerances of CLT construction produce buildings with excellent draft-proofing due to these tighter joints. Significant energy savings are just one of the benefits of this building method, and a further advantage is the reduced spending on insulation material.

Acoustic Properties

CLT boards have excellent absorption qualities for sound and are ideal materials for theatres and cinemas. The stringent joint requirements play a significant role in reducing both airborne and impact noise. This creates an echo-free acoustic environment.

Aesthetics

Wood has a beautiful appearance. It’s soft, comforting and brings nature’s beauty into our living areas. The finish on CLT boards is conducive to the application of a variety of interior decorating effects.

Drawbacks of Cross Laminated Timber

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Let’s look at some of the aspects of CLT that are not so good. At the outset, it is worthwhile remembering that change is disruptive, and CLT does herald considerable changes to conservative building methods. Also, innovation produces a gradual change, and it is the process of suggesting alternative methods and materials that influence prices.

Costs

CLT is still considered expensive. However, we need to take a view on what constitutes value. Is it only the money, or should we not also add environmental benefits, employment potential and aesthetics? 

How much is saving on greenhouse gas emissions worth? If a midsize building uses CLT timber as part of the construction material, it could reduce the global warming potential of the building by between fifteen to twenty-five percent.

Trees serve as giant absorbers of carbon dioxide and that carbon remains stored in their wood. In this way, carbon remains permanently stored for the life of the timber product.

Using cross laminated timber creates a greater commercial incentive to plant trees. So instead of deforestation, we see an increase in land usage for sustainably managed forests.

These factors must play a role in assessing the true costs of CLT construction.

Insurance Premiums

Part of the gradual rise in the popularity of CLT construction has resulted in Code restrictions around buildings constructed using CLT being evaluated and fine-tuned.

Aligned with the evolution of Code restrictions is the shift in views that insurers take on CLT construction. Track record serves as the fundamental guiding principle for insurers, and consequently, the relatively short history of CLT construction sits uncomfortably with insurers.

This increases insurance premiums for buildings using cross laminated timber, consequently adding to the cost. As the material’s popularity grows, the variance between the insured costs of a traditional building and a CLT one will diminish.

Fitting of Utilities

CLT-constructed walls have no cavities that allow plumbing and electrical wiring to be easily installed. This requires careful planning and has an impact on the overall costs. 

Provision must be made in flooring and ceilings to accommodate piping and cabling. Vertical panels need to be routed to accommodate piping and cabling, or an additional light frame partition needs to be added to house the pipes and cabling.

Higher Delivery Costs

Delivery from CLT production sites raises substantial costs and makes single-story construction relatively expensive. The delivery costs become more manageable with multi-story buildings where the volume of material delivered is greater.

Limits to Post-Construction Alterations

CLT construction requires significantly tighter tolerances. Planning and details need to be at a higher level, which can create time delays in production and delivery.

The more elaborate planning does result in some restrictive features in the construction, and it isn’t easy to make alterations to the design once construction has started.

Ingress of Moisture

The moisture content of CLT must range between nine and fifteen percent. The CLT boards must be protected from the weather during the delivery and construction phases.

Although the glue in CLT boards is waterproof, any water absorption by the boards may result in mold and rot.

Insect Infestation

While it seems that the moisture content parameters for CLT preclude insect infestation in subtropical regions, this may not always be the case.

A recent study by the University of Tennessee found that by vacuum-treating the boards with a borate solution, good penetration was recorded, which would resist some insect infestation but not termite attack.

CLT is used primarily as an internal material, which reduces the threat of termite attack. However, buildings utilizing CLT must comply with subterranean termite control measures where the timber may be exposed to the threat of termites.

Conclusion

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Being able to build sustainably must be high on the wish list of everyone concerned about the quality of the environment we leave behind for future generations. While the visual appeal of CLT interiors remains a personal preference, have a look at this site for some examples of what can be achieved with CLT for family homes.

So, What Did I Learn from CLT?

There are a few takeaways I can share about CLT. Probably the most important lesson was the “if only” one. There can never be enough planning in the early stages. CLT construction is all about fitting tight tolerance pieces together. If there is one lesson to learn, it is to plan as carefully as you can and then go over your work again to make sure.

The speed and silence of CLT building progress are more than impressive. The time savings associated with this building material are not overstated, so be prepared for things to move along a little quicker than anticipated.

The sequence of erection is critical and needs to be planned to the nth degree. Not only are site cranes expensive, but the integral strength of the building relies on the complete structure, so make sure your sequence takes the stresses into consideration.

And finally, while flexibility in design is limited in CLT construction, it does allow owners to move into the completed house and complete the interior fitting out themselves.

Feature image: Anders Vestergaard Jensen; Image 1: LEONG YEE FOON; Image 2: PublicDomainPictures

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