Cross-laminated timber: The Ultimate Guide to CLT & how to protect it

Cross-laminated timber, better known as CLT, is a type of mass timber panel used in construction. CLT panels are made by gluing together layers of sawn lumber. Each layer (or lamella) is comprised of lengths of solid wood that are laid side by side.

Each layer of a CLT panel is oriented with the wood in a perpendicular direction to the previous layer. This helps to achieve structural strength in both directions. CLT is sometimes called “super plywood”, because it is similar in structure to plywood, but CLT has thicker layers. CLT is most often made from spruce, though other types of softwood like pine, larch and fir are used. 

Because of its structural properties, CLT is increasingly used for the construction of tall buildings. CLT is starting to challenge the dominance of traditional heavy materials like steel and concrete, because it has a smaller carbon footprint while being lighter to build with, and delivering fast construction times. 

To learn more about cross-laminated timber, watch this video.

CLT is just one type of mass timber construction. Mass timber refers to composite wood products made by combining pieces of wood together, to increase the overall strength of the panel. 

Some other types of mass timber include: 

Glue-laminated timber, or glulam, is similar to CLT, but in this case the solid timber layers are oriented in the same direction, rather than perpendicular to each other. This makes glulam more suitable for building elements in which structural strength is required in just one direction, such as for columns and beams. CLT and glulam elements are often combined together in the same buildings. 

Glue laminated timber was used for the Tondiraba Ice Arena in Tallinn, Estonia.. In the structure several special structural solutions were devised, such as the imposing 62-metre span wooden girders.
Photo: Arcwood

Brettstapel (or dowel laminated timber) is a type of mass timber construction that requires no adhesives. Instead, wooden dowels are used to connect timber elements together. The dowels have a lower moisture content than the surrounding wood, so they absorb moisture and expand once the panels are assembled, locking the timber pieces together. Brettstapel is capable of using lower grade timber, locking the pieces together to create a load-bearing wall. 

Mass plywood panels (MPPs), also known as laminated veneer lumber (LVL), are one of the newest forms of mass timber construction. In this case, wood veneers — very thin slices of wood or bark — are glued together to form structural strength panels. One of the advantages of MPPs is that they make use of lower grade wood and smaller trees.

MPP – mass plywood panels at Freres Lumber in Lyons, Oregon, USA
Photo: Treesource

1. The production of CLT begins with planks of rough sawn solid wood, cut from single logs. The boards are loaded onto the production line and scanned for their moisture content. They are then glued together end-to-end using finger-joints to create longer boards, and left to cure.
2. The long boards are mechanically planed on all sides to the specifications required. They are then tightly packed together, side by side, to eliminate any gaps, and an adhesive is applied to their upper surface. The boards may also be glued together, edge to edge, to improve airtightness. The next layer of boards is then glued on top in a perpendicular direction.
3. The gluing and stacking process continues until all of the layers are in place, and the panel is then mechanically pressed. CLT panels typically consist of between three and nine layers, depending on the specification required.
4. Once the panel has fully cured, any openings, or holes can be pre-cut or pre-drilled in the factory.

Learn more about how CLT panels are made

CLT is lighter than traditional structural materials like concrete and steel, making it safer and lighter to build with, according to its proponents. And because the panels are precision-engineered to specification in the factory, less time is needed on the building site. Construction is usually neater too, as waste can more easily be gathered and recycled in the factory environment. 

Construction of Brock Commons House in Vancouver, one of the world’s tallest mass timber buildings. 
Photo: Acton Ostry Architects / Pollux Chung

During a fire, the outer layer of the CLT panel chars, and this ‘char layer’ then becomes a protective blanket that prevents the fire from reaching the unburnt layers inside for a period of time. Lining the CLT with gypsum wallboard internally can add further protection. CLT is increasingly approved for use in multi-storey buildings around the world, but fire regulations and guidance covering the use of CLT will vary by region. Please refer to your local building codes or contact your local fire officer for guidance.

CLT is regarded as a more sustainable material than either steel or concrete, both of which have a high carbon footprint. Concrete is responsible for between four and eight per cent of global carbon emissions, while steel is responsible for between seven and nine per cent.

According to one recent study, which modelled hundreds of different multi-storey building types, the carbon footprint of mass timber structures is, on average, 35% less than equivalent concrete buildings and almost 50% less than steel buildings. 

Building with mass timber is seen as one way to help tackle climate change, because trees absorb carbon dioxide as they grow, and this carbon can then be stored in the structure of those buildings. Recycling the timber at the end of the building’s life will help to keep this carbon out of the atmosphere for longer. For timber construction to genuinely benefit the climate and environment, trees must come from forests managed to high standards of sustainability.

Cross-laminated timber, like many forms of timber construction, is beautiful, and can make our buildings more pleasant to live and work in. We know that being out in the forest is good for our health and wellbeing, and hospital patients are even known to recover faster if they have a view of trees from their window. 

But indoor environments that feature wood can be good for us too. Studies have shown that living or working in rooms with visible wood surfaces can reduce stress and improve mood. One study found that pupils working in classrooms with exposed wood surfaces experienced lower heart rate and less stress.

Austria’s first wooden high rise building “HoHo Wien” in Vienna was built with CLT. The 84m high skyscraper is located in one of the largest urban development projects in Europe. The proportion of timber in the hybrid building is 75 per cent from the ground floor upwards. Hoho Wien is considered a showpiece in the field of sustainable, innovative construction.

In practice, the extent of exposed timber in buildings can be limited by local building regulations for fire safety reasons. But these studies show that even some exposed wood can improve our mood and wellbeing.

Because cross-laminated timber consists of multiple pieces of wood joined together, it may not be as inherently airtight as other, more monolithic materials. But with good planning and workmanship, buildings can achieve very high levels of airtightness using CLT construction, including the passive house standard. 

Watch the video to learn how to protect timber elements from moisture with Wetguard® 200

CLT can also warp, swell or become discoloured if it is exposed to moisture and unable to dry out. One recent Swedish study, which looked at 200 measurement points on four CLT buildings in Sweden that were left unprotected during construction, found mould growth at around half of the points measured. Much of the mould growth was invisible to the naked eye and could only be seen through a microscope. The study concluded that CLT is not safe from moisture during construction without weather protection.

Obtain the Weatherproofing and Airtightness for Mass Timber technical guide, which provides detailed steps for ensuring airtightness in mass timber elements, protecting building materials from adverse weather conditions, and achieving airtight construction.

Set on shores of Lake Mjøsa, north of Oslo, Mjøstårnet is currently the world’s tallest timber building, at 85.4 metres and 18 storeys high. The building houses a hotel, apartments, restaurant, and communal areas. It was constructed from load-bearing glulam trusses and beams, with CLT walls. Concrete decks were specified on the upper floors of the building to increase its weight and prevent swaying. The project featured numerous SIGA products including Wigluv and Rissan tapes.

Learn more about Mjøstårnet from SIGA.

Photo: Voll Arkitekter / Ricardo Foto

Brock Commons Tallwood House is an 18 storey student residence at the University of British Columbia in Vancouver. The building is 53 metres tall, and at the time it opened in 2017, it was the tallest mass timber building in the world. The floors are comprised of five-layer CLT panels, supported on glulam columns, while the stair and elevator cores were built with concrete. Learn more about the Brock Commons House. 

Photo: Acton Ostry Architects / Pollux Chung 

Dalston Works in Hackney, London, was the world’s largest CLT building when it was completed in 2017. The ten-storey, 121-unit apartment development was designed by architects Thistleton Waugh, and was built with CLT. According to Thistleton Waugh, Dalston Works has one fifth the weight of a concrete building of the same size. Learn more about Dalston Works. 

Photo: Waugh Thistleton / Daniel Shearing

"HoHo Wien" is currently the second tallest timber building in the world at 84 metres. It was primarily constructed with CLT, and all-in-all the complex includes five buildings ranging from six to 24 storeys tall. The project featured various SIGA products including Wigluv and Fentrim tapes.

Learn more about the project from SIGA.

Photo: KiTo Photography

While it is rare to see modern bridges built out of timber, the company Nordic Structures has specialised in building mass timber bridges in the forested region of Northern Quebec. Most of these are primarily built with glulam, but the Maiscasagi Bridge also includes a significant amount of CLT. Finished in 2011, it is 67 metres long and was designed to handle the weight of heavy logging trucks.

Photo: Nordic Structures / Chantiers Chibougamau

Stora Enso. Formed from the merger of two forestry and mining companies, Stora Enso is a Finnish company that specialises in pulp, paper and other forest products. It is one of the oldest companies in the world, its history dating back to a share in a copper mine granted in 1288. 

KLH Massivholz was one of the pioneering developers of cross-laminated timber back in the 1990s, and the company opened its first production facility in 1999. Since then it has delivered more than 30,000 projects across the globe. 

Mayr-Melnhof is a specialist in forest products, particularly paper and packaging, and has manufacturing facilities located around the world. The company makes CLT from its factory in Gaishorn, Germany, and recently announced that it would construct a new CLT plant in Leoben, Austria. 

Binderholz produces solid wood and mass timber products from 14 locations across Europe, and employs about 3,000 people. The company is based in Fugen, in the Austrian Alps, where it has been headquartered since 1963. The third generation of the Binder family still manages the company today.

Does CLT construction cost more than conventional alternatives like concrete and steel? The answer can depend on many factors, including what exactly you include in your cost comparison, and where in the world you are planning to build. 

Simply comparing the basic cost of CLT to other materials may not provide a full picture, particularly for larger buildings. This is because while the material cost of CLT can sometimes be more than steel or concrete, CLT can save money in other places. For example, CLT is a lighter form of construction, which means it may be possible to use a lighter foundation. 

Prefabrication also means that CLT systems are erected quickly, meaning less labour and less time on site. And transport costs can vary: while CLT is lighter and easier to transport, it may need to travel further depending on where your project is, given that concrete can usually be sourced locally. 

One recent cost comparison for a seven-storey residential building in London found that there is very little difference in the cost of CLT compared to reinforced concrete, when the full build costs are compared.