CLT houses - advantages, disadvantages, manufacturers, useful links

CLT is a relatively recent building material, having been in use for 20-something years, and is considered a material of the future. The qualities and performance achieved by CLT have brought it to the attention of Romanian builders and users who want to build with modern and sustainable methods. CLT is already being used in our country and interest in the material is growing all the time. We frequently receive questions about the advantages and disadvantages of building with CLT, construction time, costs and performance. Without claiming to have answered them all, we have gathered below the answers to the most important questions based on the opinions of specialists and those who have decided to use CLT in construction.

What is CLT, how to make it

CLT, Cross Laminated Timber, is considered one of the most modern, high-performance and curate industrially manufactured building materials. CLT is a panel made by cross-laminating timber. It uses what is generally considered to be softer wood, usually softwood, and the cross lamination produces a very strong material used as a wall or floor in both residential and commercial building projects. Basically, we are talking about prefabricated wooden building elements that simplify the design and construction of houses. On the building site, they fit together like pieces in a game.

CLT panels come in different thicknesses. When building a CLT house, different sizes and thicknesses of panels are used for walls, floors and roofs. The thickness of a CLT panel is determined by the thickness of each layer of timber and the number of layers. The thickness of the layers of timber in a CLT panel is between 20 and 40 mm and can be 3, 5, 7, even 9 layers placed perpendicular to each other. In order to obtain wall or floor size panels, the wood that goes into each layer is inspected, defects are removed and then finger-jointed to obtain the dimensions required by the project. It has controlled moisture content, around 12%, and is dried before use in dryers. The layers are bonded together, under pressure, with a special polyurethane adhesive for structural elements, containing no formaldehyde. Further information on CLT manufacturing can also be found at here.

It's no secret that the building materials industry is one of the most polluting. To produce 1 m³ of concrete, 800 kg of carbon dioxide is released into the atmosphere, and for 1 m³ of steel, 2500 kg of carbon dioxide. Wood is a natural material and the tree from which the wood comes uses carbon dioxide in photosynthesis. It is estimated that a tree consumes about one ton of carbon dioxide by the time it reaches maturity. The industrialization process to obtain CLT results in about 300 kg of carbon dioxide being released into the atmosphere, and overall the air we breathe gains, with a minus of 600 kg of carbon dioxide.

What are the advantages of building with CLT

CLT is a strong, lightweight and stable building material. The interlocking layers of wood make the panel stable and unaffected by outside moisture. Each wall functions as a structural element, its strength being determined by the number of layers of wood in its composition. The walls of the house will be able to be sized according to the forces they unload, resulting in a larger usable floor area. Compared to concrete or masonry structures, CLT panels are 4 times lighter, resulting in thinner walls with the same load-bearing capacity, better earthquake resistance and a simpler and cheaper foundation. These advantages are compounded by the insulating properties of wood and the fact that it is a naturally renewable material.

But the most important advantage is the speed of construction. The advanced prefabrication of the building elements (walls, floors, ceilings) and the very high precision with which they are machined in the factory (tolerances of 2 mm) results in a high degree of accuracy in on-site assembly, which drives the construction forward quickly. The large elements fit together quickly, so a single-storey house can be erected in days.

As a living space, a CLT house will bring all the benefits of an energy efficient home

In short, the advantages of building with CLT are:

• Fast construction, clean site, all-weather construction
• More floor area for the same footprint
• Architecture with large openings
• Good thermal insulation from wooden walls, minimizing thermal bridges, thermally efficient house
• High earthquake resistance
• Reduced finishing, as the wood of the walls can remain exposed
• Natural material, environmentally friendly technology

Main disadvantages of using CLT

On the downsides we move on to cost. CLT is one of the most modern building materials with a lot of embedded technology, which makes it an expensive material. The cost increases right from the design phase, when the so-called shop drawings. These are the drawings of the walls and other construction elements, with all the details of machining and joining. They arrive at the factory, and the processing of the walls is based on them. This is a very important phase because it is crucial for the elements to fit together perfectly on site. Hence the cost of the design and several times more than for an ordinary house.

Another disadvantage is the fact that everything has to be very well thought out and decided from the start because it is impossible to change the design along the way. When they arrive on site, the elements are processed to take their well established place in the project and this cannot be changed later.

The lack of CLT providers in the country is also a disadvantage. While there is a commendable start in this area at Technical Switzerland, near Timișoara, the main suppliers are in Austria, with waiting times of months. Transportation costs will also have to be added to the cost of unloading and positioning the walls on site.

As with design, installation cannot be done by just anyone. It requires specialized CLT construction teams and this again means higher costs and waiting time.

For the pros and cons of CLT construction see also hereMarius Șoflete, in a video taken by Marius Șoflete on such a construction site.

Fire resistance

I haven't listed fire resistance as a disadvantage because that is not the case, even though we are talking about wood. The fire behavior of CLT is different from that of a piece of wood. I'm not going to say it doesn't burn at all, because that wouldn't be true. But we are talking about industrialized wood, produced specifically to be used as a structural element in buildings, not beams made from a single piece of wood.

The CLT burns hard because of the layering in the panel. It is very difficult for the fire to advance towards the inside of the panel and the panel does not lose its load-bearing capacity even if a thick layer of jar jar coating appears on the surface. In fact, it is this layer that slows down the burning and protects the interior, keeping the panel strong. The adhesive also contributes to slowing the fire's progress inwards. In order to start burning, the fire would have to start from a corner, making it almost impossible to ignite the wall on the surface.

If you're convinced that an industrialized timber house burns easily, it's best not to consider CLT or timber frame variants for your own home. It will be hard to get over the idea and living in such a house will not be pleasant. However, there are many tall CLT buildings in the world. It is hard to believe that those who have approved such constructions in countries such as England, Canada, Sweden, the USA have not taken this into account.

CLT manufacturers

While we are on the subject of manufacturers, the world's leading manufacturers are in Europe, where they occupy the top 5 positions. The demand for CLT has been growing recently as a result of changes in legislation allowing the material to be used in high-rise construction. As a result, new production centers have sprung up, increasing the quantity produced and reducing waiting times.

The top spot is held by the Swedish-Finnish company Stora Enso followed by KLH Massivholz in Austria. In fact, Austria dominates the ranking, taking the next 3 positions. Find here a full feature on CLT manufacturers.

KLH is represented in our company by Costi Tistea, one of the best CLT specialists. He is based in Rome but has built with CLT all over the world, including a castle in Taiwan. Find here some of the projects he has been working on and an interview with important information about CLT, advantages, disadvantages and when to choose it.

How much does a CLT house cost?

The main disadvantage of CLT houses is the high cost. All experts agree that it is the most expensive house structure. It can reach up to 2-3 times more expensive than a concrete or masonry house.

The answer to the question "How much does a CLT house cost?" is not easy to answer. As far as the structure is concerned, you can get it from a CLT home builder after you present him with the design drawn up by a CLT structural designer. The design phase determines the types of panels to be used, with dimensions for each wall and floor. Based on this design, the builder will know exactly how many panels there will be and what size each will be. He will then be able to make the calculations and price the CLT structure. A 100 square meter house can have different costs depending on the complexity of the building architecture, the construction solutions and even the location.

The cost of the house comes not only from the material for the structure but also from what you derive from using it. The cost calculation should also include the design, the transport from abroad, the crane and equipment needed for unloading, the specialized assembly team, the routing of the installations.

On the other hand, the cost is further reduced by the simpler, and therefore cheaper, foundation work, shorter site time, no waste that would have had to be disposed of, simpler finishes. But if you're thinking of leaving the walls as they come from the factory, you should be aware that panels that can be left as they are are more expensive than those that are considered just for structure and covered with drywall.

Marius Șoflete has made a complete step-by-step calculation based on bids received from various suppliers. Find here all part of a series explaining how to calculate the cost of the same house built on different structures. Even if the materials have become more expensive in the meantime, you can see the percentage differences between the structures used.

Who builds CLT houses in Romania

Building with CLT has been going on in Romania for some time. Even if the panels are brought in from abroad, there are architects, designers and builders in the country who can make such a house for you, if you want. These very days (December 2022) a CLT house is being built near Rădăuți with the architectural design by Miolk, the technical design by Inginerie Creativă, and the wall assembly by Pro Building.

CLT has been used to build houses, company headquarters and office space in Romania. Owners House with sun have written several materials explaining why they chose CLT for their home. Find the materials here. Building with CLT is a big investment and it is essential to do your research first. Good information materials exist.

If the price issue can be resolved, a CLT house is preferable to a concrete or steel one from an environmental and energy efficiency point of view.

It's good to know that there's another way to build if you love wood. Timber frame houses are cheaper, and you still benefit from some of the advantages of building with wood: lighter foundations, greater earthquake resistance, shorter construction time (the shorter the better the prefabrication), cleaner building site, energy efficiency.

I hope you find the above information useful. As always, additions are welcome. And if you have any questions or queries, please leave them below in the space provided. We will do our best to give you the most complete answers.