Cross Laminated Timber: FAQ

CLT, or "Cross Laminated Timber," is a building material consisting of layers of wooden planks, also known as "lamellas." These lamellas are placed side by side and then glued crosswise to other layers of wooden lamellas. Due to these crosswise glued layers, the natural movement of the wood is reduced. The result is a strong and durable construction material. Other terms for CLT are Mass Timber, Mass Lumber, and Massivholz.

It is difficult to separate CLT layers from one another, so CLT components must be preserved as a whole when reused. The use of CLT allows for relatively simple disassembly and reassembly, because the construction of a CLT building is entirely "dry." It requires only the use of screws, bolts, and nuts. As a result, the building components remain largely intact, allowing them to be reused as desired and modified with openings and recesses.

TFC (timber frame construction) uses less timber per building section. In timber frame construction, a wooden framework is built, in which insulation is placed and then finished with board material. In contrast, solid wood panels are used in CLT. Wall openings are prepared and constructed in advance in HSB, while in CLT they are milled from solid board material. Cross-layer timber walls are inherently sturdier and generally require no further finishing when used indoors. An important distinction is that HSB is structurally limited in height, while CLT is already used in buildings up to 18 storeys high. This is because CLT walls and exterior walls form the load-bearing structure of the building.

With cross-laminated timber floors, special attention is required for impact sound. This aspect is taken into account even during the attachment of the floors to the wooden walls. Vibration-breaking material is used in the steel fixings to prevent the transmission of impact sound through the walls. Additional provisions are needed on the floor itself to minimize impact sound. This can be achieved, for example, by using a floating floor with sound-dampening material. There are also systems mounted to the ceiling to reduce impact sound.

CLT walls are generally less susceptible to impact sound because people rarely hit or walk on them. These walls behave like ordinary walls. Vibration-breaking material, as mentioned earlier, can also be applied here to reduce sound transmission.

This depends on the treatment of CLT. Often, a stain or coating is applied to protect the wood against discoloration, although this is not essential for the preservation of the material. However, this coating must be reapplied every few years. In general, CLT requires little maintenance, and any mechanical damage can be easily repaired. The main concern with CLT is the risk of cracking. Cross-laminated timber exhibits expansion and contraction behavior, which can lead to the formation of gaps that are often difficult to avoid.

Wood has a lambda value (thermal conductivity coefficient) of approximately 1, which is significantly higher than that of concrete, steel, or stone. To meet the insulation requirements of the Building Decree or the BENG regulations, additional insulation is necessary. However, it is important to note that the thickness of the wood layer does not affect the insulation value of the material; this value remains the same. As a result, the required additional insulation can be thinner than with other materials.

CLT is usually prefabricated, making it relatively easy to pre-install openings for pipes in the inner layers. This is the standard practice. However, some architects opt for constructions where pipes are placed on the non-visible side of the walls, which facilitates disassembly in the future.

Yes, CLT is sensitive to moisture. It is essential to keep CLT as dry as possible. Although occasional moisture, such as from a leak, is possible, the material must generally be kept dry and well ventilated. Therefore, CLT is usually not suitable for use without a facade finish.

The maximum clear span of CLT is determined by the layout of the entire building. The length of CLT panels is tied to the factory's production capacity, which in turn is influenced by the maximum length for road transport. A maximum length of 15 meters is possible. There is no specific limit on the thickness or the number of laminated layers of lamellas. Thicker CLT allows for larger spans. However, at a certain point, the span may become too thick for economic, practical, or aesthetic reasons. For large spans in timber buildings, a combination of glulam and CLT is often used.