What is Cross Laminated Timber (CLT)?
THE BASICS
Cross-laminated timber (CLT for short) is perhaps better understood by its colloquial term - super plywood. It consists of perpendicular layers of timber called lamellas adhered together using glue and a hydraulic press. CLT is an innovative construction material that can be used for floors, walls and roofs. Large panels are cut to size using accurate computer control machining and openings for windows, doors and skylights cut out in the same way. CLT is assembled in position using a crane and attached together with brackets and specialized screws according to the engineer’s and manufacturers specification.
Using CLT for house construction has been common practice in Europe for 30 years, but in the last decade it has soared in popularity around the world. It is likely to continue to rise and could very well become a dominant residential construction method in Australia. Many people that opt for a CLT home choose to leave the CLT exposed to the interior as a feature unique to the construction style. However, CLT can also be covered with plasterboard or other lining for a more traditional style.
BENEFITS OF CLT CONSTRUCTION
ENVIRONMENTAL
CLT is a great environmental choice for construction. The timber used in the CLT panels is sourced from sustainable forests. The trees have absorbed carbon out of the atmosphere which remains locked away in the CLT panel. The CLT can be referred to as a “carbon sink” because it takes carbon out of the atmosphere rather than putting carbon into it, such is the case with the manufacture of concrete or steel.
COST
In Australia, CLT is roughly 20% cheaper than lightweight construction. This is even considering European manufacture with international shipping, though there are domestic options too. This is because the cost of the product and operation of the crane is offset by the reduced labour costs of a traditional build.
The standard construction process of a CLT dwelling is to have the foundations prepared and ready by the time the CLT and windows are delivered. Typical assembly of the structure for a single dwelling takes 4-10 days including taping and fitting of windows and doors, by which time the building is airtight and watertight. After this, trades will conduct the internal fit out, services, insulation, cladding and roofing. This method reduces a lot of labour which is a significant proportion of Australian building cost.
COMFORT
CLT is very easy to make airtight which goes a long way to preventing heat gain in the summer and heat loss in the winter. Passive houses, which many consider to be the most efficient and comfortable way to build, use airtightness as one of their tenets to achieve such high performance. Since CLT homes use such large panels, there are very few joins in the building fabric, and these are easily sealed with airproof tape. Unsurprising then, that CLT should feature in many examples of certified passive houses.
HEALTH
Because CLT construction has an air-proof barrier and breathable insulation and cladding, there is no chance of mould growth in the building fabric if constructed correctly. Also, because nearly all the cutting is done offsite there is very little dust created during construction. Reputable CLT manufacturers will use non-toxic glues. But it is important confirm the technical specifications with your CLT manufacturer.
PEST
Termites, if given the chance, will attack CLT. However CLT does still have an advantage in that it can maintain structural integrity much longer than the studs in lightweight or brick veneer construction ever could. The same termite protection measures that are mandatory for lightweight and brick veneer construction under the National Construction Code and the Australian Standards are also applicable to CLT homes including ant caps and visual barriers.
FIRE SAFETY
CLT performs very well against fire attack. Because CLT is often constructed to an airtight standard, and the product is solid without any cavity, ember and flame penetration are lower than lightweight stud construction. When subjected to flames the outer layer of the CLT will char and protect the core, drastically slowing down the rate of incineration and maintaining structural integrity for longer. This is one of the reasons why CLT has better fire performance than steel, as it can maintain it’s structural integrity under intense heat.
THINGS TO CONSIDER
There are numerous benefits to using CLT as a residential construction method, but it is important to be aware of these limiting factors.
BUILDER AVAILABILITY
CLT is not mainstream yet, meaning that searching for builders who are willing to take on a CLT project will be harder than finding a traditional builder. Furthermore, it is also wise to track down more quotes than usual as greenhorn builders will find it hard to price, either under or overestimating the job. Builders that have experience in CLT may be very busy and have long lead times.
The good news is that builders do not need to be specialists or need additional training to construct a CLT building like they might for a more unconventional construction like hempcrete or rammed earth. For those technical or unfamiliar tasks, most CLT panel producers can either recommend or provide a consultant to liaise with the builder onsite during erection.
Your designer or architect may have contacts for builders who are interested or experienced in a CLT build. They may also know CLT panel suppliers, engineers, logisticians. If not, then this person/s will also need to be found. Something that is not necessary for a traditional build.
EMBODIED ENERGY
There are some differences in the European products compared to the domestic. If you happen to prefer the products from Europe they will have higher embodied energy than sourcing domestically due to the energy required for shipping. However, CLT will still be carbon negative, taking out more carbon from the atmosphere than adding to it even when factoring in transport, something that cannot be said about traditional construction materials.
FUTURE RENOVATIONS
A CLT home, properly looked after, will last centuries. Due to the permanency, strength, and solidness of CLT it is very hard to adapt or renovate later. CLT panels are all accurately cut to size offsite with computer aided machinery, so any adaptations that need to be made once the house is complete will be done by less accurate hand-held power tools carving up the reconciled building unit.
It is important to get it right the first time. Changes are not as simple as traditional construction where walls are easier to knock out, studs can be repositioned for wider openings, or additions integrated as smoothly. Because the CLT method is designed to be transported, relocating a CLT house offsite to build a new one might even be easier on some occasions than doing a major renovation to an existing CLT house.
ACCESSIBILITY
Cranes are required to lift the CLT panels in place during assembly. Cranes have considerable reach to navigate inaccessible terrain, tight situations and to assemble well behind any structures that may be in front, such as when erecting a granny flat behind a main dwelling. There are some circumstances however where a crane simply cannot access. In this case CLT is not a viable option for construction. Your designer or architect may want to conduct a preliminary assessment to confirm accessibility.
MORE INFORMATION
Below are a number of YouTube clips that provide more information and show video and photos of CLT construction.
The Rubix. is not affiliated with any of the companies or content creators - these links are provided as supporting material only.
PASSIVHAUS TESTING ON A CLT HOUSE
SEE CLT DESIGNS
Take a look at houses that The Rubix. has designed using CLT here.