Project stakeholders were interviewed to determine the contextual parameters fostering the successful employment of solid timber. The following are the qualitative findings on the advantages of timber.
One project was erected in as little as 4.5 days. Additionally, multi-story structures can begin subcontractor work once the first story is erected (i.e. electrical, mechanical, etc.)—this cannot be done as efficiently in steel or concrete. Bridport House reduced its schedule by eight weeks compared to traditional methods, and Forté reduced its by three months.
Due to the ‘dry process’ of mass timber buildings, they can be assembled during any season. Most CLT is sealed and is unaffected by snow, or water during construction.
A majority of solid timber comes fully finished, with the ability to be exposed as an interior surface. This can cut back on finishing materials (e.g. gypsum wallboard and paint).
Wood is a carbon-sequestering material, and it greatly minimizes the carbon footprint for each project.
Panels are fabricated offsite, and then shipped or trucked to the site. Once delivered, they can be assembled very quickly. This method is especially helpful in rural locations, or places with a minimal labor force.
With manufacturing taking place in a factory, there is a reduced need for onsite labor or site preparations.
Foundations can be smaller, and buildings can be built taller for the similar costs as traditional methods of construction. This lightweight structure can also help with special site conditions such as near waterfronts and where soils may not be as favorable. Using CLT for Bridport House made it possible to double the replacement structure with only a 10 percent increase in overall weight.
With tolerances within millimeters, the connections and envelope are tight. This also increases energy efficiency.
Given the finished nature of the panels, there is less potential for injury. There are fewer parts to assemble and transport. For example, Forté, a nine-story structure, did not have a single first aid incident during construction.
Qualitative findings on the disadvantages of mass timber included various challenges.
Knowledge and labor
Solid timber is a very different form of construction compared to stick framing, concrete, or steel. Most general contractors, designers, and engineers are not familiar with solid timber and how it is constructed. The majority of the time, a consultant was hired, or a structural engineer familiar with the technology was procured for design assist. Special construction crews were recruited to assemble the system.
Due to the lack of experience and projects completed in North America with mass timber, there is still a lack of information on construction methods, connections, and delivery methods.
Most panels are shipped either on a truck or in a container, and each method has its own transporting capacity. Shipments must be transported to the site in order from foundation to vertical termination, with the first piece to be erected on top of the shipment. Re-arranging panels and temporarily storing them is costly and wastes time.
Due to the finished nature of CLT panels, the mechanical and electrical systems are located before fabrication. Knowing where and how these chases will affect finishes and design is crucial. In other words, designing for mass timber is a completely different process—all design work must be front loaded and completed before information is sent to fabrication. The scheduling of this process is then front-loaded, compared to traditional construction.
Acoustics and vibration
Due to the rigid nature of the panels, this construction is susceptible to sound and vibration that can be transferred through walls and floors. Extra sound-proofing is usually needed to mitigate that sound.
Less manpower and fewer labor hours are required for solid timber. This decreases the amount of jobs that on-site construction currently provides.
Codes and permits
Given mass timber’s newness in North America, many building officials are unfamiliar with the associated structural, fire, and acoustic performance.
This often requires more documentation, engineering, and longer time frames to get permits.
Wind is a concern when craning CLT panels from the truck to the site. Given their wide surface area, they are greatly affected by high winds. Construction can be halted to wait for weather conditions to improve.
The massive panels are too heavy to handle by hand, thus requiring heavy machinery and cranes to install. This limits the amount of onsite adjusting that can be done.