What Exactly is "Mass Timber" and Why Should We Care?

Look closely and you will see that a lack of new home construction is behind almost every problem facing housing today.  Residential construction has simply not recovered from the financial crisis and the experts see it only growing worse.  Freddie Mac's economists estimated the long-term shortfall between the supply of homes and the demand could be 2.5 to 4.0 million units each year. The National Association of Homebuilders' (NAHB's) Paul Emrath says since 2006 builders have never matched the average of 1.5 million homes they built each year from 1961 to 2000.

While there are many reasons builders aren't building, two of them are the costs of construction making it risky for builders to assume they can make a profit, and the lack of appropriate skilled labor. One type of building that is attracting a lot of attention with its ability to help with both is the use of mass timber.

Mass timber construction isn't new, think log cabin as one rather primitive example. More high-tech types of mass timber construction have been in use in Europe and Canada for years. Now it is the big buzzword in the U.S.  However, thus far there are only 221 projects completed or being built nationwide.

Right now, it seems most likely to first be a factor in multifamily and commercial construction, but it seems inevitable its use will migrate toward single-family housing, and probably fairly.  Its large-scale adoption has been hampered so far by fear of fire. This is reflected in building codes which have restricted its use to projects of six stories or less. However, a new code that will come into use in 2021 eliminates that restriction and a 23-story, 223-unit residential building is already planned for Milwaukee.

The code is being changed after extensive tests found a mass timber panel was able to withstand temperatures exceeding 1,800° (F) for 3 hours and 6 minutes. The current code requires a two-hour burn time. Its mass actually seems to inhibit the spread of fire. In addition, mass timber buildings weigh approximately 1/5 as much as comparable concrete buildings which reduces their foundation size, inertial seismic forces, and embodied energy. This allows them to perform well during earthquakes.

A mass timber building is one in which the primary load-bearing structure is made of either solid or engineered wood.  The framing is typically characterized by large solid wood panels for wall, floor, and roof construction. The panels are formed by mechanically fastening or adhesive bonding of smaller wood components such as dimensional lumber or wood veneers, strands or fibers from large prefabricated wood elements.

The most common product at this time seems to be cross laminated lumber (CLT). Other products include glue-laminated timber (glulam), cross-laminated timber (CLT), glued-laminated timber (GLT), laminated strand lumber (LSL), laminated veneer lumber (LVL), nail-laminated lumber (NLT) and other large-dimensioned structural composite lumber (SCL) products.

What are the advantages? There appear to be a lot. Mass timber building is much faster than stick construction and building with either concrete or steel. While experience is still limited, most sources indicate that a mass timber project goes up 25 percent faster than a similar one made of concrete.  There is ample evidence that use of modular components speeds up building a single-family home and experience in Europe indicates a detached three-bedroom house built with CLT can be completed in five to eight days.

Use of prefabricated materials is estimated to reduce construction site traffic by 90 percent. It also reduces workplace accidents.

The relative costs are still uncertain. One study estimates the cost of a hypothetical 10-story residential building in the Pacific Northwest using CLT would be $48 to $56 per gross square foot plus another $2 to $ for fire protection and acoustical dampening. The same building constructed with cast-in-place reinforced concrete would run $42 to $46 plus an additional $1 to $2 for sound and safety modifications. This makes mass timber at this point 15 to 29 percent more expensive.

This, of course doesn't take into account the impact of time saved on soft costs such as financing nor that some builders report needing fewer skilled workers. It also ignores the transportation costs involved in getting mass timber products from areas like the timber-rich Pacific Northwest where they are likely to be manufactured.

There are a ton of environmental benefits to mass timber.  First of all, carbon dioxide emissions are reduced by 15 to 20 percent compared to steel construction.  One study estimates the near-term use of CLT and other wood technologies in buildings of 7 to 15 stories (which will be possible after 2021) could have the same degree of emissions control as taking more than 2 million cars off the road for one year.  There is also strong thermal performance because the components are fabricated with high levels of precision and fit tightly. One source says the construction is so precise that window glazing can be ordered from construction drawings. There also reduced off-gassing inside the completed structure.

According to one source, mass timber also works, in the same manner as live trees, to sequester carbon.  Since such structures can have a lifespan of hundreds of years, this ability, the source says, could reverse climate change effects at a large scale.

Nothing is without some downsides and, in addition to the risks inherent in other forms of construction there are others, some due to the newness of the process.

• Insurers may require more information about construction plans, exposures to catastrophes, details of exterior and interior design.
• Dealing with the moisture content of wood and construction sequencing.
• Resistance to water damage
• Insect, pest, mold, and fungus exposure
• Types of adhesives used
• Replacement costs
• Long term performance

While it might not eliminate what is being referred to in many quarters as a "housing crisis," it seems likely that mass timber construction will at least be part of the solution. Unfortunately, even if it becomes commonplace for large-scale projects, it may take time to filter down to single-family construction, especially by smaller builders.