Mass Timber is Great, but It Will Not Solve the Housing Shortage

A short policy memo recently posted on the Federation of American Scientists’ (FAS) website argues that “mass timber can help solve the housing shortage.” It was posted as part of the Housing Ideas Challenge, a collaboration between FAS, Matt Yglesias and a few other organizations to generate new, innovative ideas for building more housing. 

The memo calls for the federal government to incentivize states to adopt the most recent version of the International Building Code. The latest version of the Code includes new provisions for building tall timber buildings. According to the memo, because mass timber allows buildings to be built faster and cheaper, changing state building codes to allow tall mass timber buildings will unlock more housing construction:

Compared to concrete and steel, mass timber buildings are faster to build (therefore often cheaper), just as safe in fires, and create fewer CO2 emissions. Single- and multi-family housing using mass timber components could help with the 7.3 million gap of affordable homes.

Broader adoption could meaningfully increase productivity and thereby reduce construction costs. A 25-story mass timber building in Milwaukee completed in 2022 took 75% less time compared to traditional construction techniques. Developers have reported cost savings of up to 35% through lower time and labor costs.

I don’t have particular objections to accelerating mass timber adoption. Mass timber is an interesting technology, and it has some decided advantages over constructing buildings out of steel or concrete. And in general, I’m in favor of expanding the palette of technologies available for constructing buildings. That said, there is not much reason to think that accelerating mass timber adoption will meaningfully increase the volume of homebuilding in the US. 

What is mass timber?

Mass timber is exactly what it sounds like: construction that uses large, heavy timber elements as the structural frame of the building. In contrast, light-framed timber construction uses small, thin timber elements – 2x4s, 2x6s, thin sheets of plywood or OSB – as the structural frame.

There’s a variety of different products that can be used for mass timber construction: Cross Laminated Timber (CLT), Nail Laminated Timber, glue-laminated timber (glulam), mass-plywood panels (MPP), and so on. Mass timber products are typically made by taking smaller timber elements (2x4s or 2x6s, sheets of plywood, etc.) and attaching them together to form much larger elements. Glulams, for instance, are made by taking smaller pieces of dimensional lumber (long pieces of lumber such as 2x4s or 2x6s) and gluing them together to form large beams or columns. CLT is likewise made by gluing together layers of dimensional lumber to form large, solid panels that can be used for walls or floors.

Mass timber building consisting of glulam beams and columns, and CLT floor panels, via Matt Kantner

Light-framed wood construction consisting of 2x lumber, via Understanding Building Construction

It’s the large size of the structural elements that makes mass timber an interesting building system. With light-framed wood construction, the risk of fire limits how tall you can safely build; typically 5 stories in the US (hence the “5 over 1” construction type so popular in multifamily construction). And some argue that even these limits are too high, based on the fact that the US has a much higher rate of fire death compared to countries that use less light-frame wood. 

But large mass timber elements behave differently than smaller light-framed ones during a fire. Specifically, with mass timber a layer of charred wood forms around the outside that protects the wood on the inside from further combustion. Size your structural elements correctly, and a mass timber building will survive a fire much longer than a light-framed wood building. This gives more time for people to evacuate to safety, which makes it possible to safely build much taller buildings out of wood. While light-framed wood is typically limited to 5 stories in the US, mass timber buildings have been built up to 19 stories.¹

Besides its better fire performance, there are other reasons to like mass timber. It sequesters a huge amount of carbon because of the volume of wood it uses, so it's far less carbon-intensive than concrete or steel. It’s often possible to leave mass timber elements exposed in the finished building, which creates an attractive finish for walls or ceilings that some tenants are willing to pay extra for. Because it’s prefabricated, the structure of a mass timber building is built very fast, and the construction is much quieter than other building methods.

Mass timber challenges

But mass timber has some challenges, particularly cost. Contra the policy memo, mass timber will in general be more expensive than other types of construction, at least in the US.² The article the memo cites as evidence that mass timber can reduce construction cost in fact mentions twice that mass timber is generally more expensive than steel or concrete. The evidence of savings presented in the article is a single developer who claimed they saved 35% on a project by using mass timber. Even if this is true (and I’m skeptical), it’s not much evidence. Construction costs vary enough between projects that for any two building materials, it's probably possible to find an example of someone who saved money by switching from one to the other. But the overall trend is clear: a survey of commercial builders in the US found that the median mass timber project had a cost premium of 2%, and in some cases as high as 15%.³

Mass timber is still somewhat nascent in the US, so it’s reasonable to believe that costs will come down as more projects are built, designers and developers gain experience, and the supply chain becomes more robust. It wouldn’t surprise me if it eventually becomes competitive with structural steel or concrete in the US. But the challenge for mass timber as an enabler of housing construction isn’t with steel or concrete, but with light-framed wood.

In 2022, there were just over 1 million single family homes built in the US, and 368,000 multifamily units. Of these, the overwhelming majority were built using light-framed wood construction: 94% of the single family homes, and 73% of the multifamily units.

This is a problem for the idea of mass timber as a housing enabler, because mass timber has some inherent cost disadvantages compared to light-framed wood construction. First, for a given structure, mass timber will require more material to build it than light-framed wood will. Consider, for instance, the framing for a building’s floor. Per Smartlam’s span tables for their CLT panels, a 5-ply panel supporting residential loading can span 21 feet under typical residential loading. If we were framing the same floor using light-framed wood, one option would be to use wood trusses: for a 21 foot span, under residential loading, we can use 16 inch deep wood trusses spaced at 24 inches on center, with a 3/4 inch thick sheet of plywood on top.

Both CLT and wood trusses are made from dimensional lumber stitched together. But the mass timber framing uses four to five times the volume of wood as the light-framed wood framing.

Mass timber vs light-framed wood volume of material for a 2-ft wide section of floor framing. The input wood for the CLT is slightly higher than the in-use wood because the CLT is made from dimensional lumber planed down to smaller dimensions.

These calculations get more complex for other parts of the structure. But the basic relationship holds: mass timber uses much more wood than light-framed timber to support the same load.

The second cost disadvantage of mass timber compared to light-framed wood construction is that it adds extra processing steps. To produce construction lumber, trees are cut down and brought to a sawmill, where they’re cut into dimensional lumber (2x4s, 2x6s, etc) or processed into other timber products such as plywood or OSB. Much light-framed wood construction, such as walls and roof rafters, uses dimensional lumber that comes directly from the sawmill. In other cases, such as with floor trusses, the lumber must go through another processing step between the mill and the construction site, but the equipment for this processing is relatively simple and inexpensive. Here, for instance, is a used floor truss machine going for less than the price of a fully loaded F-150.

With mass timber, on the other hand, all of the lumber must go through a second processing step to turn it into mass timber products, and this processing step is more expensive than the comparatively simpler truss fabrication. Fabricating CLT panels, for instance, requires an involved production line that includes conveyor belts, jointing machines, gluing machines, presses, planing machinery, and (probably) CNC cutting, the combined cost of which can easily be in the millions.⁴

Because it uses more material and requires more, costlier processing steps, mass timber has some inherent cost disadvantages compared to light-framed wood construction. And because the vast majority of housing in the US is currently built from light-framed wood, there’s little reason to think that even a highly-developed mass timber supply chain would meaningfully unlock large amounts of housing construction by way of reduced costs.⁵

Mass timber is of course superior to light-framed wood construction for taller buildings, since its fire resistance means it's safe to use where light-framed wood isn’t. But most construction (residential or otherwise) isn’t tall enough for this to matter — over 90% of housing in the US is in buildings 3 stories tall or less, and just 2.5% of housing units in the US are in buildings seven or more stories tall.

Canada as a mass-timber case study

Canada’s experience with mass timber can give us a glimpse of what full-throated support of mass timber might mean for US housing construction. Like the US, Canada has enormous supplies of wood, and builds much of its housing using light-framed wood construction. Canada, especially the province of British Columbia (BC) has led the charge for the use of mass timber in North America. The Canadian forest product non-profit FPInnovations published a widely-used CLT design handbook in 2011, and in 2018 Canada built what at the time was the world’s tallest mass-timber building, the 18-story Brock Commons. Canada’s national and provincial building codes have steadily increased the height allowance for timber buildings (though they often fall short of what the current version of the IBC allows), and many provinces have passed legislation requiring wood be the first material considered for publicly funded projects. Many prominent mass timber manufacturers and designers, such as Seagate, Nordic, Equilibrium, Michael Green Architecture, and Structurecraft are located in Canada. As of 2022, the province of BC alone had constructed nearly as many mass timber buildings as the entire US.⁶

But government enthusiasm and a robust ecosystem hasn’t translated into large volumes of residential mass timber construction. Per the 2021 State of Mass Timber in Canada report, between 2007 and 2021 Canada constructed nearly 500 mass timber projects, but just 39 of these were residential projects. This is a tiny fraction of the thousands of apartment buildings built in Canada using other construction methods over the same time period.⁷

Like in the US, mass timber is still somewhat nascent in Canada, and I expect the number of mass timber buildings to continue to trend upward over time and costs to continue to fall. Nevertheless, the small residential penetration of mass timber in Canada should give pause to anyone who expects it to drive increased volumes of housing construction in the US.

It's also worth noting that not adopting the latest building code doesn't necessarily prevent mass timber buildings from being built now, though it certainly makes it easier. Older codes have provisions that allow for shorter mass timber buildings (called Type IV construction), and they can also be built using the code provisions that current light-framed wood structures use (Type III or Type V construction). The updated codes are specifically required only for tall mass timber buildings, or where the mass timber will be left exposed in the final condition. 

But even here, it’s often possible to get taller mass timber buildings constructed without updating the code. Because the most recent version of the IBC has allowances for tall mass timber buildings, building officials are often willing to accept them even if their state code doesn’t technically allow them. Several tall mass timber buildings in the US, such as Ascent in Milwaukee, have been built without the state adopting the 2021 IBC’s mass timber provisions. 

Conversely, just because a state has adopted the 2021 IBC doesn’t mean that there won’t be permitting roadblocks for tall mass timber buildings. Building officials are cautious by nature, and they’re often reluctant to allow taller wood buildings without very careful consideration, despite what the code says. And what they say goes. Adopting the most recent IBC would reduce the permitting barriers to mass timber adoption, and I’m in favor of it, but it’s not like flipping a switch where suddenly mass timber buildings are now possible.

I like mass timber; I spent several years at a startup that was trying to make mass produced mass-timber construction happen. It's good for a lot of things, particularly for low-carbon construction, and I'm happy that it's slowly becoming more popular in the US. I think it can ultimately become a popular, competitive construction technology. But it won't jumpstart US homebuilding.

Thanks to Matt Kantner of Equilibrium Consulting for reading a draft of this. All errors are my own.

1. Ascent has 19 stories of timber and 6 stories of concrete.

2. This can vary depending on local labor costs. Because of their high degree of prefabrication, mass timber structures require many fewer people on-site to construct than concrete or steel, and so in high labor cost areas it can end up cheaper, a mechanism that we’ve looked at before.

3. This is somewhat higher than it seems, since the mass timber is driving up the cost of the entire project by ~2% despite being responsible for just a portion of costs. But the actual cost calculations get complex, since a mass timber structure will reduce costs elsewhere: lighter foundations, fewer finishes and drywall, etc.

4. Some light-framed wood materials, such as plywood, OSB, and TJIs, do require expensive equipment and specialized production lines to produce. But these products are standardized, mass-produced products made in very large volumes. Mass timber products, on the other hand, are produced on a smaller scale, typically for a specific project (though we might expect this to change as its popularity increases). And some mass timber products, like mass plywood panels, will add another processing step on top of these production lines.

5. A counterargument to this is that light-framed wood construction, with its numerous small parts and pieces that must be handled and assembled together, has some inherent disadvantages compared to mass timber, which uses a smaller number of components. Basic Design for Manufacturing and Assembly (DFMA) practice, after all, suggests that all else being equal the fewer the parts the lower the cost. This is a reasonable point, but it's weaker than it seems. For one, mass timber still requires most of this handling work – mass timber elements are made from many pieces of dimensional lumber stitched together – it simply occurs at an earlier stage in the process, albeit in a place that is easier to mechanize and streamline. For another, mass timber construction will still require a lot of additional small part and piece handling for the construction of the partition walls and possibly the exterior cladding walls. Finally, this disadvantage isn’t inherent to light-frame construction: there are light-framed wood construction systems that can greatly reduce the number of individual parts required, such as SIPs.

6. For reference, BC has roughly 1.5% the population of the US.

7. Between 2007 and 2021 Canada built roughly 70-100,000 apartment units each year, or perhaps a million apartments altogether. If the average apartment building had 100 units (likely much higher than actual average), this would be 10,000 apartment buildings built over the period.