This course is part of the Mass Timber Academy

Light-Frame Wood and Mass Timber Construction

Light-frame wood construction is a popular building method that uses dimensional lumber, typically made from softwoods like pine, spruce, and fir, to create the structural frame of a building. Wood is an abundant resource in North America, and because tradespeople are familiar with the materials and methods of wood construction, labor for light-frame construction projects can be cost-effective.

Dimensional lumber—lumber cut to standardized dimensions—also tends to be cheaper than custom-cut wood because it is produced in large quantities and is readily available from a variety of sources. The manufacturing process for dimensional lumber is also highly efficient since it involves cutting and planing the wood to specific dimensions, which can be done quickly and accurately with modern machinery. This results in a lower cost per board foot compared to other types of lumber, such as specialty or custom-cut pieces.

Additionally, because dimensional lumber is standardized, it can be produced and distributed in large quantities, which further reduces the cost of production and transportation.

It is this combination of high efficiency in manufacturing, large-scale production, and standardized dimensions that makes dimensional lumber a cost-effective option for construction projects.

Light-frame wood construction has been especially popular in single-family and low-rise multifamily homes in North America, but it is now also being used in a variety of larger projects, including mixed-use retail and residential and midrise construction. Cities are getting denser, spurring mixed-use projects that combine ground-level retail and several stories of residential units. Urban dwellers, millennials in particular, are seeking affordable dwellings that are close to work and include plenty of amenities. The process of using dimensional lumber to fasten a skeleton-like assembly of floors, walls, roofs, and stairs makes it a quick and structurally efficient approach. In addition, light-frame construction techniques can be used to manufacture prefabricated components that can be transported to construction sites, providing some cost and scheduling advantages for a wide variety of applications, including those with site-specific challenges.

Adaptive Reuse, Infill, and Overbuilds

New construction and major land-development projects can’t be the only solutions to the housing crisis. Planners and developers are turning to alternative ways of building up the housing stock through adaptive reuse, overbuilds, and infill development to make the most of available space. While infill is the development of vacant lots surrounded by existing buildings, overbuilds refer to building additional stories on top of an existing building, maximizing development area per square foot. Overbuilds often use as much of an existing building as possible, adding extra support as needed. Because limited demolition is required, overbuild projects often have a quicker turnaround than new construction. They allow for higher density while retaining the economic value and historical significance of the original building footprint.

Adaptive reuse refers to redevelopment that makes use of existing, often obsolete building stock for the purposes of contemporary living. Adaptive reuse projects have been on the rise for a few decades, but trending even more since the 2010s. The U.S. has a multitude of underused, abandoned, or functionally obsolete structures, with the U.S. government owning about 45,000 of them in 2014.¹⁷ Research from Yardi Matrix and RentCafe¹⁸ suggest that factories, hotels, schools, and warehouses are the most popular to repurpose into affordable housing.

Using light-frame wood or mass timber for infill or overbuild projects can increase the advantages of these types of builds. Since timber is 20% of the density of concrete,¹⁹ it proves advantageous when building on sites with poor soil conditions, requiring less foundational systems. Its lightweight nature makes it optimal for infills because it tends to use less heavy construction equipment than other building materials, and therefore can facilitate infill projects in tight, complex urban spaces.

With recent surges in these building trends, wood is helping to meet the needs of housing demand, satisfy developers, and achieve architects’ vision for housing that is beautiful, sustainable, and socially conscious.

Performance-Based Design and Codes

The pressure for architects to innovate, implement budget-friendly processes, and create repeatable designs coincides with and supports the need to solve both the housing crisis and address climate change. More stringent building and energy codes and an emphasis on resilience are leading to performance-based designs—a goal-oriented approach that specifically addresses performance-related criteria, such as energy use, operating cost, occupant comfort, daylighting, and HVAC size and cost, among others.

Light-frame wood construction and mass timber satisfy these value propositions, from environmental sustainability and resilience to creating distinctive buildings that appeal to the next generation of apartment dwellers, while meeting tight budgets and construction timelines.

In January 2019, the International Code Council (ICC) approved a set of proposals to allow tall wood buildings as part of the 2021 International Building Code (IBC).

Based on these proposals, the 2021 IBC includes three new construction types:

• Type IV-A – Maximum 18 stories, with noncombustible protection on all mass timber elements.
• Type IV-B – Maximum 12 stories, limited area of exposed mass timber walls and ceilings allowed. In the 2024 IBC, 100% exposed ceilings will be compliant in Type IV-B construction.
• Type IV-C – Maximum 9 stories, all mass timber permitted to be exposed (with a few exceptions e.g. shafts) and designed for a 2-hour fire resistance.

These new types are based on the existing Heavy Timber construction type (renamed Type IV-HT) but with specified hourly fire-resistance ratings for building elements and added levels of noncombustible protection. The code includes provisions for up to 18 stories of Type IV-A construction for residential occupancies (and business occupancies, too).

Podium construction is another way both light-frame and mass timber contribute to housing solutions. It refers to a light-frame or mass timber structure built over a one- or multi-story “podium” of another construction type. The International Building Code (IBC) Section 510.2 considers these constructions as separate structures built on top of each other; consequently, area limitations, continuity of fire walls, and allowable number of stories are considered separately.

Buildings must still comply with the maximum allowed building height for that construction type. The podium is typically concrete construction, with the upper slab acting as a three-hour fire-resistance-rated fire-separation and structural-transfer slab for the framing above it. This enables higher-density projects with additional stories while allowing project owners to take advantage of the lower cost and speedier construction of wood framing.

Some common configurations enabled by podium construction include four or five stories of residential use over retail, commercial, office, and/or parking. Mezzanines can be used in conjunction with podium construction to achieve even greater densities. Previous versions of the IBC only allowed podiums to be one story above the grade plane. The 2021 IBC allows multi-story podiums, opening the door for buildings with double podiums and five stories of wood framing above.

THE BENEFITS OF WOOD FOR TRANSITIONAL HOUSING

Transitional housing helps shelter people who are houseless and people transitioning to homes. It is usually temporary—lasting from two weeks to two years—and works to stop people from spiraling into chronic crisis. As a temporary space, transitional housing often does not get the same level of focus that more permanent housing does and, as such, it may not offer the same indoor environmental quality (IEQ) elements that provide occupants light, acoustics, and aesthetic beauty. But all people deserve quality environments, even in transition, and wood is one material that can provide IEQ elements in a way that supports health, safety, and wellness for people who especially need it.²⁵

One element of IEQ is aesthetics, in which the design contributes to an occupant’s sense of well-being. In recent years, architects have been achieving good aesthetic IEQ by using materials and designs that evoke nature. One of the most well-known of the approaches to nature-based and wellness-focused architecture is biophilic design. The premise behind biophilic design is the idea that incorporating natural elements—water, natural light, green plants, and exposed wood where you can see and feel the grain—into buildings can contribute to the health and well-being of occupants.²⁶

Biophilic design has been associated with improvements in cognitive function, physical health, and psychological well-being.²⁷ In the case of residential construction, biophilic design can be utilized for everyone from young people starting out on their own to retired occupants aging in place to people living in transitional spaces.

In so many ways, occupants in crisis need spaces free of stress as much as anyone. Stress is one of the leading health problems²⁸ in our modern society, causing anxiety and difficulty focusing or interacting with others. According to the Canadian Observatory on Homelessness, people with poor mental health are more susceptible to the factors that can lead to homelessness: poverty, disaffiliation, and personal vulnerability. An inability to sustain employment leads to little income. Delusional thinking may lead them to withdraw from friends and family. The loss of support leaves them with fewer coping resources. But houselessness also amplifies poor mental health, and the stress of experiencing houselessness may exacerbate previous mental illness and lead to a vicious cycle.

“The needs of people experiencing homelessness with mental illnesses are similar to those without mental illnesses: physical safety, education, transportation, affordable housing, and affordable medical/dental treatment, the COH states on its website. When providing care to those experiencing homelessness, it is essential to create a non-threatening and supportive atmosphere, address basic needs (e.g. food and shelter), and provide accessible care.”²⁹

Some research in the United States, Canada, Austria, and other countries suggests that humans automatically relax when surrounded by elements from the natural world.³⁰ This is the type of aesthetic design architects are thinking about as they address the myriad housing crisis problems.

The housing crisis is not affecting all places and people in the U.S. equally. Cities with higher rents and lower rental-vacancy rates see higher per capita rates of houselessness.³¹ That means, while individual risk factors affect individuals in any given city, housing markets are the context. Seattle has a higher rate of houselessness than Chicago or Dallas. Research shows that individual vulnerabilities are more severe in places like Seattle where housing markets are less accommodating. In those less accommodating areas, houseless resources such as shelters and other transitional spaces become more overwhelmed.³² In response to the unique demands of short-term housing projects, architects and developers are increasing their focus on efficient construction methods, while incorporating thoughtful yet simple design considerations.