Speed, Precision, and Wood  

Photo courtesy of  Mithun

Mithun Forterra modular CLT prototype. 

Prefabricated wood buildings can help address some of the most pressing issues facing the AEC sector, from the need to cut carbon emissions and build more affordable housing¹ to a growing demand for faster, more energy-efficient, airtight, passive house construction.²  Building and construction is one of the world’s largest sectors, so a shift to off-site manufactured buildings could have a big impact: recent prefabricated modular projects show that it can reduce project timelines by 20 to 50%.³

When combined with building information modeling (BIM) and computer numerical control (CNC) machines, prefabricated modular wood solutions can help boost efficiencies, reduce costs, and cut waste.⁴ Conceiving buildings as a prefabricated kit of parts enables easier disassembly and re-use at the end of their service lives.⁵ Increasing the use of wood in prefabricated buildings can contribute to a lower carbon footprint because of wood’s lower embodied carbon and because wood also stores carbon.⁶

In this course, you’ll explore the foundational concepts of prefabricated construction, along with its potential advantages. Materials cover the unique benefits of prefabricated light wood frame and mass timber construction, including types of prefabricated timber systems, assemblies, and wood products used in off-site manufacturing. Case studies throughout demonstrate a wide range of sustainable prefabricated building examples using advanced light-frame and mass timber construction.

Upon completion of this course, you will have an understanding of the products and components that make up sustainable prefabricated timber buildings, as well as design and engineering challenges that wood can help solve. This includes how prefabricated wood construction can contribute to speed and efficiency of construction, design flexibility, waste reduction, environmental performance, and improved life safety.⁷

OVERVIEW OF PREFABRICATION

What is a prefabricated building? A prefabricated (or prefab) building is a building that is constructed and manufactured using factory-made components or units that are transported and assembled on site to form 

the complete building.⁸ In contrast to conventional construction that takes place almost entirely on site, prefabrication methods mean that entire sections, modules, or panels are off-site manufactured (OSM) in a factory setting prior to assembly (or, in some cases, using a nearby yard and staging area). While many prefabricated buildings are made out of wood,⁹ there are also applications of prefabricated steel and concrete structures.¹⁰ 

Distinctions are sometimes made between permanent prefabricated construction and relocatable prefabricated buildings. Relocatable buildings are designed to be reused or repurposed multiple times and transported to different building sites. In some cases, permanent prefabricated buildings may include some components that are more easily demountable or designed for disassembly, reuse, or recycling.¹¹ While prefabrication was once limited to certain building types in the past,¹² today, advanced prefabrication could benefit almost any building typology, from commercial offices, hotels, multifamily, and student housing to cultural, civic, and public facilities.¹³

What is a panelized kit of parts vs. finished volumetric modular construction? There are generally two types of industrialized approaches to prefabricated wood buildings: panelized building kits (kit of parts) and volumetric finished modules.

Panelized Kit of Parts

Panelized building kits include prefabricated elements or sections that are then delivered and assembled on site, much like life-sized toy blocks.¹⁴ These may include the roofing package (roof panels, fascia, gutters, etc.), roof structure (ceiling deck and beams), glazing package (windows and entrances), and building structure (wall panels, beams, columns, and shear paneling). The kit-of-parts approach, via panelization, is typical for mid-rise wood buildings.

Within panelization, it is helpful to understand the difference between open structural panels and closed structural panels. Open structural panels are a pre-assembled wall, floor, or roof framework that is later fitted with other elements such as insulation, exterior cladding, and weather barriers on site. While this offers time savings and flexibility, there is still some site work involved. By contrast, closed structural panels are complete, pre-assembled wall panels that may include windows, doors, plumbing, ducting, electrical, finishes, etc.

Finished Volumetric Modular Construction

Volumetric modular construction involves the off-site prefabrication of individual three-dimensional units of enclosed space that are then connected on site to form a single building. They are built in a factory and transported to the site, and, when on site, the modules can be placed side by side, end to end, or stacked, allowing a wide variety of configurations and styles in the building layout.

“Prefabricated construction technology innovation is pivotal in addressing the compounding crisis of climate change and housing shortages. Now more than ever, clients are asking, ‘What does it take to go modular?’” This is particularly well suited for replicable designs such as multi-unit residential projects like hotels, dormitories, condos, and apartment buildings.¹⁵ In some cases, volumetric modules include everything from the fundamental structure to walls and ceilings, flooring, fixtures, and finishes.¹⁶

Unlike traditional on-site construction, in which the building is generally completed from the outside in, prefabricated volumetric building modules are typically constructed from the inside out. In such cases, the box frame of the module is completed first, then interior finishes are added, the mechanical, electrical, and plumbing components and insulation are installed, and, finally, exterior sheathing and cladding are applied.¹⁷

Finishing on modular units leaving the factory generally includes plumbing, electrical, paint, flooring, fixtures, cabinets, and appliances. After the modules are craned into place, licensed sub-trades make electrical, plumbing, mechanical, and structural connections before finish work is completed and the building is prepped for occupancy.

What technologies support prefabricated modular construction? Advancements in digital technology are increasingly used in prefabricated building design and help to expand the possibilities of off-site construction. Prefabricated projects benefit from tools such as 3D modeling, BIM, and CNC machines—and make prefabrication and communication among building professionals easier.

BENEFITS OF PREFABRICATION

Shifting construction away from traditional sites and into factories could significantly improve the way we build, reduce costs, and boost efficiencies, according to McKinsey & Company.¹⁸ And, developers and governments are increasingly looking to prefabrication as an important component of making homes more affordable in the face of record housing shortages and rising prices.¹⁹ While the benefits of prefabrication will vary from project to project, the following are often cited as common advantages of streamlined OSM construction—with wood being particularly well-suited to modular designs.²⁰

Quality

The monitored manufacturing setting, the use of precise fabrication tools, and the ability to automate processes allow for a high level of quality control and consistency. The increased precision in the fabrication of exterior wall components can also deliver a much tighter building envelope with fewer air leaks.²¹

Productivity

Labor productivity is also increased when using an off-site approach. A crew working in a plant is less affected by adverse weather. Additionally, workers equipped with precise tools and machinery, working in a space designed to provide ideal conditions for manufacturing, are able to achieve higher levels of quality in a more efficient manner. OSM also means a more consistent crew and a more controlled workflow that is less prone to disruption.²²

Safety

A prefabricated approach is also generally safer for construction crews. Employees work in a controlled setting and are not exposed to the hazards of extreme weather and other construction site dangers, such as those related to noise and air quality. According to the U.S. Department of Labor’s Bureau of Labor Statistics, rates for fatal injuries are, overall, substantially lower in manufacturing than in traditional on-site construction.²³

Schedule and Speed of Construction

Prefabricating the bulk of a building in an off-site facility means that the construction process is much less vulnerable to delays due to poor weather and other site-related disruptions. Off-site construction also allows for work to be performed simultaneously. Compared to the linear process most common for traditional on-site construction, with trades working sequentially, off-site construction allows trades to work concurrently.²⁴

Cost-Efficiency and Savings

The cost for any particular modular project will vary according to a wide range of 

factors. In many cases, modular construction can be more cost-efficient than on-site construction. Nonetheless, this does not automatically mean it will result in a reduction in overall labor and material costs. The costs of a prefabricated building are often more predictable than those of traditional construction methods, however, with opportunities to scale up replicable components for further savings.²⁵ The time savings offered by prefabrication can reduce an owner’s carrying costs and provide the ability to have faster cash flow with access to a finished building sooner.²⁶

Sustainability

Prefabrication can give a boost to the sustainability of a project. The off-site production of building components allows for optimal control of material use, reducing material input and waste compared to traditional on-site construction. Additionally, surplus material and fall-off can be reused or recycled back into the inventory for use on other projects. More precise quality control and the consistency of off-site production of building envelope components under controlled manufacturing conditions can reduce a building’s operational energy costs and improve its thermal performance.²⁷

BENEFITS OF PREFABRICATED WOOD CONSTRUCTION

Wood is a good fit for prefabricated and modular construction, realizing the benefits of OSM from quality, safety, and efficiency to faster construction and sustainability.²⁸ Along with these benefits, prefab wood construction also offers additional advantages, including aesthetics, better environmental performance, strength, rigidity,²⁹ and energy efficiency.³⁰ Wood’s versatility, lighter weight, and the ease with which it can be cut and worked make it a good material choice for factory-built components.³¹ As a material, wood offers the structural simplicity needed for cost-effective projects and the design versatility to be rapidly installed or modified with less waste.³²

Prefabricated Wood Building Systems Can Improve Safety, Boost Efficiencies, and Speed Up Construction

Because wood panels and products are manufactured for specific applications, they’re well-suited to a high degree of prefabrication and just-in-time construction methods. Relatively lightweight panelized wood-frame and mass timber components can be prefabricated, complete with pre-cut openings for doors, windows, stairs, service channels, and ducts, and shipped directly from the manufacturer to the jobsite so they can be quickly and efficiently lifted into place—shaving months off the construction schedule.³³ The use of CNC machining technology³⁴ can offer further savings.³⁵ Many manufacturers ship wood panels with pre-installed lifting straps; contractors then use cranes to lift panels into place.

Along with faster construction, prefabricated wood systems mean crews spend less time on site, fewer workers are needed to climb high on scaffolding, and projects are less likely to experience weather conditions that can delay construction or make it more dangerous.³⁶

Controlled Environment Delivers Airtight, Energy-Efficient Prefabricated Wood Construction

A safe, controlled, dry factory environment is a boon for high-quality prefabricated wood construction.³⁷ Impacts from moisture, weather, and exposure to the elements are minimized. 

Wood panels, manufactured using CNC equipment, can achieve precise tolerances, and panel joints fit tightly and deliver better energy efficiency for the structure. As a result, an extremely tight building envelope can be achieved. This is particularly helpful when fabricated wood buildings are targeting air-tight, high-performance Passive House standards.³⁸ Airtightness

tests can be conducted within the factory setting, followed by modifications and improvements made before shipping completed assemblies to the site—translating into long-term energy savings for owners and occupants.

Solid mass timber panels also provide thermal mass—the key measures of their thermal performance are U-value (coefficient of heat transfer) and R-value (insulating ability). Thick wood and mass timber panels have lower U-values and higher R-values as natural insulators.³⁹

Wood’s Light Weight and Versatility Offers Flexible Prefabricated Construction

Wood’s light weight and ease of cutting using CNC fabrication makes it a good candidate for off-site factory-built modules. Framing, roofing, and window installation, drywall, fire separation, plumbing, electrical, and appliance installation can all be completed in a factory, or if needed, some of these components can be later assembled on site.

The relatively reduced weight of wood products makes them highly adaptable to different types of prefabricated projects, designs, and site conditions like soft soils or tight proximity to neighboring buildings. Prefab kiln-dried wood products have excellent dimensional stability and ensure good accuracy upon installation.⁴⁰

Another advantage of wood over materials such as precast concrete is the ability to accommodate on-the-fly changes. This adaptability of wood construction—and its ability to be modified—means workers can make changes in the fabrication facility or on site as needed. Combining light-frame wood construction with mass timber products offers further flexibility and versatility.

TYPES OF PREFABRICATED WOOD PRODUCTS AND COMPONENTS

Prefabricated wood components may include light-frame walls, floors, and roof trusses or mass timber elements such as cross-laminated timber (CLT) panels, nail-laminated timber (NLT) panels, dowel-laminated timber (DLT) panels, and glued-laminated timber (glulam) columns and beams.

Photo courtesy of Think Wood

CLT panels are formed by stacking and gluing together successive perpendicular layers of wood. Often, all machining and milling of holes is performed at the CLT producer’s plant based on the fabrication drawings and CNC machine files, and the product is shipped straight to the site.

Light Wood Frame

Light wood frame construction has long been the go-to framing choice for low- and mid-rise and, increasingly, commercial buildings. Cost-effectiveness, efficiency, ease of assembly, minimal environmental impact, and the relative availability of labor and materials make light wood frame construction the most common type of wood construction in North America. Typical light-frame roof and floor systems consist of repetitive framing members such as rafters or trusses with wood structural panel decking. Framing components include solid-sawn dimension lumber, I-joists, structural composite lumber, and parallel chord and pitched trusses. Oriented strand board (OSB) and plywood are used interchangeably as decking and sheathing material for floors, walls, and roof decks. There are several approaches to light wood frame construction, and each is suited for a specific application, most often in Type III and Type V categories. While on-site framing for light-frame construction is widely used and makes sense in certain cases, increasingly, elements of these buildings are prefabricated off site and assembled on the job.

Cross-Laminated Timber

Mass timber is a category of framing styles typically characterized by the use of large solid wood panels for wall, floor, and roof construction. CLT panels are formed by stacking and gluing together successive perpendicular layers of wood. The layered stacks are then pressed in large hydraulic or vacuum presses to form an interlocked panel. The panel is sized and shaped with a CNC machine into a construction-ready component. The number of layers in a panel can range from three to seven or more, and panels can have prefabricated door and window openings as well 

as routings for electrical and mechanical systems that are installed before shipment to the building site. In addition to glued CLT, manufacturers have also developed mechanically fastened CLT using carefully engineered fastening patterns rather than adhesives and pressure.

The cross-lamination process provides improved dimensional stability to the product, which allows for the prefabrication of long, wide floor slabs, long single-story walls, and tall plate-height conditions needed for clerestory walls or multi-story balloon-framed configurations. By the nature of its design, CLT has inherent load-bearing strength and can serve as material for both vertical and horizontal assembly applications. Since wall, floor, and roof sections made of CLT are formed off site in a factory, on-site construction time is much shorter. CLT can be used as a structural system in Type III, IV, and V buildings today and in tall wood buildings.

Often, all machining and milling of holes is performed at the CLT producer’s plant based on the fabrication drawings and CNC machine files, and the product is shipped straight to site. Other times, the product is shipped to a fabrication shop for further fitting of steel connections to speed up site erection.

Nail-Laminated Timber

NLT is a mass timber panel system that can be used for floor, wall, and roof structures. NLT floor and wall assemblies have been used for more than a century, particularly in warehouses where solid, sturdy floors were required; it is now being recognized again as a valid substitute for concrete slabs and steel decking in commercial and institutional buildings and residential buildings, where it is often exposed to create a unique aesthetic. NLT is created from dimensional lumber—2x4, 2x6, 2x8, 2x10, or 2x12—stacked on edge at 1-1 /2 inches on center and fastened together with nails. Plywood or OSB sheathing is often added to one top side to provide a structural diaphragm. Plywood sheathing also allows the product to be used as a wall panel element. NLT can be prefabricated in a staging site nearby or within a factory setting and, in some cases, is assembled using an automated process.⁴¹

Photo courtesy of Think Wood

NLT is created from dimension lumber stacked on edge and fastened together with nails. Plywood sheathing is often added to one top side to provide a structural diaphragm and allow the product to be used as a wall panel element. 

Photo courtesy of Think Wood

Glulam is stronger than steel at comparable weights, and it is stronger and stiffer than dimensional lumber.⁴³ That makes the material a cost-effective choice for long, structural spans and tall columns with minimal need for additional support.

Glued-Laminated Timber

Glued-laminated timber (glulam) is a structural engineered wood element commonly used for beams and columns in residential 

and commercial applications. To form a glulam component, dimensional lumber wood laminations are positioned according to their stress-rated performance characteristics. In most cases, the strongest laminations sandwich the beam in order to absorb stress proportionally and ensure the member’s longevity. The laminations are jointed end-to-end, allowing for long spans, and are bonded with a durable, moisture-resistant adhesive. The laminations’ grains run parallel with the member’s length to improve its strength.

Glulam is a highly visible form of mass timber in contemporary projects, with long spans framing signature designs that have been left exposed to take advantage of wood’s natural aesthetic. In addition to being used in floors, decks, and roofs, glulam and NLT mass timber panels are now used for timber elevators and stair shafts in six-story light wood frame residential mid-rise apartment buildings.

Glulam also offers the advantage of being fabricated in controlled environments based on certified manufacturing standards. Like CLT, the machining and milling of holes is often undertaken at the producer’s plant before the product is shipped to the site. Other times, the panels are shipped to the fabricator’s shop for further fitting of steel connections.

Dowel-Laminated Timber

DLT is a mass timber product that uses wood dowels as connectors. From larger prefabricated panel sizes (up to 12 feet by 60 feet) for faster erection times to a wide variety of wood species, DLT is a mass timber product that is ideal for floor, wall, and roof structures. DLT does not include any glue, chemicals, or volatile organic compounds (VOCs), equating to a healthier indoor environment. And because there are no nails or metal fasteners, DLT is easy to process through CNC machines. Unique to DLT as a mass timber product, a wide variety of profiles can be integrated inexpensively into the bottom surface of the panel. Profiles are fully customizable to suit the particular performance and aesthetic requirements of each project.

Photo courtesy of Think Wood

DLT is a mass timber product that uses wood dowels as connectors and is ideal for floor, wall, and roof structures.

IN THE FACE OF HOUSING SHORTAGES, HOW CAN PREFABRICATED WOOD CONSTRUCTION BOOST AFFORDABILITY?

The demand for affordable housing is outpacing supply nationwide, with new multifamily units renting at prices cost-prohibitive for middle- and low-income renters.⁴⁴ Nearly two-thirds of renters across the U.S. say they can’t afford to buy a home, and saving for a down payment is out of reach when home prices are rising at twice the rate of wage growth.⁴⁵

Making housing more affordable is a complex issue with no single solution. Nonetheless, innovative approaches are demonstrating how prefabricated light-frame and mass timber wood construction, along with fresh approaches to design, can offer flexibility and play a role in curbing costs by streamlining fabrication, assembly, and installation.

In the face of America’s housing crisis, Star Apartments (see case study) is a good example of the rising trend of thinking outside the box by taking advantage of prefabricated, sustainable building systems—like modular wood construction—to deliver more affordable housing solutions.

HOW SINGLE-FAMILY HOMES CAN BENEFIT FROM PREFABRICATED WOOD CONSTRUCTION

Prefabrication is gaining ground not only in multifamily and commercial projects but also in single-family home construction—driven by advancements in technologies and labor shortages as well as an increasing need to improve housing affordability while cutting carbon emissions. Examples of single-family homes taking advantage of prefabricated light wood frame and mass timber construction are popping up across the country, from Seattle and Los Angeles to Pittsburgh.

Prefab CLT Solutions in Seattle

CLT is one of the most common prefabricated building materials making its way into residential construction. Besides the environmental benefits of CLT, most of its appeal stems from the time saved during the framing process. The panels are prefabricated and delivered to the jobsite already cut to size and ready for installation. Few adjustments, if any, need to be made on site, assuming the original plan was correct and followed properly.⁴⁶ 

Architect and mass timber pioneer Susan Jones of atelierjones oversaw the building of one of the first CLT single-family residences in 2015—her own home in Seattle. Situated on a small, 2,000-square-foot triangular lot across from an alley, the 1,500-square-foot CLTHouse was built to Passive House standards.⁴⁷ It was constructed from prefabricated CLT panels and clad in Shou Sugi Ban, a technique where wood is treated with fire, leaving a charred finish that can protect it for years.

The panels were routed using standard CNC technology, and the floors and walls were designed to fit within 16 panels of 8-feet-by-40-feet. Left almost raw with a light whitewash, the walls of CLT are exposed, the interior trim is kept to a minimum, and utilities are hidden in a central service core. “The acoustics are incredibly rich; there’s a beautiful tone,” said Jones.⁴⁸

Photo courtesy of Lara Swimmer

CLT House by Susan Jones.

Totally Turnkey Wood Construction in Los Angeles

Turnkey housing is another growing trend in the prefab/modular market. Los Angeles-based CLT Architecture is a full-service architectural studio and contractor that specializes in designing, engineering, and building CLT single-family homes, including luxury models.

According to the firm, the precision of prefabrication enables quick assembly, averaging only one to three days on the construction site, and generates a 6 to 10% savings when compared to traditional methods.⁴⁹  The multi-layered CLT also allows the production of panels with various applications—visual, acoustic, and fireproof—and makes it easier to achieve passive environmental standards.⁵⁰ From a performance perspective, panels have the ability to store heat in the winter and protect the house from overheating in the summer. They can also be used to produce structures of unusual shapes, allowing for innovation in design.⁵¹

Photo courtesy of CLT

An example of a turnkey wood building by Los Angeles-based CLT Architecture.

Right-Size Prefab Housing in Pittsburgh 

What if your starter home could grow along with you and your family? This is the vision of Pittsburgh-based startup Module.⁵² The company sees right-sized houses as one way to help address the problem of affordability. Units start as small as 500 square feet, which keeps the initial cost down, but if the owners ever need more space in the future, the roof can be removed, and another box can be set in place in a day to create an additional floor. These modular homes are built in a factory using wood frame modular construction. Building in a controlled environment means the homes are more precisely constructed and are 25% to 40% quicker to build than a traditional home. Thirteen models are available to choose from with 1 to 3-bedroom options.⁵³ The homes are built to be zero energy ready.

Photo courtesy of Module Design

Prefab housing that can grow with the owner’s needs. 

Rent the Backyard with Prefab Modular Wood ADUs

Anderson Anderson Architecture has been designing prefabricated modular building systems for many years, including residential, school, and commercial applications. In partnership with start-up Rent the Backyard, homeowners can purchase free-standing modular accessory dwelling units (ADUs). The structures are prefabricated off site, then lifted by crane into place in a backyard or other location. 

Units are constructed in two months and installed in as little as two weeks. The entire structural module and primary enclosure is constructed with just six massive, cross-laminated timber panels, including four wall panels, one floor panel, and one roof panel. 

Affordable housing production has been a big focus of California government programs and planning efforts, with zoning code revisions allowing ADUs on single-family sites throughout the state. This mass timber version is the first of its kind and offers a wide range of design options. Pricing starts at $185,000.⁵⁴

Photo courtesy of Anderson Anderson

Modular ADU solution by Anderson Anderson Architecture. 

CONCLUSION

Prefabricated wood construction has advanced considerably over the last decade, opening up new opportunities for sustainable, more affordable, and climate-smart buildings. OSM wood construction techniques can make a significant contribution to lowering embodied carbon. When combining the environmental benefits of wood—and its ability to lock in carbon over 

the life of the building—with OSM methods, design teams can deliver fast, cost-effective, and eco-friendly solutions.⁵⁵ “As with any nascent industry change, we must look at every opportunity to utilize off site or modular construction with eyes wide open. Although there will be cases where the benefits do not outweigh the risks, we believe this method of design and construction will offer significant advantages as the market matures.”

The positive environmental impact, coupled with the introduction of mass timber into many modular construction techniques and the ability to produce high-performance building enclosure systems in a controlled factory environment, are key contributors to the growth of the prefab building industry.⁵⁶ Along with environmental benefits, prefabricated wood construction technology can make multifamily and single-family construction more efficient, helping to address the ongoing housing crisis in many cities across the country.⁵⁷ Design teams are looking to prefabricated light-frame and modularized wood construction and just-in-time delivery to streamline all stages of construction, which in turn can help reduce overall project costs.⁵⁸ As the market matures, expertise expands, and technologies progress, the possibilities of sustainable prefabricated wood construction are only set to grow in the years to come.⁵⁹

END NOTES

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51. https://www.modulehousing.com/
52. https://www.modulehousing.com/why-module
53. https://www.rentthebackyard.com/
54. https://mithun.com/2021/05/19/prefabrication-and-modular-approaches-rise-to-the-challenge/
55. https://mithun.com/2021/05/19/prefabrication-and-modular-approaches-rise-to-the-challenge/
56. https://mithun.com/2021/05/19/prefabrication-and-modular-approaches-rise-to-the-challenge/
57. https://www.mckinsey.com/business-functions/operations/our-insights/modular-construction-from-projects-to-products
58. https://www.ecohome.net/guides/3503/prefab-modular-construction-poised-for-dramatic-growth-in-2020/; http://www.wooddesignandbuilding.com/a- modular-revolution/; https://www.nytimes.com/2020/12/15/business/modular-construction-pandemic-coronavirus.html