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Credit: Autovol

Modular construction is promoted as an opportunity to combat rising interest rates and construction prices through greater efficiency, address skilled labor shortages, and reduce jobsite waste. However, some architects and engineers are hesitant to embrace this approach because they don’t want their designs to be compromised, and they don’t think it has the flexibility or functionality to execute certain project typologies.

So where does the truth lie? Keep reading for an overview of modular systems, applications, and design considerations. If you don’t find what you’re looking for, contact the WoodWorks Regional Director nearest you to ask project-specific questions or request a lunch and learn.

What is modular construction?

Modular construction is a type of off-site construction, which simply means that portions of a project are fabricated and constructed at a dedicated off-site facility (e.g., a modular production facility) or local shop (e.g., a carpentry crew’s workshop). The off-site umbrella includes:

Modular construction – Volumetric units each consisting of a floor, walls, and ceiling, which typically include structural elements as well as some degree of finishes, MEP, etc. Modular construction can be used for small or substantial portions of a building (e.g., bathrooms or entire hotel units).

Panelized construction – Flat-pack building components (usually wall panels, but also floor or roof cassettes) that are prefabricated off site and consist only of structural components—typically framing and sheathing.

Prefabricated construction – Flat-pack building components that are prefabricated off site and include structural members (framing and sheathing) as well as other components such as finishes, MEP, etc.

Modular wood construction in the U.S. has mostly been light-frame. However, with mass timber becoming more mainstream, both suppliers and developers have been exploring the use of CLT in volumetric modular applications.

What are the benefits? (Are they real?)

Although the choice to use on-site wood framing versus some level of prefabrication depends on many factors, prefabricated and modular construction can offer a number of benefits, including faster construction, improved material efficiency and worker safety, enhanced quality assurance, and reduced material, labor and interest costs.

Depending on the project, taking a modular approach can reduce the duration of construction by 30% to 50% compared to on-site methods—improving the overall time equation (i.e., speed to market) and potentially reducing costs. Modules are manufactured in the plant while site and foundation work is underway and, if optimally scheduled, installed—complete with most finishes—as soon as the foundation is complete. It is this combination of construction occurring simultaneously with site/foundation work, as well as faster install time, that results in significant schedule compression. Below is an illustration of the schedule process.

Other benefits include:

Quality assurance – A controlled fabrication and manufacturing environment is easy to monitor and inspect; depending on the level of prefabrication, multiple inspections may take place throughout the process to ensure a high-quality assembly. Because components and systems are built in a climate-controlled plant, there is less weather-related damage to materials and fewer potential moisture issues.

Reduced risk / improved safety – The chance of injuries on the jobsite is reduced because assembly takes place on the ground in a familiar, monitored environment without hazards caused by weather. There is also less risk to materials once they arrive at the jobsite because prefabricated components, systems and modules are typically delivered and installed within a day or two. Less material storage on site also means fewer risks associated with arson, theft, vandalism, etc.

Environmental – Because components and systems are prefabricated, on-site waste is reduced. Less than five percent scrap is typical for modular construction, which means less material going to landfills.

How do I know if modular is the right choice for my project?

The financial viability of modular construction will vary from project to project. Some examples where modular might create a situational advantage include:

Jobsites with high on-site labor costs and/or low availability of on-site labor. Modular units are built off site, in controlled environments, at lower labor costs. On-site labor time and crew size are minimized, reducing the impact of high on-site costs or labor shortages.

Jobsites with difficult access, remote locations, or other location-specific constraints. When jobsite access is difficult or cumbersome, a modular approach can help to minimize material deliveries and crew travel to the site. Similarly, projects being built in harsh weather conditions or with short windows of construction opportunity can benefit from the inherently compressed construction schedule.

Jobsites where the construction process will have a significant impact on the surrounding neighborhood. Some construction projects will have an inevitable impact on neighbors, streets and other public thoroughfares, etc. The speed of modular construction can lessen the impacts on the neighborhood.

Photo: Zeta Design + Build

Doesn’t modular construction reduce architectural design flexibility?

The short answer is no. Prefabricated and modular designs can accommodate architectural aesthetics such as building offsets, angled walls, balconies, pitched roofs, and more. In fact, in a well-designed structure, it can be impossible to tell that any level of prefabrication was used.

Credit: Jackson|Main Architecture

How steep is the structural design learning curve?

The structural design process for a modular project is very similar to the design of projects built on site. Within each module, 2x walls function as both load-bearing walls supporting gravity loads and shear walls resisting seismic and wind forces. Sheathed 2x floor and ceiling framing spans across each module in simple span conditions and functions as the building’s diaphragm. However, there are several unique factors associated with structural modular design.

First, modules are usually designed structurally in a manner that allows most framing and sheathing to be installed in the fabrication facility while allowing for on-site inspection of critical structural connections. Examples include shear wall hold downs and nailing patterns. Allowing this kind of access on site may require designing removable panels—which are temporarily installed for transit, removed on site for inspection, and then installed in their final condition.

Adjacent modules may also need to be stitched together once they’re set in place on site. This allows them to function together for lateral loading purposes, and avoids the need to design each module as an independent box. Stitching of details and connections is typically called out on the structural drawings, which are developed with input from the modular manufacturer. Another unique consideration is transportation and erection loading. The modular manufacturer will provide input on structural impacts of these conditions and what, if any, supplemental framing or connection detailing may be necessary to design for them.

With modular construction, it’s important to establish roles and responsibilities of the structural engineer(s) early in design. Some projects will have a modular structural Engineer of Record (EOR) who is responsible for design of the factory-built portions. This engineer will provide building loads from the modular structure(s) to the site EOR, who is responsible for designing the foundations and podium.

Where does modular construction fit under the IBC? How are inspections handled?

All prefabricated building materials—whether components, assemblies, or modular structures—must be designed to current International Building Code (IBC) requirements applicable to the jobsite location.

Wood-frame modular projects are typically Type V or III construction. Type V permits the use of wood framing throughout the structure, and up to four stories for multi-family and hospitality occupancies. Type III permits the use of wood framing for all interior elements, requires fire retardant-treated wood for exterior walls, and allows up to five stories for residential occupancies (and six for offices). In many cases, the wood-frame portion of a project (whether modular or site built) is constructed (or installed) on top of a single or multi-story noncombustible podium. Before starting any design layout, it’s important to research local code amendments or other considerations that could impact the design or module production. WoodWorks Regional Directors are well-versed in specific code applications and can help navigate construction type and material options for modular and other wood projects.

Inspection requirements, on the other hand, depend on the type of component. Some inspections are done in the modular plant by a third party on behalf of the project location’s applicable jurisdiction. Some inspections are performed on site after modules are installed.

Additional Resources:

International Code Council. (2022). Primer on Off-Site Construction, Codes, Standards and Compliance.

International Code Council. FAQs on Off-Site Construction.

American Institute of Architects. National Institute of Building Sciences. Design for Modular Construction: An Introduction for Architects.

Fannie Mae. (2020). Multifamily Modular Construction Toolkit.

Modular Building Institute. (2020). Maximizing Modular Design and Construction to Create Affordable Housing.

WoodWorks. Off-Site Construction in Wood: Reinventing the Role of the Design Professional.

Laase, M. Jackson | Main Architecture. Modular Wood-Frame for Multi-Family: Design, Details and Delivery.

Pinon, J. RDH Building Science. Building Enclosure Design for Modular Construction.

Miller, A. DCI Engineers. Structural Design: Detailing Strategies for Modular