Prefabricated buildings offer all the benefits of wood-framed construction, and the inspection process is evolving.

A fundamental change is underway in the building industry. The Pittsburgh-based Housing Innovation Alliance, a nationwide coalition of builders and manufacturers, predicts a major shift in the coming decade to an off-site, decentralized model in which large-scale components are built in a factory then assembled on-site by small crews. Increasingly, code officials will encounter off-site (also known as prefabricated) construction as the building industry embraces its benefits.

Benefits of off-site construction

Off-site construction addresses some of the building industry’s most challenging issues, such as a short supply of skilled laborers.

The factory setting allows efficient delivery of components built with advanced technologies. Building schedules are shorter, more predictable and not subject to weather delays. And prefabricated products and processes help architects and builders to meet sustainability and energy-efficiency goals.

Built with wood

Most prefabricated buildings are constructed with wood. Although most use conventional light framing, mass timber products such as cross-laminated timber (CLT) and glulam are increasingly specified in commercial and multi-family projects.

Prefabricated wood components contribute to a faster construction process, are easy to ship across the road, and can increase profits for developers and building owners alike.

But while off-site construction may be faster than traditional building methods, the buildings are safe and durable, as they are subject to the same code requirements as traditional site-build construction.

Modular and panelized construction

There are two types of off-site construction: modular and panelized.

Modular. In modular construction, three-dimensional building modules are built in a factory then shipped to the jobsite. Modular buildings can include pre-installed plumbing, electrical, drywall, trim, windows, doors and flooring.

Marriott International, for example, is in the process of switching much of its low-rise North American construction to wood-framed modular. Hotel rooms arrive on the site with interior finishes and even furnishings such as beds, tables and chairs wrapped in plastic and strapped together in the center of the room.

Panelized. Wood-framed, panelized construction consists of prefabricated wall, stair and even floor panels that workers install on the jobsite. An important consideration here for code officials is whether these prefabricated components are “open” or “closed.” Open components would allow for framing inspection on the jobsite similar to inspection of conventional framing. If components are manufactured with interior finishes, then the framing would be hidden and a different inspection approach would be required.

Common Ground High School in New Haven, Conn., was framed in just four weeks by a crew of five, with a combination of prefabricated CLT and glulam panels. In addition to demonstrating the speed and efficiency of the building process, the building also serves as a national model of what is possible in green school construction.

Common Ground High School, New Haven, Conn. Courtesy of Gray Organschi | Photo: David Sundberg

Common Ground High School is one of the first cross-laminated timber (CLT) buildings in the United States. Construction workers install a prefabricated CLT panel as part of the school’s grand staircase. | Courtesy of Gray Organschi | Photo: David Sundberg

Modular codes — an evolving process

In conventional construction, an inspector typically visits the building site to approve every step of construction, including the plumbing and electrical infrastructure and the insulation. With modular construction, most inspections occur in the factory.

Although modules are sometimes built in one state and shipped to another, they need to comply with local codes in the jurisdiction where they will be assembled. While architectural or factory drawings are approved by the state or local authority, the inspection process varies by state and even by city. Inspections are done either by a third-party inspector or by a state inspector sent to the factory.

According to the National Institute of Building Sciences, 35 states have some type of state-level program that handles modular projects. Of these:

  • 15 accept third-party approval, then review the approval in house
  • 10 accept third-party approval as the final approval
  • 10 review in-house and don’t use third-party approvals, (plans are reviewed by a state agency, and then an inspector visits the factory to do spot checks at different stages of construction).

In the remaining 15 states, each project has to be approved at the local level.

There have been moves to standardize modular codes. The Interstate Industrial Buildings Commission promotes reciprocal recognition of states’ industrialized buildings programs via uniform rules and regulations. So far, this has only been adopted by four states — Minnesota, New Jersey, Rhode Island and North Dakota.

Government and non-profit construction industry agencies also want to encourage the growth of off-site construction. During its 2018 summer meeting, the International Code Council Board of Directors approved the development of a standards project in cooperation with the National Institute of Building Sciences and the Modular Building Institute to support the off-site construction industry.

The Code Council has also noted the need for conformity assessment with regard to off-site products. Currently, these products are evaluated via plan-check, but manufacturers may need to seek conformity assessment via the ICC Evaluation Service (ICC-ES) process.

Presently, there is enough variation in the inspection process for modular construction that all parties — the architect, builder, factory manager and inspector — need to understand and adhere to local code requirements.

The views and opinions expressed in this article are those of the Think Wood and do not necessarily reflect those of the International Code Council, or Hanley Wood.