Factory Construction in Homebuilding
Worker installs an interior wall panel in half of a modular home in factory. Note the panel suspended by the lift between the two units. Because the homes are indoors, the exterior sheathing is installed last.
Most homes in the United States are built onsite; the lumber, bricks, drywall, and flooring and roofing materials are transported to the home’s final location before construction begins. Although over the past century the production of some housing components, such as doors, windows, and cabinets, has moved almost universally to factories, for the most part, the homes themselves continue to be built onsite.
When the U.S. Census Bureau started tracking housing construction methods in the early 1990s, the share of single-family homes built onsite was about 94 percent, and that percentage has increased in recent years. The fact that the percentage of housing built onsite remains so high is noteworthy given that the industry has recently focused considerable attention on the efficiencies of factory construction. Factory-constructed housing, such as modular housing and panelized homes, are mostly fabricated at another location and then transported to the homesite for final installation. Although manufactured housing built under the HUD Manufactured Housing Code is a form of factory construction, it is not addressed here. Factory construction leads to more efficient and consistent building practices because the factory setting allows builders to use labor-saving tools and processes in a controlled, clean, and safe environment protected from the weather.
Factory construction can include smaller modular homes that are virtually complete (transportation routes often limit the maximum building size) or components that are assembled onsite. At the simplest level, factory-built homes are either modular or panelized. Modular homes consist of connectable “boxes” that typically have walls, wiring, plumbing, siding, windows, and doors already installed and finished. Upon arrival at the homesite, the boxes are installed on a foundation and connected. Simple homes consist of a single box, although the homes can be much larger, with more than four boxes and more than one story. The onsite effort includes constructing the foundation, setting and connecting the individual boxes, and finishing the areas where the units connect.
Worker checks the plans during construction of this unit.
Constructing panels in a factory is a less complex process than manufacturing modular housing because the panels typically include only framing and sheathing. Because they can be stacked onto a truck, these panels (or structural insulated panels) are easily transported to a construction site for installation and finishing. Onsite construction using panels is faster than conventional site construction but still requires field installation of wiring, insulation, drywall, windows, and siding.
Factory building addresses many of the inefficiencies of site building by providing more consistent working conditions, reduced material handling, tools that improve production efficiency, and a clean and safe workspace. Factories are generally in large, warehouse-style buildings with heaters and fans to provide more comfortable working conditions. Forklifts deliver construction materials to the point where they are used, easing the burden on the workforce. The factory allows the tools (such as nail guns and ladders) to remain at the worksite, eliminating the morning setup and evening shutdown procedures on a conventional jobsite and saving up to 30 minutes per day. Factories work hard at minimizing scrap to encourage the efficient use of materials and reduce waste.
The “factories” where the components are constructed vary widely in sophistication. The simplest style might be a nearly bare warehouse with several worktables where lumber is assembled into panels. Many panel plants use “jigs” with spacers to hold the lumber so workers can build the panels without frequent measuring. In this approach, the tooling provides improved quality as well as greater construction efficiency. In site building, panels are generally fabricated on the floor deck, which requires carpenters to bend over to work at floor level, stressing their backs and bodies. In a factory, the work surfaces are elevated, and overhead cranes are often used to lift the finished panels, reducing stress and improving the speed and accuracy of the work.
Two halves of a modular unit on a production line. Electrical cables in the foreground are awaiting installation from the panel box to the individual outlets
Modular home plants consist of a series of stations, and the finished walls are installed on the floors as the unit moves down a production line much like an automotive assembly line. The work crews assemble components and then install them on the unit at their station. Time waiting for the next unit can be spent building the walls or other components, increasing efficiency. The unit comes off the line virtually ready to occupy.
The factories where the homes or panels are constructed are usually in states or jurisdictions some distance where the home will be installed; however, a home constructed in a factory must still meet the construction code requirements of the jurisdiction where it will be placed. Often a factory regularly builds homes that meet the building codes of several jurisdictions. Experience has shown that these factories handle multiple sets of requirements easily, and the construction quality has been found to be more consistent. This result is due to the experience of the workforce and the presence of supervisors on the factory floor, with no travel time necessary between projects.
Carpenter building a wall panel using a jig to hold and square the boards.
Great potential remains for factory construction in the housing market. In areas prone to flooding, where homes must be raised more than 4, 8, or 10 feet above the ground, modular boxes can be lifted off a delivery truck with a crane, allowing builders to complete construction relatively easily. For post-disaster housing, builders can begin the construction of replacement housing at a factory outside the immediate disaster area even as the home sites are being prepared for reconstruction.
The National Association of Home Builders regularly highlights the potential for factory-built construction through its Building Systems Councils, which include a Modular Home Building Council and a Panelized Home Building Council. Because factory construction is so time- and cost efficient, economists and others are often puzzled about why factory-built housing holds such a small share of the new construction market. Some of the challenges to more widespread adoption include the need for builders to adopt a new business model and the need for consumers to develop a greater awareness of the advantages of purchasing and living in a factory-built home. Neither the homebuilding industry nor potential homebuyers have demonstrated a willingness to step away from the traditional housing construction model. Factory construction faces challenges that any new or innovative product faces, but after more than 25 years, it still has not overcome those challenges.