What is Offsite Construction?
Offsite construction is an over-arching term used to encompass many types or processes of construction that do not occur on the final building site. In addition to work at the building site, offsite construction allows for components, modules, or subassemblies of the building to be constructed in a controlled factory setting. These components are then transported to the final site where they are incorporated into the building. Once completed, these buildings meet all applicable building codes and are indistinguishable from traditional site-built construction.
The term itself is further defined in the ICC/MBI Standard 1200 and 1205 as:
OFF-SITE CONSTRUCTION. Any module building or component which is designed and constructed in compliance with this standard and is wholly or in substantial part made, fabricated, formed or assembled in manufacturing plants for installation - or assembly and installation - on a building site and has been manufactured in such a manner that all parts or processes cannot be inspected at the installation site without disassembly, damage to, or destruction thereof.
Offsite construction is sometimes referred to as “prefab” in some regions, but basically refers to the process of preassembling or prefabricating parts of the building at a second work site (i.e. the factory).
Types of Offsite Construction
Volumetric modular buildings and homes are prefabricated buildings or homes that consist of multiple three-dimensional units or sections called modules. These modules are constructed off-site, under controlled plant conditions, and then transported to the final building or home site. The modules are then set and/or stacked to form the completed building or home. Modules may be constructed or wood, steel, or concrete. Various type of volumetric modular construction include:
Permanent Modular Construction (PMC) - Permanent Modular Construction (PMC) is an innovative, sustainable construction delivery method utilizing offsite, lean manufacturing techniques to prefabricate single or multi-story whole building solutions in deliverable module sections. PMC modules can be integrated into site built projects or stand alone as a turn-key solution and can be delivered with MEP, fixtures and interior finishes in less time — with less waste, and higher quality control compared to projects utilizing only site-built construction.
Relocatable Buildings (RB) - A Relocatable (or portable) Building (RB) is a partially or completely assembled building that complies with applicable codes or state regulations and is constructed in a building manufacturing facility using a modular construction process. Relocatable buildings are designed to be reused or repurposed multiple times and transported to different building sites. They are utilized for schools, construction site offices, medical clinics, sales centers, and in any application where a relocatable building can meet a temporary space need.
Bathroom Pods – Bathroom pods are volumetric modular bathrooms constructed in a controlled factory setting, combining multiple trades into an efficient, assembly-line construction process. Using bathroom pods lowers construction costs by reducing construction time, improving quality, and eliminating the bathroom defects list. With traditional building, a multitude of trades need to be organized to realize the bathroom design. This requires a high degree of supervision and management on site to ensure correct sequencing and quality of work from plumbers, electricians, tilers, floor layers, sealant applicators, decorators, glaziers, carpenters, and other specialists. Bathroom pods do away with most of these issues and although the capital cost may not be less than site-constructed bathrooms, savings from waste and improved quality result in less snagging and better performance in use. More significantly, reduced construction times mean earlier income streams from the project.
Shipping Containers - A six-sided steel unit originally constructed as a general cargo container used for the transport of goods and materials. Before used as a building or building component, the shipping container must bear an existing data plate containing information required by ISO and verified by an approved agency. The building must then be designed, engineered, and approved in accordance with the same codes and standards as other modular buildings.
Container-based buildings are structures made from recycled metal shipping containers. A typical shipping container is usually about 8.5 feet wide and 10, 20, or 40 feet long. This makes container-based buildings similar in size to a manufactured home, but made out of recycled materials. These structures can be a single shipping container, or many stacked together.
Offsite manufactured components are composed of aggregates and cement-making materials formed in purpose-built factories for delivery when needed.
Structural building components are roof trusses, floor trusses and wall panels that are custom designed and built in specialized manufacturing environments. Components are delivered to the jobsite where framers install them along with permanent bracing to create the overall structural system.
Components are used in all types of construction from residential to commercial to agricultural. The structural framing of a building can be entirely components or a mix of components and conventional or "stick" framing. Wood trusses are assembled from structural framing lumber and joints are fastened with metal connector plates. Cold-formed steel trusses are built with proprietary steel shapes and fastened with screws. (Source: Structural Building Component Association).
Mechanical, Electrical, Plumbing (MEP) Subassemblies – Offsite design, engineering, and combination of mechanical, electrical, and plumbing trades into transportable units or modules. Due to the higher degree of integration between these fields, subassemblies often help identify and eliminate potential clashes in operation and location of equipment.
Wall, roof or floor components that are constructed at a location other than the building site in a manner that prevents the construction from being adequately inspected at the building site without damage or reconstruction. Panelized systems include “headwalls” for healthcare facilities and structural insulated panels or “SIPS.”
As defined in Appendix Q of the International Residential Code (IRC), a tiny house is a dwelling that is 400 square feet or less in floor area excluding lofts. Tiny Houses may be referred to as “on wheels” in which case the house is built to the RV standards, or can be affixed to a permanent location, in which case Appendix Q is the best resource.
Precast concrete is a construction product produced by casting concrete in a reusable mold or "form" which is then cured in a controlled environment, transported to the construction site, and lifted in to place. Prestressed concrete is a method of using prestressing tendons that balance the tensile stress that the concrete compression member would otherwise experience due to a bending load.
3D-printed buildings are structures that use a 3D printer to produce individual components. From there, the builders can assemble the parts and create a 3D-printed building. A relocatable 3D printer can also create the structure of a building onsite. 3D-printed buildings can be created using concrete, plastics, and wood-based materials.
Offsite Construction By The Numbers
In a recent survey conducted by the National Institute of Building Sciences' (NIBS) Offsite Construction Council, 93% of respondents in the architect, engineer, contractor, and/or owner category indicated that they had implemented some form of offsite construction process in the past twelve months. These processes ranged from precast concrete, wall panels, MEP systems, bathroom pods, and permanent modular building construction.
Other notable survey results include:
83% of respondents said they will utilize more or the same amount of offsite construction in the next 12 months.
70% of respondents identified an accelerated construction schedule as an actual benefit of using offsite construction.
60% of respondents identified construction managers/general contractors as responsible for making the decision to use offsite construction methods on actual projects.
60% of respondents identified quality as an actual benefit.
50% of respondents said cost-effectiveness was an actual benefit.