Modular Size and Design Constraints

A modular home is created when one or more modules are transported to a building site and assembled on a foundation. Each modular section is a semi-independent structural unit, essentially a box that is built to interconnect with other boxes. Whereas “sticks” are the basic building unit in stick construction, and walls are the basic building unit in panelized construction, modules are the basic building unit in modular construction. Modular design, engineering, and construction work because many home designs can be subdivided into modular sections.

Modular construction, like all construction, has design constraints. The limits to what can be built are a function of modular size (maximum width, length, and height) and structural capability. The limits are themselves determined more by what can be safely, legally, and economically built and transported than by what a manufacturer can fabricate. It is technologically possible to build almost anything as one or more modules, regardless of size. But delivering two 30-foot by 40-foot modules from the factory to the job site would be another matter, as would lifting it onto the foundation. In spite of these constraints, the design possibilities for modular homes are countless.

Modular size constraints include a maximum length of 72', width of 15'9", and height of 11'. — Modular Size: Maximum Length, Width, and Height

Modular Size: Width

Most modular manufacturers build modular sizes in at least three widths, typically 12 feet, 13 feet, and 13 feet 9 inches. Some companies also build widths of 14 feet 9 inches and 15 feet 9 inches. The widths can vary by a couple of inches between companies, depending on the size of a manufacturer’s production jigs. Maximum widths are determined by the federal and state transportation regulations as well as by each factory’s production system. For special needs, such as an existing foundation or a zoning issue, many manufacturers can build to a slightly different width for a modest charge.

Modular Size: Length

Most manufacturers will build modular sizes up to 60-feet long. Some companies will build up to 72-foot-long modules, although many states will not permit these extra-long modules to be delivered. The production line set-up and the length of a manufacturer’s carriers also play a role in what a company can build.

Modular Size: Height

Federal, state, and local regulations limit the maximum height of any vehicle and its cargo, usually to 13 feet 6 inches. They also regulate the minimum height of any object, fabricated or natural, that overhangs a road, such as a bridge, wire, or tree limb. Any exceptions to the minimum height, such as with an older bridge, have to be clearly posted. These maximum and minimum height restrictions help prevent damage to low-hanging objects, vehicles, and their cargo, including modular homes.

Modular Size: Folding Hinged Roofs

Complying with these restrictions means that each modular sizes cannot exceed the maximum shipping height. Since the measurement is taken with the module sitting on top of its carrier, the height of the carrier, which is typically 2 feet 6 inches, also counts in the calculation. This limits the actual height of the module to about 11 feet. Those modules that do not contain any part of the roof, typically the first-floor modules of a two-story, seldom approach this maximum height. However, those modules that support sections of the roof must be engineered to comply with the restrictions, since the height of the modules with the roof in an upright position exceeds the maximum restrictions. An essential requirement of most modular engineering is to design the roof systems so they lie flat on top of the module during delivery. Each section of the roof is fabricated into two or more components, which are hinged to the module and each other. Once the module is on the foundation, the set crew uses the crane to lift and unfold the roof to its correct height.

Modules need folding hinged roofs to comply with the maxium total height allowed during delivery

Sometimes the roof design makes the sectional hinging impossible, either because the design cannot be hinged or because the resulting height would exceed regulations. In those situations, the manufacturer will build one or more of the roof components as panels, or it will build a module without any of the roof components installed on top of the module. The roof will then be constructed on site either from sticks, panels, or specially fabricated modular-roof sections.
For more information about modular size, see Designing a Modular Home, Modular Home Specifications and Features, and Finding and Preparing a Building Lot for a Modular Home in my book The Modular Home.