All homes must have a structure that can support its own weight. Each section of a modular home is usually designed so that two of the exterior walls, usually the long walls, serve as the structural walls, carrying more weight than the other two.
Locations of Structural Walls
Consider a typical ranch made up of two modules that have the long walls oriented parallel to the street. Since the front and rear of each module will bear more weight, there will be three weight-bearing areas: the front of the house, the rear of the house, and the middle of the house where the two modules come together. This middle section is generally referred to as the “marriage” wall, or sometimes as the “mating” wall. Because the marriage wall is really two walls joined together, it is 2 to 4 inches thicker than most interior walls in the remainder of the modular home and in site-built homes. This is one of the many reasons that modular homes are considerably stronger than typical site-built homes. The only place you will notice this is where there are passageways between the modules, such as a framed opening with or without a door.
Replace Structural Walls with Beams
Years ago, the only affordable way to eliminate a section of the marriage wall was to install a beam that dropped below the ceiling, sometimes known as a “dropped header,” to carry the weight. While this opened up spaces between modules, it created a visible room divider in places where one was often unwelcome; sometimes, however, customers featured the beam by covering it in stained wood. Today, thanks to the development of engineered wood beams made of laminated veneer lumber, sections of the marriage wall can be easily eliminated for a reasonable cost. Best of all, because the beams are not as tall, they can be installed flush with the bottom of the ceiling framing so that they do not protrude into the room. This creates a smooth ceiling, or clear span, across the two joined modules, making them into one large room. The master bedroom and family room/living room of the Whately 2 are typical examples.
Structural Walls Projecting from Front or Rear of Home
Home designs that include a module that projects out from the long walls of a module also require support beams. For example, if a garage connects to the front of the home, as in the Gracin plan, the opening that connects the front module to the garage must be supported with a beam. Similarly, if a family room is joined to the rear of a home, as in the Minerva plan, the opening that connects the rear module with the family roommust be supported with a beam.
Limitations of Structural Walls
Laminated beams cannot always create the clear-span spaces you may desire. The more weight that bears down on the marriage wall from above, the larger the manufacturer must make the beams. The weight on the marriage wall increases as you remove more of it to increase the size of the opening. It also increases with certain roof designs and when there are modules stacked above. Some of these structural situations require such tall beams that they will not fit in the ceiling without either protruding down, creating the dropped header effect, or up, compromising the floor above of a two-story or any home with a usable attic, such as a Cape Cod. Since the structural designs of each manufacturer differ, you will need to rely on your dealer to determine how much of a clear-span opening you can create.
For more information about modular home structural walls, see Designing a Modular Home and Modular Home Specifications and Features in my book The Modular Home.