Beneath the Floor: The Structural Shift Coming to Offsite Housing

The ROAD to Housing Act has opened a door. Now the industry has to decide what to build through it.

Last week, Congress passed the 21st Century ROAD to Housing Act, and one provision may prove more important to factory-built housing than many people realize. HUD Code manufactured homes will no longer be required to have a permanent steel chassis.

That does not mean every manufactured home will suddenly leave the factory without one. It means the industry now has a choice it has not had before.

For decades, the permanent chassis was part of the identity of a HUD Code home. It carried the home down the highway, supported it at the site, and remained underneath it forever. It was practical, familiar, and required. But it also created limitations in foundation design, installation, transportation costs, resale perception, and even the way some local officials viewed manufactured housing.

Now that requirement has changed.

For the HUD Code industry, the question is no longer whether a permanent chassis is required. The question is what replaces the structural job it has been doing.

That question should also matter to the IRC modular industry.

Modular homes have long depended on wood-based rim joist systems around the perimeter of each module. It has worked, and it remains a familiar solution for factories, builders, and code officials. But manufactured housing is about to gain more flexibility, and modular factories cannot afford to assume that the way things have always been done will automatically keep them competitive.

The next battle in factory-built housing may not be over kitchens, siding colors, automation, or even production speed.

It may be fought underneath the house.

The Rim Joist Is About to Matter Much More

When a permanent chassis is removed from a manufactured home, the floor system still has to do its job. The home must be transported safely, set accurately, supported correctly, and tied into a permanent foundation system that meets the site conditions.

all photos - Uni-Frame

That means the outside edge of the floor system—the rim joist—becomes far more important.

In a traditional modular home, the rim joist is generally part of a wood-framed floor assembly. It helps carry loads, supports the edge of the floor, provides a place for the exterior wall system to rest, and connects the module to the foundation.

But wood has limitations, especially when a house is expected to carry major concentrated loads, sit in a flood zone, endure seismic conditions, span across a basement with fewer supports, or be designed for a more open foundation layout.

A stronger structural rim system changes the conversation.

Instead of treating the perimeter of the home as simply another piece of the floor framing, factories can begin treating it as a true structural frame—one designed to carry substantial loads and reduce the number of supports needed below.

That is a major difference.

Why Steel Changes the Foundation Conversation

A properly engineered steel I-beam rim system can support loads approaching 1,000 pounds per lineal foot, depending on the specific design, span, connections, and project requirements. That kind of capacity gives builders, engineers, installers, and developers more options than a conventional wood perimeter system can usually provide.

All photos - Uni-Frame

The immediate benefit is often fewer foundation supports.

In many areas, a modular or manufactured home needs a long row of piers, columns, or foundation supports beneath the floor system. Every one of those supports requires labor, materials, layout work, inspection, and the possibility of adjustment or future maintenance.

If a stronger perimeter frame can carry the loads over a greater distance, fewer supports may be needed.

That can mean fewer columns in a basement. It can mean more usable space beneath the home. It can mean less obstruction for mechanical systems, future finishing, storage, or access. For a homeowner who wants a real basement instead of a forest of columns, that matters.

For builders, it can simplify a foundation plan. For developers, it can reduce repetitive site work. For installers, it can mean fewer opportunities for one poorly placed or poorly adjusted support to create trouble later.

Flood Zones Do Not Forgive Weakness

Flood-zone construction is one of the places where stronger perimeter support can become especially valuable.

Homes in flood-prone areas are often elevated. They may sit on piers, columns, pilings, or engineered foundations that are designed to allow water to move beneath the structure. The higher the home is lifted above grade, the more important the structural path becomes from the house down through the supports and into the ground.

A system that can span farther between support points may reduce the number of piers or columns needed beneath the home. That does not eliminate the need for engineering, local approvals, floodplain rules, or proper installation. Those requirements remain very real.

But it does give the engineer and builder a better structural tool to work with.

Less clutter under an elevated home can also mean less resistance to floodwater flow and easier access for maintenance and inspection. In the right application, that can be a meaningful advantage.

Seismic Areas Need More Than Familiar Methods

Seismic zones bring a different challenge. The home, foundation, connections, and support system all have to work together when the ground moves.

Wood framing remains a proven building material, but it depends heavily on connections, bracing, fastening details, moisture protection, and the consistency of every component in the system. A steel structural perimeter can offer a more direct and rigid load path when engineered as part of the complete home-and-foundation design.

The important phrase is complete design.

No rim joist system, wood or steel, should be marketed as a magic answer for every seismic situation. The foundation, anchors, shear walls, roof system, site conditions, soil, and local code requirements all matter. But a stronger, more predictable perimeter frame gives the entire system a better starting point.

That is not marketing language. It is structural common sense.

Modular Factories Should Pay Attention

The manufactured housing industry has just received an opportunity to rethink one of its oldest requirements. Some companies will continue using permanent chassis systems because they make sense for their products, customers, transportation methods, and installation markets.

Others will see the chance to build a home that looks, performs, and installs more like permanent housing while still benefiting from HUD Code manufacturing.

That should get the attention of every modular factory owner.

The modular industry has always enjoyed the advantage of being built to the IRC, often set on permanent foundations, and accepted in many markets as site-built equivalent housing. But “we have always used wood rim joists” is not a competitive strategy.

The best modular factories are already asking harder questions.

Can we reduce foundation complexity? Can we create more useful basements? Can we make our homes easier to set in difficult terrain? Can we reduce field labor? Can we give developers a stronger structural solution for projects that require open space below the building? Can we build a better home without making it unaffordable?

Those are not small questions. They are the questions that separate a factory merely producing modules from a factory preparing for the next generation of housing.

This Is Not About Replacing Wood Everywhere

Wood will continue to be a vital part of factory-built housing. It is available, familiar, economical, renewable, and understood by nearly everyone in the construction industry.

But there are places where wood is asked to do more than it was meant to do easily. When support spacing becomes critical, when loads become heavier, when a basement needs to remain open, when flood or seismic requirements become more demanding, or when a home must be supported in ways that were not part of the original factory design, the industry needs more options.

Steel perimeter framing is one of those options.

The real opportunity is not to declare wood outdated or steel perfect. It is to stop pretending that one structural approach should be used for every home, every foundation, every market, and every site condition.

The ROAD to Housing Act may have removed a requirement. What it has really done is create an opening for better thinking.

Gary’s Observation

For years, factory-built housing has been told that its biggest problem is perception. I do not believe that is entirely true.

One of its biggest problems has been accepting old answers because they were familiar.

The removal of the permanent chassis requirement gives HUD Code manufacturers a chance to rethink how their homes are supported, set, financed, and accepted. At the same time, it should encourage IRC modular factories to take a fresh look at the wood-based rim joist systems they have relied on since the beginning.

There is no reason a factory-built home should have to settle for a foundation system that creates unnecessary piers, crowded basements, difficult flood-zone installations, or structural compromises simply because that is how it has always been done.

The companies that lead this next chapter will not be the ones talking the loudest about innovation.

They will be the ones quietly solving problems that builders, developers, installers, inspectors, and homeowners have been living with for years.