What causes a crowned floor in a mobile/manufactured home?
Wednesday, June 13, 2018
The two steel I-beams that provide the main support of a mobile home are parallel to the long walls of each unit of the home, but inset several feet from the walls. The floor joists that sit on top of the beams run perpendicular to them, and are cantilevered as they extend past the beam. In other words, they are unsupported where they end under the long walls.
The Push and Pull Of A Crowned Floor
The long walls push down on the end of that cantilever with a combination of the weight of the wall itself, the area of roof bearing on the wall, and any additional loads next to the wall such as a bathtub. Also, any large openings in the wall, like a sliding glass door or double-window, transfer the roof weight over them via a header to both sides of the opening, creating a concentrated load at these locations on the wall.
The diagram below helps explain the situation. Think of the bearing point at the chassis I-beam (blue arrow) as the center of a see-saw, with lots of weight on side “A,” and not much weight at side “B.” As the unsupported end of the joists at “A” begins to sag downward under the heavy load, the “B” side responds by bowing upward. This creates a “crown,” or hump in the center of the floor.
Perimeter Piers Help
Mobile home manufacturers overcome this problem by requiring piers at specific locations under these walls. They are called “perimeter” or “sidewall” piers to distinguish them from the piers that support the steel chassis. Although piers are needed along the walls and marriage line for a general load, typical locations for additional sidewall piers are places where there is a concentrated load, like a fireplace, inset porch, and where kitchen cabinets run along the wall—besides under the sides of sliding glass doors or large windows already mentioned.
Crowning problems occur if the perimeter piers are not set and the home leveled to the manufacturer’s specs. It can also happen if a home is stored for a long period with minimal supports before delivery to the homesite, as in the photo at the top of this page. The very obvious bow that’s visible at the end of the unit will end up permanently framed into place unless corrective work is done during the installation.
Older Homes May Lack Perimeter Piers
Older mobile homes may have no perimeter piers or not enough of them, especially at locations of concentrated loads and the marriage line near short walls. The walls sag and the floor crowns at these homes gradually over time, eventually causing problems. Here’s a list of the possible symptoms:
- Crowning/humping of floors in center of home.
- Uneven floors.
- Doors and windows difficult to open
- Drooping of roof near long exterior walls or at the ridge
- Buckling or racking of interior wallboard panels.
- Loosening of the connection between the walls and the
floor or roof.
Anything that adds more weight at the long walls will make the crown worse, such as heavy furniture, appliances, or snow buildup on the roof,
Call A Licensed Mobile Home Installer
A licensed mobile home installer can repair a crowning problem for you, but it is not a simple job that a homeowner can tackle successfully. Bringing the piers under the exterior walls and marriage line into alignment at the same level fixes the crown, but can throw the bearing at the piers under the I-beams of the chassis out of whack. Some bearing points at the I-beams may end up sitting above the piers until they “relax” back down, and a second adjustment could be necessary. An experienced, professional installer knows how far things can be pushed without causing damage, especially if the floor has been crowned for while.
It’s a simple fact of structural repair for any home, both site-built and manufactured, that leveling a foundation does not automatically fit everything above it back together correctly. Sometimes the repair causes new stresses, cracks, and problems at other locations.
For a listing of typical foundation and tie-down requirements and defects, see our blog post What are the most common defects in mobile/manufactured home foundation piers? and What are the tie-down requirements for a mobile home?
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