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Two-Story Manufactured (HUD-Code) Homes

Quality, cost control, and land use efficiency

Unloading the second story truck and putting it into place.

Here's a way to get the quality and cost control of today's manufactured housing while using available land more efficiently. The permanent chassis required by the HUD code has discouraged the manufactured housing industry from implementing two-story designs. The removal of the hitch and wheel assembly allows stacking solutions similar to two-story modular. Alternatively, two-story boxes can be built in the factory, allowing them to be shipped virtually full height.

Two-story HUD-Code homes are built as single- or double-width, or single-width over double-width. Essentially there are two approaches to realizing two-story manufactured homes. Both require a crane onsite. One approach, as practiced by Schult Homes and New Era Homes, is to manufacture and deliver single-story boxes and stack the second-story unit on the first-story unit on site, not unlike modular housing. The HUD Code requires all sections to have permanent chassis but does not specify that wheels, axles, and hitch be permanent, so these are removed before they are stacked. Traditional HUD-Code chassis are made of heavy, longitudinal, steel I-beams supporting the floor system from below. Schult Homes departs from this practice with a series of transverse, steel chassis beams positioned within the height of a perimeter girder, reducing the floor's overall thickness. These beams transfer all loads to the axle springs and drawbar. The beams form wheel wells, reducing the overall shipping height. Schult supplied two-story manufactured homes for the New Colony Village project.

New Era Homes employs the Lindsay Unified Floor System. On the road, the wheel/axle assemblies support a pair of longitudinal steel beams under the house, like a standard HUD-code chassis, with the long sides of the box cantilevered. Where this differs from convention is that the girders are contained within the height of the floor structure, passing through and unitary with transverse trusses, which support a perimeter girder. This girder is either a built-up steel channel or wood 2 x 12s (less expensive than steel, but deeper). Floor loads are picked up by the trusses, which may be all wood or wood with steel chords. The steel chords allow for a 1" reduced floor depth. Asked why there is still the option of wood, Fred Lindsay indicates that HUD-Code manufacturers choose the steel but modular manufacturers choose wood.

New Era has a cape design on the near horizon using a tilt-up roof to enclose second-story living space, thus eliminating a stacking solution yet keeping a low profile for transportation. A 12:12 roof is achieved by hinging the rafters just above the wall plate, then knee walls are folded up to enclose livable space a few feet inboard of the eaves. Options will include a water heater with space heating capability, to simplify layout, reduce equipment costs, and save energy by taking advantage of heat stored in a large water tank; compact fluorescent lighting and energy-efficient appliances to reduce water and electrical bills; a ventilation control system to maintain indoor air quality in the tightly-built home; and low-E windows to reduce outside heat transfer. With all HVAC equipment and ductwork installation within conditioned space, a major source of system leaks is eliminated by centrally locating the air distribution. Developed in conjunction with HUD's Next Generation of Manufactured Homes and Steven Winter Associates, NextGen is in the prototype stage.

Another approach, as practiced by Silvercrest Homes in their Lido Homes development, allows the box to be built two stories high in the factory and shipped to the site in this configuration (with special transportation permits). This is possible only when the factory is relatively close to the site and overhead clearance is generous. In order to clear an underpass, the box sits very low and close to the road. This is accomplished by incorporating the chassis within the height of the perimeter beam and joist system, and by installing inverted, prefabricated fiberglass wells to allow wheels to project through the floor inside the box. These openings are then patched up after the wheels, axles, and hitch are removed. The roof ridge section is attached after the house is placed on its foundation.

Silvercrest's alternate method is closer to that of Schult, with a perimeter steel chassis supporting steel Z joists. Tongue-and-groove decking of 1 1/8" is screwed to the joists for a very stiff floor. The perimeter channels are supported by the wheel/spring assemblies.


Affordability

Factory-built homes typically cost less than site-built homes.

Environmental Performance

Factory construction often reduces material waste. It also decreases the number of site visits and, hence, auto emissions associated with trips to the jobsite.


Easy

Schult Homes will deliver anywhere in the US, and has delivered two-story homes to the Midwest from its Milton, PA facility. You'll pay approximately $2 or more per mile for each long-distance truckload.

At present there are only three manufacturers that have produced two-story HUD-Code housing. Manufacturers expect demand to increase dramatically in the next couple of years as builders and potential homeowners become more familiar with its advantages (and its existence). HUD initiated the Next Generation of Manufactured Housing (NextGen) program to explore and promote advanced technologies such as two-story manufactured homes.

One of the biggest barriers to manufacturers more fully developing multi-story HUD-Code housing is lack of demand. HUD-code homes are chiefly sold through dealers without a lot of marketing, so few buyers know of advances like two-story homes. It has been said that site builders are sales-driven while factory builders are production-driven. Site builders and land developers would be quick to pick up the two-story sales slack, but they are traditionally separated from the manufactured housing industry.

Another barrier to acceptance of manufactured housing in general is zoning. Many communities still have not revised their zoning and subdivision standards that govern manufactured housing. As a result, manufactured homes are not permitted by right in these areas, especially those zoned for single family. Thankfully, this is changing, with many states requiring that local regulations not discriminate between factory- and site-built housing.

Because each box has both floor and ceiling joists to be self-supporting for transport, the second floor of each unit is almost double thick, stacking the second floor joists above the first floor ceiling joists. This adds 5½" to 7½" to the floor thickness of the stackable models. Some homeowners report reduced sound transmission between the first and second floor due to the larger cavity with increased mass.


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It is important to distinguish between manufactured homes and modular housing. Both types of housing are built with the same methods in a factory, and in that sense are both manufactured. The product in both cases is a largely completed box to be shipped to the site and usually attached to other boxes, and in that sense they are both modular. However, manufactured homes, having evolved from the mobile home industry, is built to a code administered by the U.S. Dept. of Housing and Urban Development (HUD). The 1974 Enabling Legislation, or "HUD Code", requires that the manufactured box have a permanent chassis, a leftover from the mobile home era, to assure that the home is properly supported during any subsequent relocation. Modular homes, conceived as resting on permanent sites, have no such requirement and conform to conventional building codes of the area to which they are delivered. Without a permanent chassis, they are easily stackable and are commonly built to two or more stories.

The HUD-code, born of the mobile home industry, has no provision for stairs. The manufacturer prefabricates the stair and installs it following delivery, where it is inspected, by the local building inspector, along with other site-built elements like garages. These elements must comply with local building codes.


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A crew of two to four working with one crane can erect one or two Schult homes (four to six boxes) a day. Boxes are attached to foundations by anchor bolts accessed through an open strip along the exterior wall, which is then covered with bandboard sheathing and siding. Siding is factory-installed on the longitudinal sides of the boxes, except for a few courses at the mating lines where the first floor connects to the second floor and the foundation. Siding is installed completely onsite on the shorter gable end sides, where there may be both vertical and horizontal mating lines. The second floor box is attached to the unit below by screwing through the sole plate to the header of the first floor box, accessed through the exterior wall, similar to the first floor/foundation connection.

New Era's enclosed first floor cavity is insulated with R-29 fiberglass batt. Open-web trusses allow plumbing and heating lines to run through. Lindsay Industries, manufacturer of New Era's floor, invented a novel two-story concept that can deliver both first and second stories on a single chassis. Since some chassis can be 76' in length, one can accommodate, say, a 40' first floor box plus a 36' second story. The perimeter girder is cut apart on site, or unbolted in the case of steel, freeing the boxes to stack vertically.


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By building up instead of out, a plot of land can accommodate considerably more two-story units than ranch houses, increasing saleable area and potentially builder profit. This also expands the traditional market of HUD-Code housing, most prevalent among sprawling home parks, to denser communities and city infill sites. The Manufactured Housing Institute supported a series of urban infill projects using two-story HUD-Code homes in its Urban Design Project.

Starting at $20 to $30/sf (not including foundation, services, landscaping, property, and other soft costs), manufactured housing can be less than half the cost of equivalent site-built housing. Comparing per sf costs of one-story manufactured housing to two-story is not as straightforward; the two-story will save on roofing and foundation costs, but there are tradeoffs. Two-story construction involves craning costs, steel floor, shear walls, stairs, and heavier headers, which can raise the delivered price 5% to 8%. This will be offset by economies of scale when two-story designs are more widely used and setup infrastructure is more developed.

By accommodating an equal amount of people on a smaller plot of land, two-story manufactured housing causes less environmental disruption than sprawling single-story homes.

Disclaimer: The information on the system, product or material presented herein is provided for informational purposes only. The technical descriptions, details, requirements, and limitations expressed do not constitute an endorsement, approval, or acceptance of the subject matter by the NAHB Research Center. There are no warranties, either expressed or implied, regarding the accuracy or completeness of this information. Full reproduction, without modification, is permissible.