A recurring theme of this newsletter is the failure of prefabrication (building homes in factories instead of on-site) to revolutionize the housing industry. While it’s possible to build a successful construction business with prefabrication, it hasn’t swept away the old methods of building - building a home on-site remains the most common (and in most cases, the cheapest) way of building a house in the US. (Outside the US, prefabrication is often more popular, but it’s still not a low-cost method of building.) No one has yet managed to do for housing what Ford did for cars, or what Corning did for lightbulbs, or what Arkwright, Hargreaves, and Crompton did for cotton thread.

However, one form of prefabrication is able to reliably produce housing substantially cheaper than site-built methods - the manufactured home (formerly the mobile home, also called trailer homes or HUD homes.) Manufactured homes are a particular type of factory-built housing that isn’t required to meet local building codes - instead, manufactured homes are built to the Manufactured Home Construction and Safety Standards, a federal standard administered by HUD. Manufactured homes have an average per-square-foot cost that’s less than half the cost of the average site-built home

(via the Manufactured Housing Institute)

(This is a somewhat misleading statistic, as site-built homes will be higher-end than manufactured homes on average and thus more expensive for reasons unrelated to production efficiency. Comparing like for like, the cost-per-square-foot of a manufactured home is somewhere around 10-35% less than the cost of a site-built home.)

Despite their lower cost, manufactured homes today make up a relatively small portion of the housing market. In 2021 manufactured homes were just over 6% of new housing units, with just over 100,000 manufactured homes shipped compared to 1.6 million single family and multifamily housing starts. But this wasn’t always the case - at its peak in the early 1970s, the mobile home industry (they didn’t "officially" become manufactured homes until 1980) was shipping 600,000 homes a year, and mobile homes were over 20% of new housing units.

(One statistic you sometimes see is that at one point mobile homes made up 60% of total new houses - this is incorrect. At one point mobile home units were around 50% of the number of single family homes built.)

Since lately there’s been renewed interest in manufactured homes as a potential low-cost housing solution, it’s useful to understand how the industry got to where it is. Why did manufactured homes explode in popularity only for the industry to collapse a few years later? Why aren’t manufactured homes more widely used today? Let’s take a look.

Origins of the manufactured home

The manufactured home can trace its origins to early 20th century camping trailers. In the 1910s and 1920s, car adoption was rapidly increasing - in 1910, there were around 500,000 cars in the US, which by 1925 had increased to 17.5 million cars. Car owners would often use their cars to go on camping trips, an activity which became known as "autocamping." To avoid having to pack and unpack the campsite, and to get more comfort than a tent could provide, people began to build camping trailers that could be towed behind the car - by 1936 it was estimated that there were 300,000 camping trailers in the US. It’s estimated that around 75% of those trailers were homemade, but over time commercially-built ones became more popular. By 1936 most new trailers were purchased from commercial builders, and manufacturers were producing 55,000 trailers a year.

The Curtiss Aerocar camping trailer, which began production in the late 1920s

Early on, autocampers would park by the side of the road, or in a farmer’s field, or any other available space. But as camping became more popular, this became less tenable - a small town on a main road "might expect to see fifty to sixty cars seeking sites each night" [1]. Municipalities began to build campgrounds to accommodate the campers - between 1920 and 1924, an estimated 3 to 6,000 municipal campgrounds were built. And as camping trailers became more popular, campgrounds designed exclusively for trailers began to appear. These became known as "trailer parks."

Almost as soon as trailers appeared, they began to be used for year-round living rather than camping trips, typically by traveling salesman or other itinerant workers. In the 1920s and 30s it was estimated that between 10 and 25% of trailers were used for year-round accommodation. And as unemployment soared and housing starts collapsed during the Great Depression, trailer living became more common. By 1937, it was estimated that 50% of new trailers were purchased as permanent shelter.

Then, as now, permanent trailer living was stigmatized. A 1937 article in Fortune described permanent trailer camps as "crooked rookeries of itinerant flophouses." Many municipalities, such as Detroit and Toledo, placed maximum allowable times that trailers could be parked to try to prevent the appearance of "trailer shantytowns". Lawsuits (such as People vs Gumarsol) were filed against trailer occupants who tried to occupy their trailers permanently.

The trailer industry had a delicate balance to strike - trailer living was becoming more common, which required favorable treatment by municipalities to allow trailer placement. But manufacturers also wanted to ensure that trailers remained classified as vehicles, rather than housing, to avoid having to comply with onerous building regulations and avoid conflict with building trade unions. In 1936, trailer manufacturers formed the Trailer Coach Manufacturers Association (TCMA) to advocate for legislative priorities, encourage favorable regulation, and ensure that trailers continue to be treated by governments as vehicles, rather than housing.

Trailers during and after WWII

The on-set of WWII resulted in a mass-migration of people across the country, as huge numbers of workers moved into areas of defense production. Ypsilanti, Michigan saw its population double between 1941 and 1942 as workers were brought in to staff the largest factory in the world. Orange, Texas saw its population grow from 7,400 to over 50,000 as ship production ramped up at the Port of Orange. The San Francisco Bay area’s population increased by 50% between 1940 and 1950. Altogether, nearly 1 million people migrated to defense areas during the war. This massive shift in population created a commensurate need for housing, much of which was fulfilled using trailers. 50,000 trailers were built for housing during the war, and altogether 120,000 trailers were used for housing in defense areas. In some cases (such as in Ypsilanti) up to 50% of new residents were housed in trailers.

However, the stigma associated with trailer living remained - trailer occupancy was considered another required sacrifice of wartime. One woman living in a trailer noted:

I have wondered how many times how many less employees would Willow Run Bomber Plant have if it were not for the men and women living in trailers - men and women who, like us, are praying and hoping for the day when they can go back home again, back to normal living.

Trailers were still largely considered to be substandard housing. The National Housing Agency (NHA), responsible for the supply of wartime housing, stated:

While Trailers are being used successfully as stop-gap war housing, they do not meet the standards of the National Housing Agency for duration housing for war workers. These wartime standard, moreover, have been cut to a minimum commensurate with providing adequate shelter for war workers and the NHA has no intention of going below them.

However, after the war the housing shortage remained, and full-time trailer living remained common. Large post-war government construction projects (such as dams and AEC facilities) continued to require the use of trailers to house workers. In 1947 trailer manufacturers produced 60,000 units, which rose 86,000 units in 1948 (though sales would collapse the next year.) By 1948 it was estimated that 7% of the US population was living in house trailers, and in 1950 it was estimated that 99% of new trailers were purchases as housing. In 1954, the census estimated that there were 700,000 trailer dwellings in the US, most of them located in the more than 12,000 trailer parks across the country, most of them occupied by construction workers and military personnel.

Some time during this period, trailers began to be referred to as "mobile homes" - in 1952, Trailer Park Management magazine became "Mobile Home Park Management", and in 1953 the Trailer Coach Manufacturing Association became the Mobile Home Manufacturing Association (MHMA).

Despite their increasing use as permanent housing, the stigma against trailers remained. Anthropological studies of trailer parks referred to them as "trailer slums", and one municipal official was quoted in an article for Survey magazine:

A new kind of slum, the permanent trailer camp, offering all the bad features of the urban "blight area,", none of the vacation adventures for which trailers were made.

Trailer camp slums are a very real, if as yet unrecognized, menace to our American way of life. They should be eradicated now, even in the face of an acute housing shortage, for the creation of even more slums is not the solution to the problem of housing shortage.

As part of their advocacy work, the Mobile Home Manufacturer’s Association put substantial effort towards overcoming prejudices and fostering the development of mobile home parks. Partly due to this advocacy, in 1956, Congress authorized the Federal Housing Authority (FHA) to insure loans to finance new mobile home parks, helping to legitimize them (prior to this, mobile home parks were considered unimproved land.)

Relaxing of size limitations

As mobile homes were increasingly used for permanent shelter, they were made larger and larger. Prior to WWII, few trailers were longer than 20 feet, but by 1952 74% of new trailers were longer than 30 feet. But further trailer size increases were limited by the ability to transport them. In 1954 "most states specified that house trailers could not be more than 12.5 ft high, 8 ft wide, and 35 ft long" - building within these dimensions limited a mobile home to just 280 square feet, a small fraction of what a typical site-built home (Levittown houses, for instance, were closer to 1000 square feet.) And these dimensional limitations meant that units weren’t wide enough for a corridor, meaning the only way to access the ends was through the middle rooms, limiting privacy.

Manufacturers tried to use clever expansion mechanisms to create extra space, but while these had some success, they added expense, and the joints between moving sections were a frequent source of leaks and mechanical problems.

Vertically expanding mobile home offered by Liberty in 1946

In 1954, Marshfield Homes debuted a mobile home that was 10 ft wide. The brainchild of Elmer Frey, the 10-wide was too wide to be used as a regular vehicle - it could only be transported by acquiring special permits of limited duration (the original 10-wide was permitted as a construction shack, rather than a trailer.) But the added width gave it additional space and provided enough room for a corridor. The 10-wide was an immediate success, and by 1961 98% of new mobile homes were 10-wides.

The introduction of the 10-wide meant that mobile homes were no longer especially mobile, and marked a major shift in the industry. While early mobile homes moved on average every 20 months, by 1970 they were moved only once every 5 years on average. Manufacturers of mobile trailers used for camping or vacationing (which remained within the 8-ft width limit) split off, forming the Recreational Vehicles Association in 1963. And while previously occupants of mobile homes had largely been itinerant workers (who were somewhat more affluent than average), the larger, less mobile mobile homes increasingly appealed to those in need of low-cost housing (and were thus somewhat less affluent than average.) The industry transitioned from one that supplied movable housing, to one supplying low-cost housing.

With the need to move them regularly abandoned, the only thing restricting the size of mobile homes was state highway transportation limitations. In the early 1960s, thanks to industry lobbying efforts, most state regulations were relaxed to allow the transportation of 12-ft wide units, and by 1972 12-wides made up 85% of new mobile homes. The 12-wide was followed by the 14-wide, and the "doublewide" (a mobile home created by stitching two units together.) Length restrictions were eased as well, and by the 1970s the typical mobile home unit was 65 feet long.

As mobile homes got larger, the structure of their production changed. The industry had grown up as a largely centralized one, near car manufacturing plants in Indiana, Michigan, and Ohio (taking advantage of auto part suppliers, as well as the expertise of the auto manufacturing labor pool), with another center of production in California. But as the size of units increased and transportation costs became a larger concern, production decentralized, with plants increasingly being built near where the homes would be placed. In 1952 Texas had 2 mobile home plants, and Florida had 1 - 20 years later, that had risen to 92 and 70 plants, respectively. By 1972 there were mobile home plants in 45 states.

Mobile home manufacturing locations in 1952

Industry expansion

The relaxation of highway regulations allowing transportation of larger units made mobile homes competitive as a low-cost housing option, and sales of them exploded. Mobile home sales increased from 90,000 in 1960 to nearly 600,000 in 1972, going from 8% to 22% of annual housing units produced. By 1974 mobile homes were produced by over 300 firms in 800 plants across the country, and across the country 9 million people were living in nearly 4 million mobile homes. 41% of those mobile homes were in trailer parks, which now numbered over 24,000. The largest mobile home manufacturer (Skyline) sold more than 50,000 mobile homes a year, more than any other homebuilder, and according to Forbes, the top 3 most profitable companies between 1968 and 1973 were mobile home manufacturers. As part of its efforts to encourage the creation of mobile home parks, by the early 1970s the Mobile Home Manufacturer’s Association had become the world’s largest residential land developer.

This growth occurred largely in the South - between 1960 and 1974 the South added 1.6 million mobile homes, more than every other region of the country combined.

However, this growth didn’t reduce the stigma associated with mobile homes. In the early 1970s, 60% of communities excluded mobile homes from being sited on private lots, and a survey by the American Planners Association found that residents in 80% of communities wanted to exclude mobile homes. Metro areas often tried to prevent the construction of new mobile home parks, with cities such as Des Moine and Miami putting a moratorium on new park construction. The state of Illinois approved just 1 mobile home park between 1955 and 1975. Outside of parks, mobile homes were largely relegated to areas outside of cities where there were fewer zoning restrictions (75% of rural counties allowed mobile home placement on private lots, compared to just 31% within cities and 20% within surburbs.) Where mobile homes were allowed, they were often restricted from being located in residential areas - 15% of communities only allowed mobile homes in areas zoned for commercial or industrial.

Despite this stigma, mobile homes were gradually gaining acceptance (however reluctantly) as a legitimate housing option. In the 1960s mobile homes became eligible for FHA and VA mortgage financing, and in 1970 Richard Nixon (in an effort to show that housing construction goals had been met) included mobile homes in total number of new housing units built for the first time in his address to Congress.

Increasing scrutiny

As the number of mobile homes and the visibility of the industry grew, the mobile homes themselves were subject to increasing scrutiny. Because mobile homes weren’t subject to local building codes, manufacturers could build them with whatever materials they deemed appropriate, which typically meant using lighter, cheaper materials than would be found in conventional homes. Mobile homes used thinner plywood, 2x2 or 2x3 wood studs instead of 2x4s, plastic instead of metal, and glues instead of nails or screws.

Beginning in the late 1950s the industry began to develop a minimum set of quality standards. Working with the American National Standards Institute (ANSI) and the National Fire Protection Association (NFPA), the industry developed ANSI/NFPA standards 119.1, which governed electrical, heating, and plumbing requirements. In 1969, structural requirements were added. 119.1 was a performance-based code - mobile home manufacturers could use any method of construction as long as certain performance parameters were met. By 1975, 40 states had adopted some version of the 119.1 standard as state regulations for mobile homes.

However, these provisions were often lightly enforced, and quality problems with mobile homes often remained. A 1973 survey of Ohio mobile home owners found that 65% had problems with waterproofing the first year, and a survey of mobile homes on dealer and factory lots in California found that 94% failed to meet state requirements. A manufacturer that tried to offer an extensive 1-year warranty found that they "nearly lost their shirts", as their warranty servicing costs were 4x higher than the industry average. Complaints documented by the Center for Automotive Safety included mobile homes that had one-inch of insulation instead of the advertised 3 inches, and aluminum wiring instead of copper.

Mobile homes often lost their value quickly compared to site-built homes. Via "Mobile Homes: the low-cost housing hoax":

A survey of the industry’s own "blue book" indicates that the average mobile home is worth 63% of its purchase price after 4.5 years and only 28% of its original price after 10.5 years. In 1971 the Whirlpool corporation funded a study of mobile home longevity based on census data, which indicates the average life of a mobile home is 16 years.

Mobile homes were also more susceptible to other kinds of damage. Early mobile homes were apparently often called "ten second trailers" because of how quickly they would burn in a fire, and insurance studies found that, while mobile homes were no more likely to be in a fire than conventional homes (and in some cases they were less likely), the average losses from fire damage were up to 50% higher in mobile homes than site-built homes (despite the fact that mobile homes were on average smaller and cheaper than conventional construction.) And mobile homes were more than 30 times as likely to be destroyed in a windstorm as conventional homes (though the absolute number of homes affected remained low.)

In response to the perceived need for greater regulation, in 1974 Congress passed the Mobile Home Construction and Safety Standards Act. This gave HUD the power to create and regulate mobile home safety standards, marking another major industry change.

This will continue next week with Part II

Unless otherwise noted, information from this article is from Arthur Wallis’ "Wheel Estate" and Arthur Bernhardt’s "Building Tomorrow"

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