The roof as a design element has been greatly reduced in the twentieth century. Modernism, with its roots in both notions of efficiency and technology, along with its penchant for Mediterranean forms (clear, cubic houses of Greece, Italy), greatly reduced the role of the roof as one of, if not the most significant, element in the design of buildings. Gabled or hipped, gothic or ranch, the roof of a building was not simply a thing that sat on top of the walls. Rather, the building in a sense was the roof - it sheltered and protected, and the walls held it up. Walls for centuries in Western architecture were for protection from enemies - walls around cities, walls around castles. Within these walled compounds, people made areas with roofs to shelter and protect, to gather. The roof was the building.
Victorian, Colonial revival, Gothic revival
Modernism, or more specifically European modernism, eschewed the varied and messy geometry of roofs in favor of pure platonic solids. Walls made the building. A roof needed to be there to keep the rain out, but it was diminished to point of invisibility. Frank Lloyd Wright's Prairie School is the glaring exception to this, recognizing the fundamental sheltering aspect of the roof and making that the primary expression of the building.
FLW's Ward Willits house
Nevertheless, the technology of flat roof construction and the efficiency of construction colluded to make the roof disappear in modern architecture. A building was a series of walls. If you could figure out how to make a roof into an expressed horizontal wall, then that was acceptable.
LeCorbusier, Mies van de Rohe, Walter Gropius
In the United States, modernism, and especially flat-roof modernism, was hijacked as the official architectural style of corporate institutions. This is partly because of timing (modernism was the latest style as these institutions built masses of buildings) and partially a desire to seem forward-looking and progressive. Largely the adoption of the flat roof as a corporate style, sometimes with or without modernism, is due to its cheapness of construction. So now, most Americans see the flat roof as an extension of their workplace and so many certainly don't want their homes to be part of that 9-5 working world. And most importantly, because a house is a relatively small building, making a roof out of cheap wood trusses and asphalt shingles is the cheapest way to cover a small building. (City houses can be both traditional and flat-roofed, no one seems to be bothered by that)
steel flat roof v. wood roof trusses
So, cost trumps all. Houses have gables and hips because it is cheaper for that size of building. Commercial buildings have flat roofs because it is cheaper for that size of building. We can justify the expressed sheltering aspect of a gabled roof as a fundamental identification with notions of home, but if the large home builders across the county can save some pennies doing it flat, then flat we shall have.
This is all a very long-winded way of getting at the crisis of diminishing resources of lumber and the potential impacts for architecture. Wood is increasingly scarce and of increasingly poor quality. As we begin to switch to alternate materials, will we hold on to the forms that wood made efficient and ubiquitous or will new forms arise from the forced implementation of new materials? In the world of high, international architecture, the roof as a element of design has not only come back but maybe has begun to dominate the discussion as computer modeling has allowed easier visualization of the plasticity of objects, diminishing the distinction between walls and roofs, to the walls detriment.
Hadid, Libeskind, Gehry