How roof conscious are you? Do you recall parents reminding you to be grateful for having "a roof over your head?"

The universal role of roofs as providers and symbols of shelter has changed little since human beings first made buildings, whether primitive huts or sophisticated domes. But like facades, roofs also influence how we perceive cities and buildings, contributing more to the architectural experience than just keeping out the rain and snow.

Roofs shape the silhouettes of buildings, streetscapes, neighborhoods and cities. The configuration, detail, proportions, color and texture of roofs clearly affect a building's overall image. Certain roof types have historical and cultural associations. And most important, roofs define and give form to the spaces contained under them.

Some roofs are no more than a horizontal plane behind parapets or gravel stops on a building's top story. Many flat-roofed office buildings, schools, apartments, shopping centers and factories depend primarily on their floor plan geometries and facade elaboration for attaining aesthetic character. Their roofs simply don't participate symbolically or formalistically in the exterior composition.

However, flat roofs don't have to be neutral. The great French architect Le Corbusier saw the flat roof as a special opportunity, another site in the form of a new piece of constructed, horizontal real estate given free of charge. His vision, and many of his buildings, transformed flat roof planes into lofty terraces and landscapes, usable like a beach or patio. As if making a microcosmic village, he positioned skylights, penthouses, stair towers, ramps, trellises and railings on roofs and transformed the profile of his cubistic buildings.

The Watergate complex, designed by Luigi Moretti, does likewise with its rectilinear and curvilinear penthouses arrayed along the rooftop and varying in height and shape. Seen from the air, these elements establish a spatial and volumetric realm totally different from the one below.

Roofs can become entire buildings, dramatic and all-encompassing. They can diminish or even eliminate the presence of facade walls. An Indian tepee or the A-frame vacation house represent such forms. The Filene Center at Wolf Trap and Eero Saarinen's Dulles Airport exemplify roofs constituting buildings, as do arched roof airplane hangers and Quonset huts.

On the campus of Mount Vernon College, Hartman-Cox designed chapel and dormitory buildings whose most sculptural facades are in fact sloping roofs that come nearly to the ground. The roof planes are penetrated by openings that admit light to the chapel sanctuary and to individual dorm rooms. Light enters both directly and indirectly, through sloping roofs to bring light into attics. Skylights in a variety of shapes can penetrate roof surfaces, and entire roof structures can be glazed. With premeditation, rich and constantly changing patterns of light and shadow can paint a building's interior.

Children (and perhaps adults) drawing a house will usually draw a simple shape topped by a peaked roof as viewed from the "gable" end, like "Monopoly" game houses. This undoubtedly represents our quintessential roof memory, the symbol of "home."

Sloping roofs have "ridges" at their top edges, and their bottom edges are frequently defined by a facia and gutter. "Valleys" occur where two sloping roof planes intersect concavely. Roofs turning corners convexly are "hipped," and a roof plane sloping only one way is a "shed" roof. Many buildings appearing to be flat-roofed actually have very shallow sloping roofs. Conventional sloping roofs cover rectangular volumes and spaces most easily, but with appropriate ridging, valleying and hipping, almost any geometry can be fitted.

Roof overhangs, or their absence, are dictated by climatic demands, construction customs, taste and desires for symbolic expression. In one-story buildings, overhangs protect walls and windows from both sun and dripping rainwater. But designers often stop roofs at the top edges of facade wall planes. Without overhangs, a roof appears to be part of the cladding system, conforming to the surfaces and edges of the building volume. By contrast, an overhanging roof can dominate and obscure such geometry and solidity. Some roofs resemble removable brimmed hats, hovering over and casting shadows on facades.

Compare houses designed by Frank Lloyd Wright and Hugh Newell Jacobsen, which illustrate different roof strategies.

Wright's "Prairie Houses" use cantilevered, overhanging, horizontally expressive roof forms, along with projecting balconies and terraces, to claim and control both exterior and interior spaces. Inside ceilings become outside ceilings (soffits). From the interior, dominant roofs compress and squeeze views horizontally outward, stressing connections to the landscape in metaphor and in actuality. But from outside, it is hard to comprehend the volumes comprising the whole building.

On the other hand, some of Jacobsen's houses clearly expose and dramatize the pristine quality of simple, white, cubic volumes ingeniously juxtaposed. Roof surfaces are differentiated only by slope, texture and color -- roof as skin. Not only are there no overhangs, there also are no gutters visible; they are concealed within the roof over the exterior wall. Only inside does the space-making power of Jacobsen's roofs become manifest in rooms with "cathedral" ceilings.

The dome is one of architectural history's most wondrous shapes. Usually combined with other roof geometries, domes and cupolas almost always surmount buildings and significant spaces for purposes of grandeur and exaltation, to demarcate and proclaim. Even when small in scale, they can monumentalize and enrich otherwise mundane buildings. Domes have been idealized for centuries because of their geometry, their centrality and axiality, the techniques of their construction, and their ability to soar vertically while spanning large spaces.

In Rome, Florence and Istanbul, domed churches and mosques are among the great landmarks and skyline determinants. In the District, the domes of the Jefferson Memorial, the Library of Congress, the Museum of Natural History and the National Gallery of Art complement the Capitol Dome as profile makers. A beautifully tiled dome caps the Byzantine-styled Shrine of the Immaculate Conception at Catholic University.

Vaulted roofs, typically circular in cross section, span between parallel rows of columns, beams or walls. Notable Washington examples of vaulted roofs expressed on facades include a house on Ordway Street in Cleveland Park, designed by I. M. Pei, and town houses in Southwest, designed by Charles M. Goodman.

Mansard roofs, popular during the 19th century, are often really parts of facades, inclined walls that surround attic stories. Because we habitually perceive the bottom edge of a roof as the uppermost edge of a building's rectangular mass, the mansard roof can lower the apparent scale and height of a structure. The actual, normally invisible roofs of many contemporary mansard-roofed buildings are nearly flat.

Look again at "Washington Harbor's" roofscape, designed by Arthur Cotton Moore and nearing completion on the Georgetown waterfront. It's a veritable exposition of roof forms -- flat, shallow -- sloped, mansard -- sloped, domed, vaulted, terraced, chimneyed -- as if a new elevated Potomac mini-city and skyline were created. Like it or not, it's an unforgettable silhouette.

Architects must worry not only about how a building and its roof look, but also about resisting the forces of gravity, the transfer of heat, the penetration of water and the threat of fire. No wonder people look up to well-designed roofs.

NEXT: Going through the roof