Winter is when roofs really matter, when we all hope that the geometric forms and assembly of materials covering our buildings succeed in keeping us dry and warm.
Architects, contractors and building owners will tell you that of all the construction problems that arise after buildings are completed, roof leaks are near the top of the list. And leaks in winter can be as bad because accumulations of snow and ice can radically change a roof's geometry and drainage patterns.
Snow and ice on roofs clog gutters, block roof drains and downspouts, and create dams in roof valleys. These dams, or snow and ice piling up around pipes, vents, chimneys and skylights, can allow water from melting snow and ice to move up under shingles or over flashing, after which it can pour freely into the building interior.
But architects think about more than keeping out the weather. They are also concerned about "roof-itecture."
A building's profile and shape, how it meets the sky, are vital aesthetic issues for designers. Indeed, while many roofs are invisible or visually unobtrusive, others can be the defining characteristic of a building.
What would Rome's Pantheon or the U.S. Capitol be without their domes? How different the Old Executive Office Building would look without its French-inspired mansard roof. The main terminal at Washington Dulles International Airport is almost entirely a roof, as is the Sydney Opera House.
Tents, pyramids, sheds, vaults and domes are the oldest and most generic roof forms. In fact, the water-shedding characteristics of ubiquitous, gabled-ended and hipped roofs are basically the same as those of tents.
But why be just tent-like? In Asian architecture over the centuries, the composition and structure of steeply pitched, gently curved, gable-ended roofs, with elaborate roof ridges, fascias and eaves, became an art form.
When roofs are flat, a building's mass and facades alone define its exterior qualities. But flat roofs are not always visually passive. A memorable hallmark of many houses designed by Frank Lloyd Wright is the horizontal sweep of cantilevered roof planes reaching out toward the landscape while imparting a strong sense of shelter.
A defining characteristic of the Kennedy Center is its flat, proportionally thin, projecting roof plane. Supported on spindly columns, the roof girdles the box-like building to make a kind of lofty, wrap-around loggia.
