According to a recent publication of the American Institute of Architects, roofs of buildings are the number one source of complaints and litigation against architects, engineers and contractors. And the number one problem is roof leaks.
If creating "roofitecture" is among the most exciting parts of design for the architect, coping with leaks is among the most frustrating. Trouble-shooting roofs also is not a very inspiring activity for building contractors, owners and tenants. However, if one thing holds true in the world of building, it's the certainty that all roofs will leak somewhere, sometime.
To keep water out, the roof "sandwich" begins with a waterproof covering, the roofing. Note that "roof" and "roofing" are not the same. The former is the entire assembly of materials and structure covering the top of a building and contributing to its characteristic visual image and silhouette. The latter refers only to the outermost layer of shingles or membranes.
Most sloping roofs are covered with thin, light, inexpensive asphalt shingles finished in a variety of colors. If the budget permits it, heavier, fancier asphalt shingles, fabricated to look like slate or wood shakes, can be used. Unfortunately, asphalt shingles don't last forever -- seven to 15 years at most. On the other hand, slate shingles can last for a building's lifetime if installed and maintained properly.
Machine-cut wood shingles or rougher, thicker split-wood shakes, usually cedar, are popular for residential use. When treated to be fire-retardant, they can be expensive, too. Occasionally, you may see noncombustible cement shingles or red terra-cotta tile roofs more prevalent in warmer climates.
Roofers apply the first row of shingles to the lowest edge of the sloping roof plane and then proceed upward row by row. Nailed to the roof sheathing covered with moisture-resistant building paper or felt, each row overlaps the row below so that water must run several inches uphill to get under the shingles. Unfortunately, this can happen with enough wind or wintertime ice building up along roof edges.
Sloped roofs also can be covered with rustproof metals -- copper, lead, zinc, tin and special alloys. Standing seams between each metal sheet are folded over tightly and anchored to the sheathing by metal cleats within the folded seam. More visually prominent battens may be used to fasten and cover joints. Metal roofs, available today in a multitude of colors, also are more expensive than conventional asphalt shingles.
How steep should a sloping roof be? Most shingle manufacturers will not guarantee the performance of their product if used on a roof whose slope is less than about 20 degrees (measured from the horizontal). Metal-covered roofs can be shallower. The steeper a roof's pitch, the more quickly and positively it will shed water, but this also can be a problem for gutters and downspouts sized too small. Surprisingly, snow can stick tenaciously to well-insulated, very steep roofs.
For flat roofs, which should be pitched slightly for drainage, two kinds of roofing are in common use today. The five-ply built-up roof is the most widely employed and consists of alternating layers of mopped-on bitumen and asphalt-impregnated roofing felts. After all plies are installed, a layer of gravel or white marble chips is imbedded in the top coating, or bitumen, to reflect sunlight and protect the roofing plies from damage.
The second type of roofing consists of thin, synthetic rubber sheets applied to roof decks to form continuous, water-impervious membranes. Lapped joints are chemically "welded," or glued together, with special solvents. Once in place, a "ballast" of gravel or insulation boards is put on top of the membrane to hold it down and protect it from sun and impact.
What constitutes the rest of the roof sandwich? In wood-frame structures, the continuous sheathing or decking to which roofing is applied can be plywood or wood planks. These, in turn, are supported by structural roof joists, beams or trusses.
Steel- or concrete-frame structures support decks or slabs several inches thick. Decks often are composites of corrugated steel sheet and concrete poured on top of the sheet. For domes and vaults, the structural "shell" may be reinforced concrete, steel, cut stone or brick over which roofing is applied. Washington Dulles International Airport's roof is a suspended cable structure that supports prefabricated decking with a continuous roofing membrane on top.
Insulating a roof properly in this climate is even more critical than insulating exterior walls. Unlike walls, roofs receive intense and direct solar radiation throughout the day in summertime. In winter, rising heat in a building can be lost through the roof by conduction and radiation, especially at night. Thus, adequate roof insulation keeps summer heat out and winter heat in, conserving energy and reducing fuel costs.
Rigid insulation, such as boards of dense polyfoams, can be installed between synthetic membrane roofing and the decking layer. On flat surfaces, it also can be placed on top of membrane roofing, maintaining the waterproof membrane at a more stable, moderate year-round temperature. With either flat or pitched roofs, glass- or mineral-fiber insulation can be installed on top of the finished ceiling with a vented attic or air space between the top of the insulation and the roof decking.
Venting the air spaces between insulation and roof decking is essential, for it lets water vapor escape and reduces the risk of condensation. As with walls, moisture can get into roof or attic spaces from inside as well as from the exterior, so that vapor barriers on the underside of the insulation are needed.
Ceilings under roofs, whether drywall, plaster or some other material, can be attached directly to roof or ceiling joists, or they may be suspended from the roof structure. In some buildings, the underside of the roof structure and decking is left exposed. In this case, because there is no attic space, the only place for roof insulation is on top of the decking.
Making a continuous sandwich of roofing, decking or sheathing, insulation, vapor barrier and ceiling seems straightforward enough. This gets tricky only when we must penetrate the roof or form its edges. For almost all roof leaks occur when parapets, walls, chimneys, skylights, pipes and ventilators interrupt the continuity of the roofing surfaces.
Appropriate roof flashing is supposedly our salvation. Thin sheets of copper, lead, aluminum, galvanized steel or plastic generally are folded and lapped so that water cannot enter the building except by flowing uphill or rising to a level well above the roof plane. Asphalt or synthetic material also may be used to caulk or cover joints, especially around skylights.
At chimneys, facade setbacks, penthouses, parapets and other roof-wall intersections, continuous or lapped flashing is imbedded in the wall several inches above the roof plane. It emerges and turns down the wall until it reaches the roofing surface, where it again turns to be integrated with the roofing itself. In effect, a waterproof metallic curb is created. For pipes, special circular collar flashings are used.
There are many reasons why flashing details fail -- faulty design or installation, movement of building components, shrinkage and deterioration of sealants and other materials, to name a few. But sometimes nature can overwhelm the best roof's defenses. Severe and prolonged thunderstorms or abnormal amounts of ice and snow produce levels of precipitation and pressures that push moisture over and behind roof flashings. As on walls, sealed seams eventually can open up when caulking compounds dry out, shrink and lose elasticity.
It may sound apologetic, but the ideal roofing system has yet to be invented or built. We can make roofs beautiful, but we can't make them perfect . . . yet.