Roofs
Roofs play a key role in protecting building occupants and interiors from outside weather conditions, primarily moisture. The roof, insulation, and ventilation must all work together to keep the building free of moisture. Roofs also provide protection from the sun. In fact, if designed correctly, roof overhangs can protect the buildings exterior walls from moisture and sun. (See passive solar design.)
Different roof designs and materials are used for residential and commercial buildings. Commercial buildings typically use low-slope or flat roofs, while home roofs are sloped. The concerns regarding moisture, standing water, durability and appearance are different, reflected in the choices of roofing materials.
Roof design can impact the building's thermal performance. For example, in a metal-framed building, the metal eaves can act as thermal fins, moving heat out of the building, which is not desirable in heating climates.
A number of roofing choices are available for high-performance buildings. New roof shingles on the market can produce electricity using solar technology. Reflective roofing materials or coatings help send the heat back into the sky rather than into the building. And recycled content shingles are available that look like slate or wood. Roofing materials should be integrated into the whole building design.
A number of roofing materials are available:
Asphalt
Asphalt is the most commonly used roofing material. Asphalt products include shingles, roll-roofing, built-up roofing, and modified bitumen membranes.
Asphalt shingles are typically the most common and economical choice for residential roofing. They come in a variety of colors, shapes, and textures. There are four different types: strip, laminated, interlocking, and large individual shingles. Laminated shingles consist of more than one layer of tabs to provide extra thickness. Interlocking shingles are used to provide greater wind resistance. And large individual shingles generally come in rectangular and hexagonal shapes.
Roll-roofing products are generally used in residential applications, mostly for underlayments and flashings. They come in four different types of material: smooth-surfaced, saturated felt, specialty-eaves flashings, and mineral-surfaced. Only mineral-surfaced is used alone as a primary roof covering for small buildings like sheds. Smooth-surfaced products are used primarily as flashing to seal the roof at intersections and protrusions, and for providing extra deck protection at the roof's eaves and valleys. Saturated felt is used as an underlayment between the roof deck and the roofing material. Specialty-eaves flashings are typically used in climates where ice dams and water backups are common.
Built-up roofing (or BUR) is the most popular choice of roofing used on commercial, industrial, and institutional buildings. BUR is used on flat or low-sloped roofs and consists of multiple layers of bitumen and ply sheets. Components of a BUR system include the roof deck, a vapor retarder, insulation, membrane, and surfacing material. A modified bitumen membrane assembly consists of continuous plies of saturated felts, coated felts, fabrics or mats between which alternate layers of bitumen are applied, either surfaced or unsurfaced. Factory surfacing, if applied, includes mineral granules, slag, aluminum or copper. The bitumen determines the membrane's physical characteristics and provides primary waterproofing protection, while the reinforcement adds strength, puncture resistance, and overall system integrity.
Metal
Most metal roofing products consist of steel or aluminum, although some consist of copper and other metals. Steel is invariably galvanized by the application of a zinc or zinc/aluminum coating, which greatly reduces the rate of corrosion.
Metal roofing is available as traditional seam and batten, tiles, shingles, and shakes. Products also come in a variety of styles and colors.
Metal roofs with solid sheathing control noise from rain, hail, and bad weather just as well as any other roofing material. Metal roofing can also help eliminate ice damming at the eaves. And in wildfire-prone areas, metal roofing helps protect buildings from fire should burning embers land on the roof.
Metal roofing costs more than asphalt, but it typically lasts two to three times longer than asphalt or wood shingles.
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Wood
Wood shakes offer a natural look with a lot of character. Because of variations like color, width, thickness, or cut of the wood, no two shake roofs will ever be the same.
Wood offers some energy benefits, too: it helps to insulate the attic, and it allows the house to breathe, circulating air through the small openings under the felt rows on which wooden shingles are laid.
A wood shake roof, however, demands proper maintenance and repair, or it will not last as long as other products. Mold, rot, and insects can be a problem. The life cycle cost of a shake roof may be high, and old shakes cannot be recycled.
Most wood shakes are unrated by fire safety codes. Many use wipe or spray-on fire retardants, which offer less protection and are only effective for a few years. Some pressure-treated shakes are impregnated with fire retardant and meet national fire safety standards.
Installing wood shakes is more complicated than roofing with composite shingles, and the quality of the finished roof depends on the experience of the contractor as well as the caliber of the shakes you use. The best shakes come from the heartwood of large old cedar trees, which are difficult to find. Some contractors maintain that shakes made from the outer wood of smaller cedars—the usual source today—are less uniform, more subject to twisting and warping, and do not last as long.
A recycled content roofing material, the eco-shake® looks like wood and contains reinforced vinyl and cellulose fiber.
Concrete and Tile
Concrete tiles are made of extruded concrete that is colored. Traditional roofing tiles are made from clay.
Concrete and clay tile roofing systems are durable, aesthetically appealing, and low in maintenance. They also provide energy savings and are environmentally friendly. And, although material and installation costs are higher for concrete and clay tile roofs, when evaluated on a price versus performance basis, they may out perform other roofing materials.
Tile adorns the roofs of many historic buildings as well as modern structures. In fact, because of its extreme durability, longevity, and safety, roof tile is the most prevalent roofing material in the world. Tested over centuries, roof tile can successfully withstand the most extreme weather conditions: hail, high wind, earthquakes, scorching heat, and harsh freeze-thaw cycles. Concrete and clay roof tiles also have unconditional Class A fire ratings, which means that—when installed according to building code—roof tile is non-combustible and maintains that quality throughout its lifetime.
In recent years, manufacturers have developed new water-shedding techniques and, for high-wind situations, new adhesives and mechanical fasteners. Because the ultimate longevity of a tile roof also depends on the quality of the sub-roof, roof tile manufacturers are also working to improve flashings and other aspects of the underlayment system. Under normal circumstances, properly installed tile roofs are virtually maintenance free. And, unlike other roofing materials, roof tiles actually become stronger over time. Because of roof tile's superior quality and minimal maintenance requirements, most roof tile manufacturers offer warranties that range from 50 years to the lifetime of the structure.
Concrete and clay tile roofing systems are also energy efficient, helping to maintain livable interior temperatures (in both cold and warm climates) at a lower cost than other roofing systems. Because of the thermal capacity of roof tiles and the ventilated air space that their placement on the roof surface creates, a tile roof can lower air conditioning costs in hotter climates and produce more constant temperatures in colder regions, which reduces potential ice accumulation.
Tile roofing systems are made from naturally occurring materials and can be easily recycled into new tiles or other useful products. And, they are produced without the use of chemical preservatives, and do not deplete limited natural resources.
Single-Ply
Single-ply membranes are flexible sheets of compounded synthetic materials that are manufactured in a factory. There are three types of membranes: thermosets, thermoplastics, and modified bitumens. These materials provide strength, flexibility, and long-lasting durability. The advantages of prefabricated sheets are the consistency of the product quality, the versatility in their attachment methods, and therefore, their broader applicability. They are inherently flexible, used in a variety of attachment systems, and compounded for long lasting durability and watertight integrity for years of roof life.
Thermoset membranes are compounded from rubber polymers. The most commonly used polymer is EPDM (often referred to as "rubber roofing"). Thermoset membranes make successful roofing materials because they can withstand the potentially damaging effects of sunlight and most common chemicals generally found on roofs. The easiest way to identify a thermoset membrane is by its seams—it requires the use of adhesive, either liquid or tape, to form a watertight seal at the overlaps.
Thermoplastic membranes are based on plastic polymers. The most common thermoplastic is PVC (polyvinyl chloride) which has been made flexible through the inclusion of certain ingredients called plasticizers. Thermoplastic membranes are identified by seams that are formed using either heat or chemical welding. These seams are as strong or stronger than the membrane itself. Most thermoplastic membranes are manufactured to include a reinforcement layer, usually polyester or fiberglass, which provides increased strength and dimensional stability.
Modified bitumen membranes are hybrids that incorporate the high-tech formulation and prefabrication advantages of single-ply with some of the traditional installation techniques used in built-up roofing. These materials are factory-fabricated layers of asphalt, "modified" using a rubber or plastic ingredient for increased flexibility, and combined with reinforcement for added strength and stability. There are two primary modifiers used today: aPP (atactic polypropylene) and SBS (styrene butadiene styrene). The type of modifier used may determine the method of sheet installation. Some are mopped down using hot asphalt and some use torches to melt the asphalt so that it flows onto the substrate. The seams are sealed by the same technique.
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Solar Shingles
Thin-film photovoltaic (PV) cells are now doubling as rooftop shingles. Research on thin-film PV and a growing interest in integrating PV into buildings has resulted in this new building material that generates electricity using sunlight.
Designers need to take care when incorporating solar roofing shingles. The dark color and high absortivity can actually add the building's thermal load. The shingles need to be thermally isolated from the building so they don't heat the building. The climate and thermal insulation level will determine whether solar shingles help or hurt the building's energy performance.
The new PV shingles are being manufactured by a U.S. company, United Solar Systems Corp. The energy generated from a building's PV rooftop shingles can provide power both to the building and to the utility's power grid. Several demonstration projects, including a solar rooftop system showcased at the Southface Energy and Environmental Resource Center in Atlanta, Georgia, have proven that these innovative shingles can provide clean electricity.
PV shingles offer many advantages:
- The same protection, durability, and flexibility as ordinary asphalt shingles
- Aesthetical appeal, blending with standard roofs and normal home construction
- Low installation costs
- Lightweight handling.
Recycled Content
The roofing industry is developing products that reuse waste from other industries. For instance, waste from manufacturers of car hoses, shoes, tires, and other rubber products is now being directed to the manufacturing plant of EcoStar Inc., which makes a 100% recycled lightweight rubber "slate" tile.
Another recycled product, the eco-shake®, looks like wood and contains reinforced vinyl and cellulose fiber.
Coatings
The sun's energy inevitably finds its way into the buildings on which it falls. In the summertime, this unwanted heat energy makes buildings hot and requires more air conditioning. Since the summertime sun rises high in the sky, the sun's radiant energy falls mainly on the roof, with east and west walls of buildings also receiving a significant share.
One of the simplest ways to keep solar heat out of buildings is to use reflective roofing materials. Then, the sun's radiant energy is simply reflected back toward the sky from which it came. In fact, reflective materials also help keep the building's environment (i.e., the city) cool, by reflecting solar energy back out into space, thereby reducing the heat island effect. The Cool Roofing Materials Database provides information about the reflective properties of different roofing materials.
Look for ENERGY STAR®-labeled roofing products, they reflect heat and can reduce peak cooling demand by 10 to 15 percent.
Another way to lower cooling costs and extend roof life is to put a light colored coating (also called cool-coating system) over an existing roof. Adding a cool coating is most beneficial for a building that meets these criteria:
- Is in a climate with hot, sunny weather during at least part of the year
- Uses significant cooling energy and/or has problems maintaining comfort
- Has a large roof area compared to the rest of the building's surface area
- Has roofing which tends to crack and age prematurely from sun damage