Roofs fall into two main categories: ‘sloped’ or ‘pitched’ and ‘flat’ or ‘low-sloped’. Most urban or large-scale apartment buildings have flat or low-sloped roofs. The slope of the roof affects the types of materials used, the costs of installation and repair, and the lifespan of the system.
- Sloped roof systems typically use: composite shingle, wood shingle or shake, slate or feau-slate composite equivalents, or standing seam metal materials. Most residential roofs are typically asphalt shingles.
- Flat roof systems typically use: built up tar and gravel, PVC membranes, modified bitumen, or rubber membrane systems such as EPDM.
There are wide variations of roofing materials available in the market. The products also vary in material and installation costs. We have highlighted several common options as well as some newer technologies below:
Sleep slope systems
- Asphalt Shingles Asphalt shingles are the most widely used roofing material in the US. Asphalt shingles are available in organic or fiberglass options. Fiberglass shingles are made of a fiberglass mat that is sealed on both sides by layers of asphalt, and coated with mineral granules. Organic shingles contain a layer of cellulose-fiber (i.e. wood) that is saturated with asphalt and then coated with mineral granules. They typically have a lower fire resistance rating than fiberglass shingles. Compared to fiberglass shingles, organic options have more asphalt per square inch, providing added weight and durability.
Architectural grade shingles are more expensive but also more aesthetically appealing than the typical 3-tab shingles. Shingle quality is most often measured by warranty life. Typical shingles have a 20-year warranty but more expensive shingles that are thicker and more durable are available with 30, 40 or even 50 year warrantees. Since no additional labor except carrying heavier shingles is required for the longer-life shingles the slightly higher material cost may be an excellent investment.
- Pros: Inexpensive, easy to install and repair, variety of colors and styles.
- Cons: Asphalt shingles are large contributor of landfill waste. As a result of the widespread use of asphalt shingles, roughly 11 million tons of asphalt roofing ends up in landfills annually. Recycling efforts are slow due to local, municipal, and federal approvals to use asphalt shingle in hot mix asphalt.
- Costs: Installation and initial costs are lowest of all systems but require more frequent replacement than other options. Average lifespan of 20 + years. Lower lifespan in hotter climates. Warranties cover 20 to 50 years, but only for materials and have other limitations.
Low slope systems
- Built Up Roofing (BUR) BUR is used on flat or low-sloped roofs and consists of multiple layers of bitumen (a dark, viscous residue of petroleum) and ply sheets. Components of a BUR system include a vapor retarder over the roof deck, insulation, membrane and surfacing material. B.U.R.’s come in two basic types, asphalt and coal tar, and three basic components – (1) the waterproofing component, (2) the reinforcing component, and (3) the surfacing component which is used to protect the other components from the elements. Typically two or more plys are used.
- Pros: Used since 1840s and considered one of the most durable, long lasting and impact resistant system available. Low technology maintenance and repairs—can be repaired by custodial staff.
- Cons: BURs are typically hot applied with a heated asphalt kettle on site which is incredibly smelly which makes it difficult to use this system in tight urban neighborhoods or with renovations of occupied buildings. BURs may tend to creep, sag, or slip when placed on any significant slope, especially under heated temperatures. Due to multi-layer, bi-directional system, can be difficult to find leaks. Also, typical BUR system has dark color so while white granules can be applied to increase reflectivity, white granules are not as reflective as white roof membrane. White granules may need to be reapplied to maintain high reflectivity.
- Costs & Longevity: A 1996 survey found that coal-tar organic felt BURs had an average lifespan of 23 years although best systems can last up to 50 years. A complete BUR system including flashing, insulation, etc. costs approximately $16.00/sq ft. (2006 dollars).
- Modified Bitumen Built Up Modified Bitumen (MB) is asphalt that has been modified with polymers (ie. Styrene-Butadiene-Styrene or Atactic Polypropylene ) to give it rubber or plastic-like properties. A modified bitumen membrane assembly consists of a single or multi-layer system of saturated felts, coated felts, fabrics or mats between which alternate layers of bitumen are applied. Factory surfacing, if applied, includes mineral granules, slag, aluminum or copper. Typically only two-plys are used.
Pros: Very versatile—can be torched, or cold, hot and self-applied. Applied in single or multi-ply systems. Application methods can also be combined. Modifications to standard asphaltic based systems allow for greater durability and longer lifespans.
- Cons: Expensive. Hot applied systems create significant odors during installation.
- Costs & Longevity: Compared to BURs, total labor costs are lower due to fewer plys but material costs are higher. Thus, total installed costs for modifieds are higher than BURs. Average lifespan cannot yet be determined from existing roofs since they are relatively newer to the market place. They are expected to last at least 30 to 50 years.
- EPDM Ethylene Propylene Diene Monomer (EPDM) is a single-ply polymer membrane. EPDM is known as a thermoset membranes. This is the most popular flat roof system in New England due to its low cost. Material can withstand the potentially damaging effects of sunlight and most common chemicals generally found on roofs. Requires the use of adhesive, either liquid or tape, to form a watertight seal at the overlaps. Fully adhered systems use adhesive under the entire sheet. Ballasted systems are glued together at seams but the entire system is laid loose and held down with a layer of rounded stones on top. Ballasted systems may be more aesthetically pleasing to look at from above but cannot be walked on without pavers for maintenance and it is more difficult to locate leaks without removing the stone.
- Pros: Can withstand extreme temperatures. Minimal maintenance (remove debris, reseal or replace seams and flashings as needed). Available in reflective white. Fully exposed single-ply system makes finding and repairing leaks relatively simple. Low cost system, quick to install.
- Cons: Life expectancy is lower than other materials. More easily punctured than multi-layer BUR or modified bitumen systems. Ballasted systems require sweeping rocks off to find and repair leaks.
- Costs & Longevity: Lifespan of approximately 14 years. EPDM system costs average $9.00 - $10.00/s.f. (2006 dollars)
- PVC Polyvinyl chloride (PVC) is a form of thermoplastic membrane, or membrane composed of plastic polymers. PVC is applied as a single-ply membrane with seams that are formed using either heat or chemical welding. Most PVC membranes are manufactured to include a reinforcement layer of polyester or fiberglass for increased strength and stability.
- Pros: History of product testing, available in a variety of colors including reflective white, resists puncture and impact, good low-temperature flexibility, good high-temperature tolerance, good fire-resistant properties.
- Cons: Non-recyclable and depending on a particular manufacturer’s process chemical byproducts may be released during manufacturing that have negative impacts on human and environmental health. If ignited, high concentrations of toxic hydrochloric acid are also released.
- Costs & Longevity: Simpson Gumpertz and Heger’s 1996 survey found that PVC systems vary greatly in their longevity. Although the average lifespan for a PVC system in their survey was 13.8 years, the Sarnafil PVC system lasted an average of 25 years at an additional incremental cost of only $0.10/sq. ft..
Cool Roofs Selecting roofing materials with reflective properties can provide additional environmental benefits. Reflective roofing products, also known as “cool roof” systems, help buildings absorb less heat and stay cooler. In addition to reducing the amount of energy required for air conditioning, it reduces the “heat island effect”, a phenomenon that causes cities to be 2 to 8 degrees warmer than the surrounding countryside.
These heat islands occur because many buildings and paved surfaces use dark materials that absorb heat from the sun. This heat remains stored during the day and released slowly during the evening, keeping air temperatures high.
ENERGY STAR provides ratings for roofing products that lower roof surface temperature by 100○ F. Air-conditioned buildings, and buildings located in hot, sunny climates will yield the greatest energy bill savings from “cool roof” systems. Buildings in heating-only climates may have slightly higher energy costs if a cool roof is used but will still reduce heat island effect.
Solar Shingles These assemblies, when installed on a roof, convert solar energy to electricity reducing the demand for power from fossil fuels and nuclear power plants.
Roof Mounted PV Systems Some systems are simply mounted on top of the roof such as at the Robert White Environmental Center in Boston, MA. These do not act as a roof, holding out water, or protecting from the weather. These traditional systems are added onto the exterior envelope of the building – the window glazing or roofing system.
The Powerlight system at the Capuano Early Childhood Center in Somerville, MA lays the PV system flat on a flat roof on an integrated 2” thick pad of rigid insulation that provides additional insulation to the roof system. This provides some additional benefit but the entire roofing system is still required. It also provides additional ultraviolet and temperature swing protection to the roof, increasing the roof’s longevity.
On some sloped applications the roof system may be replaced by the PV system. Window or skylight glazing can also be replaced with see-through PV panels. These types of installations are called Building Integrated PV (BIPV), which means that the system is replacing some other part of the building envelope that you would have to buy anyway such as a sunscreen or window glazing or roofing. This may improve the economy of PV systems since the incremental cost may be reduced.
Green Roofs Green or garden roofs are systems of plant vegetation existing on top of a membrane roof system. Green roofs help manage storm water runoffs and urban heat islands. Green roofs may also help buildings save energy by insulating buildings from extreme temperatures. While this system has been used for centuries in Europe to reduce air conditioning needs, green roofs are a relatively new concept in the United States. The City of Chicago has lead the way in U.S. experiments with the impact of green roofs in urban environments.