Resources
Roofing Systems
Introduced during the 1840s, built-up ("tar and gravel") roofing remains the predominant system installed on low-slope ("flat") roofs. Built up roofing systems continue to be a durable roofing system at a reasonable cost.
A built-up roof consists of plies of reinforcing material (organic felts, fiberglass mats or polyester), inter-ply layers of bitumen (asphalt or coal tar) and a finish surfacing, such as gravel or decorative rock, mineral surface cap sheets, or coatings. Owing to energy concerns, many built-up systems today incorporate a rigid board insulation as well.
The repetitiveness of a built-up roofing system -- its layer upon layer nature -- provides both flexibility and durability. The number of plies and the materials used for surfacing can be varied to adapt the system to virtually any environmental or aesthetic concerns. For the same reason, a built-up roof is more forgiving of abuse after application than are many other roofing systems.
Polymer modified bitumen sheet membranes (SBS styrene butadine styrene and APP atactic polypropylene) were developed in Europe in the 1960s and have been used in the United States since the mid 1970s. They are composed of either asphalt reinforced with polyester or fiberglass and, in some cases, with factory-applied surfacing. The membranes are installed typically in multiple layers using hot bitumen, torch-applied or cold adhesive. These membranes are resistant to heavy traffic, puncture and tears.
An alternative to traditional built-up roofing, cold process roofing gets its name from the simple fact that hot asphalt is not utilized. In place of hot asphalt, "cold" asphalt emulsions or adhesives are applied between polyester mats or fiberglass base sheets. A surfacing is then usually applied over the membrane to provide protection from ultraviolet radiation and to achieve a fire rating.
Single Ply Roofing Systems have been used since the early 1960s. These low slopes roofing systems are designed to be installed in a single layer and are sometimes referred to as a “membrane system”. They can be manually attached using special fashioning plates, fully adhered with adhesive that secures the membrane to the substrate or ballasted, using loose gravel or interlocking pavers to keep the membrane in place. There are two main types of single ply systems: Thermoset such as EPDM that are synthetic rubbers and thermoplastic including PVC and TPO that are plastic based. Both types of systems are highly flexible and resistant to ultraviolet radiation. They are also comparatively easy to install.
EPDM (Ethyl Propylene Diene Monomer)
EPDM roofing systems where developed in the United States in the 1960s. These very large sheets have incredible flexibility that make it adaptable to many sizes and shapes for new or re-roofs. This lightweight rubber-like membrane flexes to conform to the roofs movement. Its superior weatherability makes it a good choice for regions with extreme temperature fluctuations. It is also resistant to ultraviolet light and ozone. EPDM has good chemical resistance, poor oil, grease and asphalt resistance. Typical thickness is 45, 60, 75 and 90 mils.
PVC was developed in the late 1970s in Europe. PVC applications are an excellent choice when you need a system that will stand up to common contaminants such as animal fats, grease, oil and chemicals. PVC uses heat welded seams that are recognized for their superior strength. The PVC membrane is resistant to tears, impact and punctures. They can be easily repaired. PVC systems are flexible allowing structural movement without splitting or cracking. Typical thickness is either 47 mils or 60 mils. They are available in a white “cool roof” option.
TPO (Thermoplastic Olefin Membrane)
The first TPO roofing membrane was introduced in the United States in 1987. TPO roofing systems have become the fastest growing single ply systems because of their superior weatherability and reliable heat-welded seams combining the benefits of PVC systems and EPDM systems. TPO Systems are resistant to tears, impact and punctures. Like PVC, TPO roofs can be easily repaired. Typical thickness is either 47 mils or 60 mils. They are available in a white “cool roof” option.
Metal roofs are lightweight, fire resistant and durable. Structural metal roofing systems are generally used in low-slope applications. Although still dependent upon slope to shed water, they are designed to resist the passage of water under some hydrostatic pressure, are strong enough to span joists without being supported by a solid deck and do not require an underlayment. Non Structural or Architectural metal roofing systems are designed for appearance. They shed rather than resist water and are therefore used in steep slope applications. Architectural metal roofing systems need solid under support and use underlayment felts for additional "waterproofing." The two major types of metal roofs are Corrugated and Standing Seam Metal Roofing Systems.
Metal roofing has a very long history, with roofs dating back to the 1800s still in service. The sustainability of metal roofing provides property owners with a very low "per year" cost option in roofing. Metal roofing products being manufactured today carry manufacturers' warranties lasting from 20 to 50 years. However, most products have been designed so that they can be refurbished on site for additional life once their original finish reaches its useful life. Metal roofing materials are 100% recyclable in the event that they are ever removed.
The high-quality, oven-cured paint systems used on pre-painted metal roofing are formulated to resist chalking and fading of their colors. In addition, these paint systems shed dirt and do not support the growth of algae or fungal matter.
Asphalt shingles are the most commonly used and among the least expensive of materials available for steep slope applications. They are very durable and are available in a wide variety of colors and textures.
Asphalt shingles consist of either an organic (wood fiber or cellulose) or fiberglass mat which has been saturated with asphalt and covered with colored mineral granules. In climates where fungus growth is a potential problem, granules treated with algecide are available. "Self-sealing" shingles have an additional strip of adhesive on the underside. The adhesive is activated by the sun's heat after installation and "seals" each shingle to the one below it, providing extra wind uplift resistance for the roof system.
Clay and concrete tile offer comparable benefits to slate, but with greater variety and at less cost. Tile is very durable and fire resistant. It is available in numerous profiles, styles, finishes and colors. Tile is also less expensive that slate (although it is also generally more expensive than asphalt shingles and fiber-cement products). Like slate, however, tile is relatively heavy, and its use may require roof reinforcement.
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