Common Roofing Materials
Asphalt Shingles: 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 a fiberglass mat which has been saturated with asphalt and coated with colored mineral granules. In climates where fungus growth is a potential problem, granules treated with algaecide 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 additional wind uplift resistance for the roof system.
Shingles were once available in only one thickness, but today many manufacturers offer shingles where several layers have been laminated together to produce three-dimensional profiles and appealing shadow lines. These are known as “dimensional” or “architectural” shingles.
Built-up Roofing: Introduced during the 1840s, built-up roofing remains one of the most popular systems installed on low slope (“flat”) roofs.
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 aggregate, mineral-surface cap sheets, or coatings. Owing to energy concerns, many built-up roofing systems today incorporate a rigid board insulation, as well.
The repetitive nature 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 conditions or aesthetic concerns. For the same reason, a built-up roof is more forgiving of abuse after application than are many other roofing systems.
Metal Roofing: 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.
Architectural metal roofing systems are designed to shed rather than to resist water and are therefore used in steep slope applications. Metal roof panels for both types of system come in a variety of profiles, from the traditional standing seam metal roof panel to die-formed panels that simulate the look of tile, slate or wood shakes/shingles.
Modified Bitumens: Polymer modified bitumens were developed in Europe during the 1960s and have been in use in the United States since the mid-1970s. These materials come in rolls and as the name suggests, the idea is to improve upon the natural properties of bitumens by modifying them during the manufacturing process. Typically, this means blending in either plastic or rubber.
When Atactic Polypropylene (APP) plastic is blended into asphalt it makes it more flexible and increases its resistance to ultraviolet radiation. Some APP modified bitumen materials have granules embedded on the top surface for further UV protection and to achieve a fire rating. Surfacings may be applied for the same purposes. APP modified sheets are usually torch-applied.
The other major modifier for bitumen is Styrene Butadiene Styrene (SBS), a rubber-like material which improves the ability of sheets to elongate and recover. SBS sheets are generally mopped down with hot asphalt or cold adhesive, but some can be torch-applied. Unlike APP modifieds, SBS is susceptible to damage from long-term exposure to ultraviolet radiation, so surfacings are necessary to protect them.
Because of their flexibility, modified bitumens are commonly used as flashing materials in built-up roofing systems.
Polyurethane Foam: Sprayed polyurethane foam (SPUF) was first used as an alternate roofing material about 35 years ago. Because of its excellent insulating characteristics, it has steadily gained in popularity.
Polyurethane foam roofing is applied as a liquid. Two components (isocyanate and polyol) are combined in the mixing chamber of a special gun and sprayed onto the substrate, where it immediately expands to 20-30 times its initial volume. As the foam rises, it sets into a solid and a skin forms on the surface. The closed-cell foam provides insulation, while the skin forms a water-resistant surface. The thickness of the application can be varied to create slope to drain, to meet a given R-value or both. To protect the foam from ultraviolet radiation and to add to the waterproofing qualities of the system, a finish coating is applied to the surface.
Single Ply Roof Membranes: This class of low slope roofing materials encompasses a variety of products which share the common characteristic of being designed to be installed in a single layer. Single ply materials are also highly flexible, which is why they are sometimes referred to as “flexible membrane systems.”
There are two main types of single-ply product. Thermosets, such as EPDM, CSPE and Neoprene are synthetic rubbers. Thermoplastics, such as PVC and TPO are plastic-based materials. Both types of membrane are highly flexible and resistant to ultraviolet radiation. They are also comparatively easy to install. But they are also generally incompatible and cannot be used in combination on a single roofing system.
Slate: Roofing slate is a dense natural material that is practically nonabsorbent. The color of slate is determined by its chemical and mineral composition. Because these factors vary from region to region, slate is available in a variety of colors. These same factors also influence how susceptible slate is to changing color upon exposure to the weather. Those that change color minimally are known as “permanent” slates, while those that show more marked color change are called “weathering” slates.
Although the natural beauty of slate is unparalleled, it is one of the most expensive of steep slope roofing materials. Installing slate is labor intensive and requires considerable skill. Slate is also quite heavy. Reroofing with slate may require reinforcement of the roof to safely carry its weight.
Historically, wood shingles and shakes have been very popular in California. In recent years, however, their use has declined because of concern about their fire resistance. U.L. tested “Class B” wood shakes and shingles, which have been pressure treated with fire retardants are readily available. A “Class A” fire rating for a wood roof system can be achieved by using special underlayments in conjunction with “Class B” pressure-treated shingles and shakes, but some communities nonetheless restrict or ban the use of wood roofing materials entirely, regardless of fire rating. Before considering a wood roof, check with your local building department to see if any such restrictions apply in your community.