Asphalt shingles are composed of: a base material, either organic felt or glass-fiber mat, that provides support for the weather-resistant components and gives a shingle strength; asphalt and fillers; and surfacing material, generally in the form of mineral granules, that provides protection from impact and UV degradation and improves fire resistance.
The most common form of asphalt shingles are strip shingles. They are rectangular, the most prevalent size being 12 inches wide by 36 inches long. Metric shingles are 13-1/4 inches by 39-3/8 inches. Strip shingles most frequently have three tabs that are exposed along the length of the shingle for visual effect and are called 3 tab strip shingles.
Shingles may be produced in a single layer or two or more layers. The latter generally are known as laminated strip shingles, or architectural shingles, and they have a three dimensional appearance.
Both 3-tab asphalt shingles and laminated asphalt shingles contain a strip of factory applied adhesive that is activated by the sun's heat after installation and seals each shingle to the next course. The seal strip also provides much of a shingle's resistance to wind uplift. Shingles with factory-applied adhesive have a strip of clear polyester film applied to each shingle to prevent the sealing strips from bonding the shingles together when packaged. When the shingles are installed, the self-sealing strips will not align with the plastic film strips and will bond to adjacent shingles. For this reason, the plastic film strips do not have to be removed.
NRCA recommends that asphalt shingles be applied over continuous or closely spaced wood plank decking or wood decking. The most common materials used for roof decks are plywood or oriented strand board (OSB). When plywood is used, NRCA recommends the use of a minimum 15/32 thick or 1/2 inch nominal exterior-grade plywood for 16-inch rafter spacings and 5/8 inch nominal thickness for 24-inch rafter spacings. For OSB, NRCA recommends using a minimum 15/32 inch thick or 1/2 inch nominal exterior-grade OSB for 16-inch rafter spacings.
Caution should be exercised when roof decks are constructed out of the following materials:
Oriented strand board (OSB): NRCA is concerned with potential fastener-holding problems and dimensional stability because of the effects of moisture where OSB and other nonveneer products are used as roof decking.
Preservative-treated wood: Many roofing material manufacturers recommend wood roof decks be constructed with wood that has been treated with a nonoil preservative pressure treatment or with nontreated air- or kiln-dried lumber.
Fire-retardant-treated wood: Because of the deterioration of some fire-retardant-treated wood panels caused by premature fire retardant activation caused by heat history in service, the use of fire-retardant-treated wood panel decks should be carefully considered.
Underlayment (or "felt paper" as it is frequently called) is installed over the roof deck before the application of asphalt shingles. An underlayment performs two primary functions: it provides temporary weather protection until the asphalt shingles are installed, and it provides a secondary weatherproofing barrier if moisture infiltrates the asphalt shingles.
It is not uncommon for it to rain after the contractor installs underlayment but before he installs the asphalt shingles. The underlayment gets wet and becomes wrinkled. If the wrinkling isn't severe enough to affect the shingle installation (i.e. the wrinkling won't telegraph through the shingles and they won't appear buckled or wavy once installed), the underlayment probably can remain in place. The effects of wrinkling also will be minimized by using heavier weight shingles.
Asphalt saturated, nonperforated organic felts are among the most common underlayments used for shingles; they commonly are designated as Type 15 and Type 30 or referred to as No. 15 and No. 30, which are reflective of a once used pound per square weight designation. The terms Type I and Type II now are used within the industry in lieu of No. 15 or No. 30, respectively.
Another type of underlayment is a synthetic underlayment. It is characterized as being lightweight, water-resistant and less likely to wrinkle; having high tear strength; and being easy to walk on—even when wet. Theoretically, the product may be left exposed to the elements for extended periods of time. Although synthetic underlayments and their purported attributes seem appealing, there are significant issues to consider before using them. To date, there are no applicable ASTM standards for these products. Many synthetic underlayments do not meet current building code requirements, so manufacturers need to obtain a code evaluation report for code compliance. And use of these underlayments may void some manufacturers' material warranties for certain roof coverings (such as with asphalt shingles).
NRCA recommends a minimum of one layer of No. 15 asphalt-saturated felt applied horizontally in shingle fashion on roof decks having a slope of 4:12 (18 degrees) or more. For roof decks having slopes of 3:12 (14 degrees) up to 4:12 (18 degrees), a minimum of two layers of No. 15 asphalt-saturated underlayment should be applied horizontally in shingle fashion. It should be noted that one layer of No. 30 asphalt-saturated underlayment is not the same as two layers of No. 15.
In locations where the average temperature for January is 30Âº F or less, NRCA suggests installation of an ice-dam protection membrane. An ice-dam protection membrane generally is a self-adhering polymer-modified bitumen membrane.
An ice dam protection membrane should be applied starting at a roof's eaves and extending upslope a minimum of 24 inches from the exterior wall line of a building. For slopes less than 4:12 (18 degrees), NRCA recommends a minimum of 36 inches. See Figure 1.
Figure 1 - Example of ice damming
NRCA recommends roofing nails be 11-gauge or 12-gauge galvanized steel or the equivalent corrosion-resistant roofing nails. Nail heads should be low-profile, smooth and flat. Shanks should be barbed or otherwise deformed for added pull-out strength. Nails should be long enough to penetrate through all layers of roofing materials and extend through the underside of the roof deck or penetrate at least Â¾ inch into wood plank or board decks.
Flashings for asphalt shingle roof systems fall into four categories: perimeter edge metal, penetrations, valleys and vertical surfaces. See Figure 2.
Perimeter edge metal: Depending on the severity of climate, anticipated rainfall and freeze-thaw cycling, the use of perimeter edge metal should be considered.
Penetrations: Plumbing soil stacks, exhaust vents and pipes are flashed into asphalt shingle roof systems with some type of flat flange that extends around a penetration and is installed under shingles on the upslope of a flange.
Valleys: Valleys that are called "open valleys" are typically lined with sheet metal.
Vertical surfaces: When a roof system abuts a vertical surface, there are four types of flashing commonly used: apron, step, cricket (or backer) and counterflashing.
Figure 2 - Basic sheet metal flashing components
Apron, step and cricket flashings require some form of counterflashing to cover and protect the top edges from water intrusion. In many cases, the wall covering or cladding material acts as counterflashing. When this does not occur, a metal counterflashing mounted to the vertical surface should be installed. See Figures 3, 4 and 5 for examples.
Figure 3 - Example of metal counterflashing inset in masonry mortar joint
Figure 4 - Example of through-wall metal counterflashing embedded in masonry mortar joint
Figure 5 - Example of surface-mount metal counterflashing
NRCA does not make any recommendations about which shingle product or manufacturer to use; however, NRCA does recommend asphalt shingles meet standards established by ASTM International.
Organic asphalt shingles should meet ASTM D225, "Standard Specification for Asphalt Shingles (Organic Felt) Surfaced With Mineral Granules."
Fiberglass asphalt shingles should meet ASTM D3462, "Standard Specification for Asphalt Shingles Made from Glass Felt and Surfaced with Mineral Granules."
When purchasing a new roof system, there will be two warranties to consider. First, there will be the manufacturer's warranty. In general, these warranties cover defects in the manufacture of the roof covering. The period of coverage can range from 20 years to a lifetime. Please read NRCA's consumer advisory bulletin addressing roofing warranties for more information. Once the project is complete, be sure the contractor provides you with a certificate for your records.
Second, the roofing contractor will provide you with a warranty covering his workmanship. Typically, this will cover installation and related issues. The warranty should contain what items are covered and what will void them. Many contractors offer one year or two years of coverage; however, there is no industry standard.