Asphalt shingles
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Material description
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. Some shingles have a strip of cellophane 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
cellophane strips and will bond to adjacent shingles. For this reason, the cellophane
strips do not have to be removed.
Roof deck
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 ½ 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 ½ 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 chemical reaction, special care should be given to investigate
the use of fire-retardant-treated wood panel decks.
Underlayment
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 30º 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
Fasteners
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 ¾ inch into
wood plank or board decks.
Flashings
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
Material standards
NRCA does not make any recommendations about which shingle product or manufacturer
to use; however, NRCA does recommend asphalt shingles meet standards established
by the American Society for Testing and Materials (ASTM).
- 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."
Warranties
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.
Construction details
Below are links to some common asphalt shingle construction details.