R o o f i n g 1 0 1
Principle Roofing wants you, the consumer, to feel informed and
knowledgeable regarding your roofing needs. The following
information is provided by the National Roofing Contractors
Association (NRCA) as part of their ongoing effort to educate home
and building owners about roofing and roofing contractors.
We hope you find this information useful.
Roof system components
Choosing a roof system
Ventilation and insulation are key
Even roofs have enemies
Terms you should know
Roof system components
All steep-slope roof systems (i.e., roofs with slopes of 25 percent
or more) have five basic components:
- Roof covering: shingles, tile, slate or metal and
underlayment that protect the sheathing from weather.
- Sheathing: boards or sheet material that are fastened to
roof rafters to cover a house or building.
- Roof structure: rafters and trusses constructed to
support the sheathing.
- Flashing: sheet metal or other material installed into a
roof system's various joints and valleys to prevent water seepage.
- Drainage: a roof system's design features, such as shape,
slope and layout that affect its ability to shed water.
Choosing a roof system
There are a number of things to consider when selecting a new roof
system. Of course, cost and durability head the list, but aesthetics
and architectural style are important, too. The right roof system
for your home or building is one that balances these five
considerations. The following roofing products commonly are used for
Asphalt shingles possess an overwhelming share of the U.S.
steep-slope roofing market and can be reinforced with organic or
fiberglass materials. Although asphalt shingles reinforced with
organic felts have been around much longer, fiberglass-reinforced
products now dominate the market.
Organic shingles consist of a cellulose-fiber (i.e., wood)
base that is saturated with asphalt and coated with colored mineral
Fiberglass shingles consist of a fiberglass mat,
top-and-bottom layers of asphalt, and mineral granules.
Asphalt shingles' fire resistances, like most other roofing
materials, are categorized by Class A, B or C. Class A signifies the
most fire-resistant; Classes B and C denote less fire resistance.
Generally, most fiberglass shingles have Class A fire ratings, and
most organic shingles have Class C ratings.
A shingle's reinforcement has little effect on its appearance.
Organic and fiberglass products are available in laminated
(architectural) grades that offer a textured appearance. Zinc or
copper-coated ceramic granules also can be applied to organic or
fiberglass products to protect against algae attack, a common
problem in warm, humid parts of the United States. Both types of
shingles also are available in a variety of colors.
Regardless of their reinforcing type and appearance, asphalt
shingles' physical characteristics vary significantly. When
installing asphalt shingles, NRCA recommends use of shingles that
comply with American Society for Testing and Materials (ASTM)
standards-ASTM D 225 for organic shingles and ASTM D 3462 for
fiberglass shingles. These standards govern the composition and
physical properties of asphalt shingles; not all asphalt shingles on
the market comply with these standards. If a shingle product
complies with one of these standards, it is typically noted in the
manufacturer's product literature and on the package wrapper.
Wood shingles and shakes are made from cedar, redwood, southern
pine and other woods; their natural look is popular in California,
the Northwest and parts of the Midwest. Wood shingles are machine
sawn; shakes are handmade and rougher looking. A point to consider:
Some local building codes limit the use of wood shingles and shakes
because of concerns about fire resistance. Many wood shingles and
shakes only have Class C fire ratings or no ratings at all. However,
Class A fire ratings are available for certain wood shingle products
that incorporate a factory-applied, fire-resistant treatment.
Tile—clay or concrete—is a durable roofing material. Mission and
Spanish-style round-topped tiles are used widely in the Southwest
and Florida, and flat styles also are available to create French and
English looks. Tile is available in a variety of colors and
finishes. Tile is heavy. If you are replacing another type of roof
system with tile, you will need to verify that the structure can
support the load.
Slate is quarried in the United States in Vermont, New York,
Pennsylvania and Virginia. It is available in different colors and
grades, depending on its origin. Considered virtually
indestructible, it is, however, more expensive than other roofing
materials. In addition, its application requires special skill and
experience. Many old homes, especially in the Northeast, still are
protected by this long-lasting roofing material.
Metal, primarily thought of as a low-slope roofing material, has
been found to be a roofing alternative for home and building owners
with steep-slope roofs. There are two types of metal roofing
products: panels and shingles. Numerous metal panel shapes and
configurations exist. Metal shingles typically are intended to
simulate traditional roof coverings, such as wood shakes, shingles
and tile. Apart from metal roofing's longevity, metal shingles are
relatively lightweight, have a greater resistance to adverse weather
and can be aesthetically pleasing. Some have Class A fire ratings.
Synthetic roofing products simulate various traditional roof
coverings, such as slate and wood shingles and shakes. However, they
do not necessarily have the same properties.
Before making a buying decision, NRCA recommends that you look at
full-size samples of a proposed product, as well as manufacturers'
brochures. It also is a good idea to visit a building that is roofed
with a particular product.
Ventilation and insulation are key
One of the most critical factors in roof system durability is proper
ventilation. Without it, heat and moisture build up in an attic area
and combine to cause rafters and sheathing to rot, shingles to
buckle, and insulation to lose its effectiveness.
Therefore, it is important never to block off sources of roof
ventilation, such as louvers, ridge vents or soffit vents, even in
winter. Proper attic ventilation will help prevent structural damage
caused by moisture, increase roofing material life, reduce energy
consumption and enhance the comfort level of the rooms below the
In addition to the free flow of air, insulation plays a key role in
proper attic ventilation. An ideal attic has:
The requirements for proper attic ventilation may vary greatly,
depending on the part of the United States in which a home or
building is located, as well as the structure's conditions, such as
exposure to the sun, shade and atmospheric humidity. Nevertheless,
the general ventilation formula is based on the length and width of
the attic. NRCA recommends a minimum of 1 square foot of free vent
area for each 150 square feet of attic floor—with vents placed
proportionately at the eaves (e.g., soffits) and at or near the
- A gap-free layer of insulation on the attic floor to protect the
house below from heat gain or loss.
- A vapor retarder under the insulation and next to the ceiling to
stop moisture from rising into the attic.
- Enough open, vented spaces to allow air to pass in and out
- A minimum of 1 inch between the insulation and roof sheathing.
Even roofs have enemies
A roof system's performance is affected by numerous factors. Knowing
about the following will help you make informed roof system buying
- Sun: Heat and ultraviolet rays cause roofing materials to
deteriorate over time. Deterioration can occur faster on the sides
facing west or south.
- Rain: When water gets underneath shingles, shakes or
other roofing materials, it can work its way to the roof deck and
cause the roof structure to rot. Extra moisture encourages mildew
and rot elsewhere in a house, including walls, ceilings, insulation
and electrical systems.
- Wind: High winds can lift shingles' edges (or other
roofing materials) and force water and debris underneath them.
Extremely high winds can cause extensive damage.
- Snow and ice: Melting snow often refreezes at a roof's
overhang where the surface is cooler, forming an ice dam. This
blocks proper drainage into the gutter. Water backs up under the
shingles (or other roofing materials) and seeps into the interior.
During the early melt stages, gutters and downspouts can be the
first to fill with ice and be damaged beyond repair or even torn off
a house or building.
- Condensation: Condensation can result from the buildup of
relatively warm, moisture-laden air. Moisture in a poorly ventilated
attic promotes decay of wood sheathing and rafters, possibly
destroying a roof structure. Sufficient attic ventilation can be
achieved by installing larger or additional vents and will help
alleviate problems because the attic air temperature will be closer
to the outside air temperature.
- Moss and algae: Moss can grow on moist wood shingles and
shakes. Once it grows, moss holds even more moisture to a roof
system's surface, causing rot. In addition, moss roots also can work
their way into a wood deck and structure. Algae also grows in damp,
shaded areas on wood or asphalt shingle roof systems. Besides
creating a black-green stain, algae can retain moisture, causing rot
and deterioration. Trees and bushes should be trimmed away from
homes and buildings to eliminate damp, shaded areas, and gutters
should be kept clean to ensure good drainage.
- Trees and leaves: Tree branches touching a roof will
scratch and gouge roofing materials when the branches are blown by
the wind. Falling branches from overhanging trees can damage, or
even puncture, shingles and other roofing materials. Leaves on a
roof system's surface retain moisture and cause rot, and leaves in
the gutters block drainage.
- Missing or torn shingles: The key to a roof system's
effectiveness is complete protection. When shingles are missing or
torn off, a roof structure and home or building interior are
vulnerable to water damage and rot. The problem is likely to
spread-nearby shingles also are ripped easily or blown away. Missing
or torn shingles should be replaced as soon as possible.
- Shingle deterioration: When shingles are old and worn
out, they curl, split and lose their waterproofing effectiveness.
Weakened shingles easily are blown off, torn or lifted by wind
gusts. The end result is structural rot and interior damage. A
deteriorated roof system only gets worse with time-it should be
replaced as soon as possible.
- Flashing deterioration: Many apparent roof leaks really
are flashing leaks. Without good, tight flashings around chimneys,
vents, skylights and wall/roof junctions, water can enter a home or
building and cause damage to walls, ceilings, insulation and
electrical systems. Flashings should be checked as part of a
biannual roof inspection and gutter cleaning.
Terms you should know
Deck/sheathing: The surface, usually plywood or oriented strand
board (OSB), to which roofing materials are applied.
Dormer: A small structure projecting from a sloped roof, usually
with a window.
Drip edge: An L-shaped strip (usually metal) installed along roof
edges to allow water run off to drip clear of the deck, eaves and
Eave: The horizontal lower edge of a sloped roof.
Fascia: A flat board, band or face located at a cornice's outer
Felt/underlayment: A sheet of asphalt-saturated material (often
called tar paper) used as a secondary layer of protection for the
Fire rating: System for classifying the fire resistances of
various materials. Roofing materials are rated Class A, B or C, with
Class A materials having the highest resistance to fire originating
outside the structure.
Flashing: Pieces of metal used to prevent the seepage of water
around any intersection or projection in a roof system, such as vent
pipes, chimneys, valleys and joints at vertical walls.
Louvers: Slatted devices installed in a gable or soffit (the
underside of eaves) to ventilate the space below a roof deck and
equalize air temperature and moisture.
Oriented strand board (OSB): Roof deck panels (4 by 8 feet) made
of narrow bits of wood, installed lengthwise and crosswise in
layers, and held together with a resin glue. OSB often is used as a
substitute for plywood sheets.
Penetrations: Vents, pipes, stacks, chimneys-anything that
penetrates a roof deck.
Rafters: The supporting framing to which a roof deck is attached.
Rake: The inclined edge of a roof over a wall.
Ridge: The top edge of two intersecting sloping roof surfaces.
Sheathing: The boards or sheet materials that are fastened to
rafters to cover a house or building.
Slope: Measured by rise in inches for each 12 inches of
horizontal run: A roof with a 4-in-12 slope rises 4 inches for every
foot of horizontal distance.
Square: The common measurement for roof area. One square is 100
square feet (10 by 10 feet).
Truss: Engineered components that supplement rafters in many
newer homes and buildings. Trusses are designed for specific
applications and cannot be cut or altered.
Valley: The angle formed at the intersection of two sloping roof
Vapor retarder: A material designed to restrict the passage of
water vapor through a roof system or wall.