A new roof system is a big investment, and
you should get a quality roof system at a fair price from a professional
roofing contractor. Hopefully, this information will make you a more knowledgeable
consumer and, when the time comes, a smart roof system buyer.
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 Sytem
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 steep-slope structures.
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 granules.
- 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
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 machinesawn; 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
Ventilation and Insulation are of Key Importance
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 attic.
In addition to the free flow of air, insulation plays a key role in
proper attic ventilation. An ideal attic has:
- 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 freely.
- A minimum of 1 inch between the insulation and roof sheathing.
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
Your Roof's Enemies
A roof system's performance is affected by numerous factors. Knowing
about the following will help you make informed roof system buying decisions:
- 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
- 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
- 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
- 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.
||The area between your ceiling and the roof structure
||The surface, usually plywood or oriented strand board (OSB), to
which roofing materials are applied.
||A small structure projecting from a sloped roof, usually with a
||An L-shaped strip (usually metal) installed along roof edges to
allow water run off to drip clear of the deck, eaves and siding.
||The horizontal lower edge of a sloped roof.
||A flat board, band or face located at a cornice's outer edge.
||A sheet of asphalt-saturated material (often called tar paper) used
as a secondary layer of protection for the roof deck.
||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.
||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.
||Metal troughs at the edge of the roof which collect and carry away
|Ice and Water Guard
||A 36-inch wide sheet of self-adhering underlayment that helps prevent
water leakage caused by ice dams and wind-driven rain. It is applied
directly to the decking in potential leak zones prior to shingle application.
||Slatted devices installed in a gable or soffit (the underside of
eaves) to ventilate the space below a roof deck and equalize air temperature
|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.
||Vents, pipes, stacks, chimneys-anything that penetrates a roof deck.
||The supporting framing to which a roof deck is attached.
||The inclined edge of a roof over a wall.
||The top edge of two intersecting sloping roof surfaces.
||The boards or sheet materials that are fastened to rafters to cover
a house or building.
||Windows in the roof structure which allow additional natural light
into the home
||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
||The common measurement for roof area. One square is 100 square feet
(10 by 10 feet).
||Engineered components that supplement rafters in many newer homes
and buildings. Trusses are designed for specific applications and
cannot be cut or altered.
||The angle formed at the intersection of two sloping roof surfaces.
||A material designed to restrict the passage of water vapor through
a roof system or wall.
|Vents or ventilation
||Vents are openings which allow air movement into the attic or roof
structure to remove excess heat and moisture. Vents should be located
at the soffett areas to let air in, and on the top of the roof to
let the air out.
This information was prepared by the National Roofing Contractors
Association (NRCA) as part of their ongoing effort to educate home and
building owners about roofing and roofing contractors.