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An asphalt shingle is a type of wall or roof shingle. They are one of the most widely used roofing covers in North America because they have a relatively inexpensive up-front cost and are fairly simple to install.
Asphalt shingles are an American invention first used in 1901, in general use in parts of America by 1911 and by 1939 11 million squares of shingles were being produced. The forerunner of these shingles was first developed in 1893 and called asphalt prepared roofing which was similar to asphalt roll roofing without the surface granules. In 1897 slate granules were added to the surface to make the material more durable. Types of granules tested have included mica, oyster shells, slate, dolomite, fly-ash, silica and clay. In 1901 this material was first cut into strips for use as one-tab and multi-tab shingles.
All shingles were organic at first with the base material, called felt, being primarily cotton rag until the 1920's when cotton rag became more expensive and alternative materials were used. Other organic materials used as the felt included wool, jute or manila, and wood pulp. In 1926 the Asphalt Shingle and Research Institute with the National Bureau of Standards tested twenty two types of experimental felts and found no significant differences in performance. In the 1950's self sealing and manually applied adhesives began to be used to help prevent wind damage to shingle roofs. The design standard was for the self sealing strips of adhesive to be fully adhered after sixteen hours at 140 degrees Fahrenheit. Also in the 1950's testing on the use of 3/4 inch staples rather than roofing nails was carried out showing they could perform as well as nails but with six staples compared with four nails. In 1960 fiberglass mat bases were introduced with limited success, the lighter more flexible shingles proved to be more susceptible to wind damage particularly at freezing temperatures. Also in the 1960's research into hail damage which was found to occur when hail reach a size larger than 1.5 inches.
Asphalt Roofing Manufacturers Association (ARMA) formed the High Wind Task Force in 1990 to continue research to improve shingle wind resistance.
Two types of base materials are used to make asphalt shingles: A formerly-living organic base and fiberglass base. Both types are made in a similar manner with asphalt or modified-asphalt applied to one or both sides of the asphalt-saturated base, covered with slate, schist, quartz, vitrified brick, stone, or ceramic granules and the back side treated with sand, talc or mica to prevent the shingles from sticking to each other before use. The top surface granules block ultra-violet light which causes the shingles to deteriorate, provides some physical protection of the asphalt and gives the shingles their color. Some shingles have copper or other materials added to the surface to help prevent algae growth. Self-sealing strips are standard on shingles to help prevent the shingles from being blown off by high winds. This material is typically limestone or fly-ash-modified resins, or polymer-modified bitumen. American Society of Civil Engineers ASTM D7158 is the standard most United States residential building codes use as their wind resistance standard for most discontinuous, steep-slope roof coverings (including asphalt shingles) with the following class ratings: Class D – Passed at basic wind speeds up to and including 90 mph; Class G – Passed at basic wind speeds up to and including 120 mph; and Class H – Passed at basic wind speeds up to and including 150 mph. An additive known as styrene-butadiene-styrene (SBS), sometimes called modified or rubberized asphalt, is sometimes added to the asphalt mixture to make shingles more resistant to thermal cracking, as well as more resistant to damage from hail impacts. Some manufacturers use a fabric backing known as a "scrim" on the back side of shingles to make them more impact resistant. Most insurance companies offer discounts to homeowners for using Class 4 impact rated shingles.
Organic shingles are made with a base mat of formerly living (organic) materials such as paper (waste paper), cellulose, wood fiber, or other materials saturated with asphalt to make it waterproof, then a top coating of adhesive asphalt is applied and ceramic granules are then embedded. Organic shingles contain around 40% more asphalt per square (100 sq ft.) than fiberglass shingles. The paper-based nature of "organic" shingles leaves them more prone to fire damage, and their highest FM rating for fire is class "B". Organic shingles are less brittle than fiberglass shingles in cold weather.
The older organic (wood and paper pulp product) versions were very durable and hard to tear, an important property when considering wind uplift of shingles in heavy storms. Also, some organic shingles produced before the early 1980s may contain asbestos.
Fiberglass shingles have a base layer of glass fiber reinforcing mat. The mat is made from wet, random-laid fiberglass bonded with urea-formaldehyde resin. The mat is then coated with asphalt which contains mineral fillers and makes the fiberglass shingle waterproof. Fiberglass shingles typically obtain a class "A" fire rating as the fiberglass mat resists fire better than organic/paper mats. Fiberglass reinforcement was devised as the replacement for asbestos paper reinforcement of roofing shingles and typically ranges from 1.8 to 2.3 pounds/square foot.
Fiberglass shingles are slowly replacing organic felt shingles and by 1982 the production of fiberglass shingles overtook organic shingles. Widespread hurricane damage in Florida during the 1990's prompted the industry to adhere to a 1700-gram tear value on finished asphalt shingles.
Per 2003 International Building Code Sections 1507.2.1 and 1507.2.2, asphalt shingles shall only be used on roof slopes of two units vertical in 12 units horizontal (17% slope) or greater. Asphalt shingles shall be fastened to solidly sheathed decks.
Asphalt shingles have varying qualities which help them survive wind, hail, or fire damage and discoloration.
Shingles tend to last longer where the weather stays consistent, either consistently warm, or consistently cool. Thermal shock can damage shingles, when the ambient temperature changes dramatically within a very short period of time. "Experiments...have noted that the greatest cause of asphalt shingle aging is thermal loading." Over time the asphalt becomes oxidized and becomes brittle. Roof orientation and ventilation can extend the service life of a roof by reducing temperatures. Shingles should not be applied when temperatures are below 10°C (50°F), as each shingle must seal to the layer below it to form a monolithic structure. The underlying exposed asphalt must be softened by sunlight and heat. A few shingle types utilize release tape which must be removed just prior to installation.
|Left: Example of faster asphalt shingle wear along eaves due to channeled water running down the roof. Right: Severe shrinkage resulting in tearing away of entire flaps. Water running down the roof can seep around the exposed nails into the interior space.|
The protective nature of asphalt shingles primarily comes from the long-chain hydrocarbons impregnating the paper. Over time in the hot sun, the hydrocarbons soften and when rain falls the hydrocarbons are gradually washed out of the shingles and down onto the ground. Along eaves and complex rooflines more water is channeled so in these areas the loss occurs more quickly. Eventually the loss of the heavy oils causes the fibers to shrink, exposing the nail heads under the shingle flaps. The shrinkage also breaks up the surface coating of sand adhered to the surface of the paper, and eventually causes the paper to begin to tear itself apart. Once the nail heads are exposed, water running down the roof can seep into the building around the nail shank, resulting in rotting of roof building materials and causing moisture damage to ceilings and paint inside.
Moss and debris which remains on the shingles will cause premature deterioration and should be physically removed from the roof or with a copper sulfate, zinc chloride or other solution carefully applied and thoroughly rinsed. To prevent the growth of algae and moss install zinc or copper strips or wire at the ridge and every four to six feet down the roof. Black algae growth can be cleaned with a bleach solution.