|This article does not cite any references or sources. (December 2009)|
An asphalt shingle is a type of roof shingle. They are one of the most widely used roofing covers because they are relatively inexpensive and fairly simple to install.
Two types of asphalt shingles are used: organic and fiberglass or glass fiber. Organic shingles are generally paper (waste paper) saturated with asphalt to make it waterproof, then a top coating of adhesive asphalt is applied and ceramic granules are then embedded. In the case of algae-resistant shingles, a portion of the granules contain leachable paint ceramically coated, designed to protect against discoloration from algae on the roof. This does not protect from moss growth but does slow the growth. Moss feeds on algae and any other debris on the roof. Most manufactures offer a 5- to 10-year warranty against algae growth.
Shingles are judged by warranty and ASTM test standards. Organic shingles contain around 40% more asphalt per square (100 sq ft.) than fiberglass shingles. But this extra needed asphalt makes them less environmentally friendly (despite its "organic" nickname). The paper-based nature of "organic" shingles leaves them more prone to fire damage, and their highest FM rating for fire is class "B". Shingle durability is ranked by warranted life, ranging from 20 years to 50 years; in some cases lifetime warranties are available.
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.
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. Fiberglass is slowly replacing felt reinforcement in Canada and has replaced mostly all in the United States. Widespread hurricane damage in Florida during the 1990s prompted the industry to adhere to a 1700-gram tear value on finished asphalt shingles.
A newer design of fiberglass asphalt shingle, called laminated or architectural, uses two distinct layers which are bonded together with asphalt sealant. Laminated shingles are heavier, more expensive, and more durable than traditional 3-tab shingle designs. Laminated shingles also give a more varied, contoured visual effect to a roof surface.
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.
Shingles tend to last longer where the weather stays consistent, either consistently warm, or constistently cool. Thermal shock can damage shingles, when the ambient temperature changes dramatically within a very short period of time. Proper roof ventilation can extend the service life of a roof. Shingles should not be applied when temperatures are below 10ºC (50ºF), 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.
An additive known as SBS (styrene-butadiene-styrene) can be added to the asphalt mixture to make the shingles more resistant to thermal cracking, as well as more resistant to damage from hail impacts. Most insurance companies offer discounts to homeowners for using impact-4-rated shingles.
Aging and failure of asphalt shingles
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.