Asphalt Shingle Roofing: Composition, Installation, and Key Considerations

A modern home with a multi-gabled roof covered in asphalt shingles, illustrating the popularity of this roofing material in residential construction. 

Asphalt shingles are the most common roofing material in North America, protecting countless homes with a durable and cost-effective shelter from the elements. First developed over a century ago, asphalt shingles have remained popular due to their relatively inexpensive upfront cost, ease of installation, and reliable performance in varied climates. In this post, we will explore what asphalt shingles are made of, how they are installed (including the role of underlayment and roof pitch), the importance of special ridge cap shingles, how granule loss is part of the natural aging process, and why a single layer of shingles is preferable to multiple layers. We’ll also touch on the critical role of flashing in a roofing system. Throughout, the tone will remain factual and technical, reflecting the standards of professional home inspection and roofing industry guidance.

Overview of Asphalt Shingles

Asphalt shingles are a type of composition roofing – meaning they are composed of multiple layered materials. Modern asphalt shingles use a tough fiberglass mat as the core or base, which is then saturated and coated with asphalt for water resistance, and topped with mineral granules for UV protection and color. In short, each shingle is a composite of fiberglass reinforcement, asphalt (a tar-like hydrocarbon derived from petroleum) for waterproofing, and ceramic-coated granules on the surface. The granules give the shingle its color and shield the asphalt from sunlight and weathering, while the asphalt binds everything together and repels water. The back side of the shingle usually has a sand-like material to prevent sticking in the bundle, and self-sealing adhesive strips of asphalt that activate from the sun’s heat to bond shingles together once installed.

Originally, asphalt shingles were made with organic felt (cellulose) mats, but modern shingles almost exclusively use fiberglass mats, which are lighter and more fire-resistant. This shift improved stability in moist or warm climates since fiberglass shingles absorb less moisture and deform less with age compared to the old “organic” shingles. Regardless of these manufacturing nuances, asphalt shingles today remain the overwhelmingly preferred choice for residential roofing – by some estimates, over 80% of homes in the United States have asphalt shingle roofs. Homeowners and builders favor them because they offer a compelling balance of affordability, ease of installation, and respectable longevity (typically 20 to 30 years for quality products). They are also relatively lightweight (avoiding the structural challenges of heavier roofs like tile or slate) and come in a wide array of styles and colors to suit different architectural tastes. In many neighborhoods, an asphalt shingle roof can mimic the look of more expensive materials (such as wood shakes or slate tiles in the case of architectural shingles) at a fraction of the cost while still providing reliable protection.

Installation Process and Roof Pitch

Installing an asphalt shingle roof is a methodical process that builds up a water-shedding system in layered components. The process begins with preparing the roof deck (usually plywood or OSB sheathing) and applying an underlayment over it. Underlayment is a continuous layer of material – traditionally asphalt-saturated felt paper, or more commonly today a synthetic polymer underlayment – that covers the entire deck before shingles are applied. Its purpose is to serve as a secondary moisture barrier and protect the roof deck from any wind-driven rain or ice that might work under the shingles. Starting at the eaves (roof edge), rolls or sheets of underlayment are laid “shingle fashion,” meaning each higher course overlaps the one below so that water runs off without penetrating . At the very bottom edge, a special ice and water shield (a self-adhering waterproof membrane) may be used in colder regions to further guard against ice dam leakage. Once underlayment is in place (with proper overlaps at seams and nails fastened flat), the roof is ready for shingles.

Roofers installing asphalt shingles over a felt underlayment. The black underlayment is visible covering the roof deck, and bundles of new shingles are staged for application. Shingles are installed starting from the eaves (lower edge) upward in overlapping rows. 

Asphalt shingles are installed in an overlapping pattern of horizontal rows (courses), starting from the roof’s lower edge and working upward. The first course typically begins with a starter strip shingle (a modified shingle or roll designed to provide a clean edge and seal along the eaves). Each subsequent shingle course is staggered relative to the one below it so that no butt joints align between adjacent rows – this prevents continuous seams that could admit water. The lower edge of each shingle row (except the first) overlaps the top of the row below, covering the nails and seams of the row beneath it. In this way, the roofing is layered like shingles on a wall or fish scales, ensuring water sheds downslope. Nails (usually four to six per shingle, depending on shingle type and wind zone) are driven near the top of each shingle, penetrating through that shingle and into the one below it, which secures both layers to the deck. As shingles are laid, care is taken to maintain straight rows and the correct exposure (the part of each shingle left uncovered by the one above, often about 5 inches for standard shingles). Valleys (where two roof planes meet at an interior angle) and penetrations (like chimneys or vents) require special techniques – typically metal flashing or woven/closed valley shingle techniques – to ensure these vulnerable areas are sealed. All these details are crucial so that the finished roof system is a continuous, overlapping mosaic that channels water off the roof with no opportunity to leak through.

Roof pitch (slope) plays an important role in how asphalt shingles are installed. Asphalt shingles are intended for roofs with sufficient slope to shed water; they cannot be used on a flat roof. The minimum recommended slope for standard shingle application is 2:12 (2 inches of rise for every 12 inches of run). On roofs with a slope between 2:12 and 4:12 (often called “low-slope” for shingles), building codes and manufacturer guidelines require extra precautions, usually a double layer of underlayment for added protection. This typically means applying an initial 19-inch strip of underlayment along the eaves, then overlapping full-width rolls above that by about 19 inches, resulting in two layers covering the whole roof deck in a shingle-lapped fashion. The reason for this is that water drains more slowly on shallower slopes, and wind-driven rain is more likely to push up under the shingle laps; having two layers of underlayment helps guard against leaks in these conditions. If shingles are installed on a roof sloped less than 2:12, manufacturers will not guarantee performance – such a low slope is generally incompatible with shingle roofing because water could puddle or flow backward beneath the laps. On the other extreme, very steep roofs (for example, slopes above 60° or near-vertical mansard sections) often require additional fasteners or adhesive to keep shingles secure. Shingle manufacturers specify that on slopes above a certain steepness (often 21:12 pitch, which is 60°), the shingles should be fastened with extra nails and sometimes roof cement spots to ensure they don’t slip before the self-seal strips bond. In all cases, following the manufacturer’s installation instructions for the appropriate roof pitch is critical for the roof’s integrity and warranty.

Special Ridge Cap Shingles

One specialized component of an asphalt shingle roof is the ridge cap shingles. The ridge (the horizontal line at the top of a sloped roof where two roof planes meet) and hips (the inclined intersection of two roof planes on an outside corner) are high-stress areas that need to be covered by shingles, but standard flat shingles don’t conform well to these sharp angles. Originally, roofing crews would take regular 3-tab shingles and cut them into thirds to create cap pieces, bending and nailing these over the ridge or hip. While this method covers the ridge, those cut shingles are not purpose-built for this application – they can be prone to cracking or blowing off, since a standard shingle is thinner and not reinforced for bending. Today, manufacturers produce dedicated hip and ridge cap shingles that are designed specifically for these areas. Ridge cap shingles are typically pre-cut into shorter lengths that are the right width for capping, and they often have a thicker or multi-layer construction and more flexible composition (sometimes modified with polymers like SBS rubber) so they can fold over the ridge without breaking. They are installed overlapping one another along the ridge, each piece covering the nails of the previous one, in the direction of prevailing wind (to resist uplift). These caps serve a critical role: they seal the top of the roof against water intrusion and wind, which is especially important because the ridge is a point of converging roof planes where wind-driven rain or snow could otherwise sneak in.

Ridge cap shingles also contribute to the finished look of the roof and are usually made to complement or exactly match the color and texture of the field shingles. Using the correct ridge caps has practical benefits beyond aesthetics. Manufacturers often specify that if standard shingles are cut and used as ridge caps, the warranty on those shingles may be reduced (since they are being used in a way not intended). For example, one manufacturer notes that using a 3-tab shingle as a ridge cap will only carry the warranty of a 3-tab (perhaps 20-25 years and lower wind rating), even if the field shingles are a higher-tier product. Purpose-made ridge cap shingles, on the other hand, are rated for the same longevity and wind performance as the rest of the roof system. In short, ridge cap shingles are thicker, tougher, and more resilient than ordinary shingles used in that role, ensuring the roof’s most exposed seams (ridges and hips) are well-protected against leaks and wind damage. Home inspectors will often check that appropriate ridge caps are installed, since using improvised caps can be a sign of corner-cutting in a roof installation.

Granule Loss and Shingle Aging

One of the most visible signs of an asphalt shingle roof’s aging is granule loss. The colored mineral granules on the surface of asphalt shingles are not just for appearance – they protect the underlying asphalt from ultraviolet sunlight and physical wear. Over time, it is normal for some of these granules to loosen and wash away. In fact, granule loss is an ongoing process that begins from the moment shingles are manufactured and installed. New shingles often shed a small amount of excess granules initially (the extra granules that didn’t get firmly embedded at the factory will come off in the first rains and collect in gutters). This early shedding is common and not a defect. After this initial phase, a well-performing roof will then lose granules very gradually over its lifespan due to weather exposure. Everyday factors like sunlight, temperature changes, rain, and wind all contribute to granule wear. Long-term uniform granule loss – where the granules wear away evenly over the shingle surfaces – is considered part of the natural aging of the roof and not a cause for alarm. In fact, insurance companies and roof experts typically do not count uniform granule erosion as “damage” if it occurs late in the shingle’s life, because it’s expected as the shingle nears the end of its service life.

However, the granules are crucial to the shingle’s function, so as they disappear, the shingle’s ability to perform is diminished. The granules shield the asphaltic material from UV rays; once a shingle has lost a lot of its granule coating, the exposed asphalt will accelerate in drying out and cracking. Therefore, significant granule loss is a sign of an aging (or damaged) roof. If you notice bare spots on shingles or large amounts of granules collecting in gutters and downspouts long before the roof is near its expected age, it could indicate a problem. Ideally, granule loss should be slow and uniform over decades. When a roof is, say, 20 years into a 25-year shingle, it’s common to see the granules have worn thin – the roof may start to look patchy or the color less vibrant. This is a signal that the roof is approaching the end of its useful life. On the other hand, premature or heavy granule loss (especially if localized to certain areas) can result from other causes such as hail impacts or a manufacturing defect, but those are separate issues beyond normal wear. The key point for homeowners is that some granule loss is expected over time – small sand-like granules in your gutters are normal as years go by – but once shingles become very smooth or bald in spots, they cannot effectively protect the roof and replacement should be planned. Regular roof and gutter inspections can help monitor granule accumulation so you have an idea when your shingles are deteriorating. In summary, granule loss is to asphalt shingles what tread wear is to tires: an inevitable gradual wear-down that, at a certain point, tells you the material is nearing the end of its serviceable life.

Single-Layer vs. Multiple Layer Shingle Roofs

When installing a new asphalt shingle roof, best practice is to completely remove the old roofing down to the deck before installing the new shingles. This results in a single layer of shingles on the roof. In some cases, however, homeowners or roofers choose to install a new layer of shingles directly over an existing layer – this is called a roof-over or overlay. While codes in many areas allow a second layer of asphalt shingles (typically two layers maximum are permitted; more than two is almost always prohibited), there are important reasons why an overlay is usually not a good idea. A single-layer shingle installation is superior in terms of longevity and performance, and is the only method that will fully maintain the manufacturer’s warranty in most cases.

Layering new shingles over old can seem tempting to save on tear-off labor and disposal costs in the short term, but it introduces several problems. First, multiple layers add extra weight to the roof structure. Asphalt shingles are relatively lightweight per square foot, but an additional layer can add 200-300 lbs per square (100 sq.ft.) to the roof. This added dead load can strain the roof framing, especially on older homes or in regions that experience heavy snow (where the roof might already be under stress in winter). Secondly, installing shingles over an existing uneven surface can prevent the new shingles from laying flat and sealing properly. The old shingles beneath may be curled, buckled, or just irregular enough that the new layer doesn’t get a perfectly flat substrate. This can lead to warping and reduced lifespan of the new shingles, as they are more likely to have air gaps or not adhere uniformly, making them more susceptible to wind damage or premature cracking. Moreover, the trapped space between layers can hold heat. An overlay roof tends to run hotter, as the heat of the sun has to penetrate two layers to dissipate. The extra heat can accelerate the aging of the shingles (asphalt breaks down faster at higher temperature), meaning the new layer could age more quickly than it would on its own deck.

Another major drawback is that adding a layer makes it difficult to inspect or address problems below. Any existing damage to the decking (like rot or hidden leaks) or issues with the old shingles (like trapped moisture) will be concealed under the new layer. This means underlying problems can fester and worsen, unnoticed until they possibly affect the interior of the home. By stripping off the old roof, the installer can examine the decking and replace any rotten wood, and also install proper underlayment and flashing anew. When an overlay is done, some of those steps are skipped or compromised. Additionally, most shingle manufacturers explicitly state that their product’s warranty will be void or diminished if installed over an existing roof. They generally warrant their shingles when applied to a proper deck with underlayment and so on; if you shingle over old shingles, you likely won’t get the full warranty coverage (for instance, a 30-year shingle might not carry that 30-year warranty on a re-roof installation). Finally, consider that the next time the roof is replaced, the tear-off will be more costly, as it will involve removing two layers of shingles (and potentially paying double disposal fees).

For all these reasons – structural load, shingle performance, thermal effects, inspectability, and warranty – a single-layer installation is strongly recommended. Roofing professionals often summarize it this way: “One good layer is better than two mediocre layers.” It’s usually worth the extra cost and effort to do a complete tear-off and install the new shingles on a clean deck, ensuring the best chance for the new roof to achieve its full lifespan. In fact, some local building codes and homeowners’ insurance policies require tear-off for a new roof, precisely to avoid those risks. In short, while an overlay might save money up front, it can lead to increased damage risk down the road, and it may ultimately cost more in reduced roof life and voided warranties.

The Importance of Flashing in Shingle Roofs

No discussion of asphalt shingle installation is complete without mentioning flashing – the pieces of metal (usually aluminum or galvanized steel) that waterproof the junctions and edges where shingles alone are insufficient. Flashings are critical components installed at roof penetrations and transitions: along edges (eave and rake drip edge), in valleys, around chimneys and skylights, at roof-to-wall intersections (step flashing), vent pipes (boot flashing), and anywhere the continuity of shingles is broken. The role of flashing is to divert water away from seams or joints and onto the shingle surface so it can drain off safely. For example, step flashings are used where a roof meets a vertical wall – each course of shingles has a corresponding piece of L-shaped metal tucked under the siding or counter-flashing on the wall and lapped over the shingle course, effectively creating a shingled metal barrier that prevents water running down the wall from getting under the shingles. Around a chimney, base flashings and counter-flashings interlock to keep the intersection leak-free. These metal flashings work in tandem with the shingles: the shingles cover most of the field of the roof, but the flashings fortify the weak points. If flashings are missing, damaged, or improperly installed, even the highest-quality shingles will not prevent leaks. That’s why in any roof installation or replacement, a professional roofer will typically replace or at least carefully inspect all flashings. They are relatively small pieces of the system, but they do outsized work in preventing water intrusion. As one technical bulletin puts it, metal flashings “are critical components of an asphalt shingle roofing system… installed at roof terminations, junctions, and penetrations to provide protection against leaks”. In a proper shingle roof job, flashings are integrated with the shingles and underlayment, following manufacturers’ guidelines and best practices. Homeowners should be aware that flashing is not an accessory; it’s a necessity. Whether you’re getting a new roof or repairing an existing one, insist that all the appropriate flashings (drip edges, valley metal, step flashings, vent flashings, etc.) are installed or renewed. This will ensure your asphalt shingle roof performs as intended, keeping your home dry and secure at all the vulnerable joints for many years.

By understanding the components of asphalt shingle roofing – from the materials that make up a shingle to the details of installation and maintenance – homeowners and buyers can better appreciate why asphalt shingles have earned their place as a trusted roofing solution. With proper installation (including quality underlayment, ridge caps, and flashing) and by avoiding shortcuts like overlaying shingles, an asphalt shingle roof can provide a home with durable and effective protection through decades of weather. It is this combination of practicality and performance that keeps asphalt shingles as the go-to choice for residential roofs.