What Happens to Asphalt Shingles as They Age?

The asphalt itself oxidizes

The asphalt layer in a shingle is a petroleum-based material blended with fillers and reinforced with a fiberglass mat. When it's manufactured, it contains oils that keep it pliable. Sunlight, heat, and oxygen react with that asphalt over years of exposure in a process called oxidation, similar to what happens to a rubber gasket or an old garden hose left outside. Oxidation is not damage from a defect. It's a normal chemical reaction that every asphalt shingle roof goes through, just at different speeds depending on climate and sun exposure.

In Maine and New Hampshire, that process is shaped by a wide seasonal temperature swing. Shingles bake in summer sun, then go through repeated freeze-thaw cycles all winter. That expansion and contraction accelerates the same oxidation process that would happen more slowly in a milder, more stable climate.

Loss of oils and flexibility

As oxidation progresses, the asphalt gradually loses the lightweight oils that make it flexible. A new shingle can flex slightly with temperature changes and minor deck movement without cracking. An older, oxidized shingle has lost much of that give. It becomes stiffer, and stiff asphalt is asphalt that cracks instead of bending.

This is one of the main reasons roofers avoid walking heavily on an older roof, especially on a cold morning. The same shingle that would compress harmlessly when it was five years old can crack under foot traffic once it's lost most of its flexibility.

Granule loss

The ceramic-coated granules embedded in the shingle surface serve two jobs: they shield the asphalt from direct UV exposure, and they give the shingle its fire resistance and color. Granules are bonded to the shingle using asphalt as an adhesive. As that asphalt oxidizes and loses its bonding oils, the grip on the granules weakens and they shed more easily, especially during wind, hail, or heavy rain.

Some granule loss is normal, particularly right after installation when loose manufacturing granules wash off. But steady, ongoing granule loss on an older roof is different. It's a sign that the asphalt underneath is losing its protective coating, which speeds up further oxidation in a self-reinforcing cycle.

Cracking and curling

Cracking usually shows up first at the tabs, where the shingle is thinnest and gets the most direct sun and thermal movement. Hairline cracks widen over repeated freeze-thaw cycles as water gets into them and expands when it freezes.

Curling happens when the shingle edges lift away from the roof deck, either curling upward at the corners or across the whole tab. It's typically driven by a combination of oxidized, stiffened asphalt and, in some cases, poor attic ventilation trapping heat and moisture against the underside of the roof deck. Curled shingles are more vulnerable to wind uplift and water intrusion at the edges.

UV exposure and color fading

UV light breaks down the asphalt's surface chemistry and fades pigment in the granules. This is why a south- or west-facing slope often looks visibly more worn and faded than a north-facing slope on the same house. The sun-facing side is doing more work and aging faster, even though both slopes were installed the same day with the same materials.

Why this matters for maintenance decisions

None of these changes mean a roof needs to be replaced the moment they appear. Early- to mid-stage oxidation, some granule loss, and light surface fading are common on roofs that still have years of useful service left, especially if the underlying shingles are still mechanically sound and properly sealed. That's the window where routine maintenance and, for eligible roofs, rejuvenation treatments are worth evaluating.

Advanced cracking, widespread curling, exposed fiberglass mat, or shingles that crumble when handled are a different story. At that stage the asphalt has lost too much of its structural integrity for surface treatment to help, and replacement becomes the realistic path.