Hail vs. Wear: Why Missing Granules Don't Prove a Storm

Granules in the gutter

After a storm, plenty of homeowners take their first close look at the roof in years. They find granules in the gutters and spot dark patches where the granules are gone from their roof shingles. The natural question follows: was it the storm?

It's a fair question, and it deserves a careful answer. Those granules are the shingle's sunscreen, protecting the underlying asphalt from the weathering effects of the sun's ultraviolet (UV) rays. How, where, and why they came loose is a separate question.

Puzzle pieces

By itself, a gutter full of granules tells you very little. Shingles shed granules for many reasons, and many of them have nothing to do with hail. New shingles often shed loose "hitchhiker" granules early in their life. Older shingles tend to shed more with every hard rain.

Loose granules are also not the same thing as damage. In itself, missing granules do not necessarily mean a shingle sheds water any worse or will wear out sooner.

That's why we treat a roof like a puzzle. No single piece decides it. A blemish is one piece. The slope it sits on is another. So are its size, its shape, how many there are, and the weather data for that date. The answer comes from how the pieces fit together.

Hail has a direction

Hail usually doesn't fall straight down. Research on severe hailstorms shows hailstones travel at an angle, pushed by the storm's movement and the winds inside it. Those winds high up are typically much stronger than the wind you feel on the ground. So a calm afternoon at your house doesn't mean the hail came straight down.

That matters, because a roof has slopes facing different ways. Slopes that face the oncoming storm tend to take square, solid hits. Slopes that face away tend to get glancing blows with less energy.

On the slopes that face the storm, hail marks tend to be consistently random: scattered with no pattern, but spread across the whole slope. The swath, the band of falling hail, usually covers a full slope, but it can cover only part of one.

So we compare the slopes. When every slope looks similar, including slopes that would have faced away, that's a reason to look harder at other causes.

The usual suspects

Hail is one name on a long list. Here are several others that can leave a shingle looking worn or spotted, and what each one typically looks like.

Age and weather

Sun, heat, and rain break shingles down a little every year. The granules loosen and wash off slowly, and the gutters collect them after every hard rain. This wear tends to spread broadly across a slope. It's often heaviest on the slopes that get the most sun.

Foot traffic

Roofers, satellite installers, inspectors, and chimney sweeps all walk on roofs. Their shoes scuff granules off as they go. These marks tend to follow a path: near the ladder spot, around vents and chimneys, and along the ridge. They usually look scraped or irregular, rather than round.

Heat blisters

Blisters form when gases or moisture inside the asphalt are heated, expand, and push the surface up. They tend to show up on newer shingles over a hot, poorly vented attic. When they pop, they leave small round pits of bare asphalt, sometimes deep enough to show the mat at the shingle's core. Blisters tend to appear in clusters and on the hottest slopes, and they are often discerned by their well-defined edges. Round and dark does not automatically mean hail.

Ventilation problems

A typical attic needs air moving through it, in at the eaves and out near the ridge. The International Residential Code sets a minimum prescription of 1 square foot of open vent area for every 150 square feet of attic. It allows 1 for every 300 when the vents are balanced between high and low and its other conditions are met. The asphalt roofing manufacturers' trade group notes that excess roof heat speeds up the aging of roofing materials. Poor attic airflow causes built-up heat that cooks the shingles from underneath. That can speed up wear and blistering. It tends to show up across large areas of the roof rather than in spots. It's often most apparent on the south- and west-facing slopes that face the sun during the hottest part of the day.

Maintenance and the yard

Tree limbs rubbing a roof wear the granules away in patches under the branches. Leaves and debris that sit on a roof hold moisture and wear the surface beneath them. Pressure washing can strip granules if not done correctly. Moss and lichens tend to intertwine with the granules and cause their detachment as the moss/lichen die or are otherwise removed. Animals tend to tear, scrape, or otherwise abrade the shingles as they forage for food, and bird droppings can break down the asphalt surface and cause granules to dislodge.

The pattern that fits

No single blemish settles anything. A dark spot on a shingle is a question, not an answer.

What we look for is a pattern that fits:

Marks heavier on the slopes that faced the storm.
Marks in a consistent range of sizes, in line with the hail on record, and consistently random across the roof.
Gutters, vents, and other impressionable surfaces around the house telling the same story.
A storm with hail on a known date.

Size matters for another reason. Laboratory testing by Marshall, Herzog, Morrison, and Smith found that the hail size needed to damage a shingle depends on the shingle's type and age. In their field observations, damage typically began around 1 inch for three-tab shingles and 1.25 inches for thicker dimensional shingles. Aged three-tab shingles were the most vulnerable.

What a Hail Report Can and Can't Tell You

References

This article is general education. It is not an engineering opinion about any specific property. This is not a meteorological opinion, a site observation, or an engineering conclusion.

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