Factors Affecting Hail Damage to Roofs in Minnesota

The damaging power of hail on on asphalt shingles

Hail damage to shingles of home in mn
Hail damage to a roof in MN.

Minnesota summers bring storms! While it can be fun for some to watch the lightening and feel the thunder from big cumulonimbus clouds clouds, hail often accompanies these natural displays energy. Hail damage to roofs affects thousands of homes each year all over Minnesota. Hail damage can affect all types of roofing materials and is particularly damaging to asphalt shingles.

hailstones from storm in minnesota
Hail stones can get as big as a golf ball.

Hail Physics

Several components affect how much damaging force hail has. Hail size, weight, density, and speed are all factors that combine to deliver a certain amount of potential energy. We will discuss in greater detail these factors.

The size of hail may be as small as a pebble to larger than a baseball. It doesn’t take a degree in physics to know that the baseball sized hail is going to do more damage than the pea sized hail stone, but if you have to hail stones of equal diameter, the other factors come into place. Shape, for example, can have an impact on the impact force. While round is the most common shape, hail can also develop into an oval or elongated shape, which can achieve a greater speed, will have greater weight per diameter, and will usually hit with more force and result in more damage.

hail impact force increases with velocity

Density plays a huge role in the energy transmission potential of a hailstone. Counterintuitively, in colder air temperature conditions the hailstone will develop softer and usually smaller compared to warm weather hail storms.

diagram of hail force

In the Clouds

Hail forms in thunderstorm clouds that have strong updrafts, often over 100 mph, high liquid water content, and where a good portion of the cloud layer is below freezing. Hail begins as water droplets, like rain in any large cloud. As the droplets rise and the temperature goes below freezing, they freeze on contact with other water droplets.

green hail producing cloud in Minnesota
Green hail producing cloud in Minnesota

If you looked at a cross-section of a large hailstone it looks like an onion. Because the stone increases in size as it moves vertically through a cloud, the higher altitude collisions yield a colder denser layer. So, the density of these stone scan vary from 0.5 to 0.9 grams per cubic centimeter. As a reference point, water is a 1 gram per cubic centimeter in liquid form, and as we know, ice is less dense than liquid, at about 92% the density of water. Air bubbles trapped in the water yield a less dense hailstone that has less impact force.

Example of layers in a hailstone
Example of layers in a hailstone

The hailstone will keep rising in the thunderstorm until its mass can no longer be supported by the updraft. This may take at a half-hour depending upon on the force of the updrafts and the top vertical height of the clouds, whose tops may be greater that usually greater than 6 miles high!

a hail shaft from storm cloud in minnesota
A hail shaft from storm cloud in Minnesota

Eventually the weight of the stone is greater than the force of the updrafts and it falls out of the cloud, gaining speed until terminal velocity is reached. The terminal velocity of a hailstone is the speed of falling (gravity vs wind resistance) plus the speed of the wind. In a windless condition a hail stone will hit at, say, 72mph. Add in a forty mile per hour gust and that hailstone is going over 80mph. The impact force with the wind vs no wind is 30% greater.

Then there is the impact angle of the hail. If a stone is falling straight down on an angled roof, the impact is less than if a falling onto a flat roof. The same is true if a hailstone falls sideways into an angled roof, resulting in a direct perpendicular impact.

diagram of hail impact angle on a roof

Roof age has an effect too. An older roof tends to be more brittle. The shingles will crack or puncture more easily than a newer roof. Furthermore, a south facing roof will sustain more damage because the shingles will tend to be more weathered from the heat and UV damage of the sun. And speaking of temperature, a roof that is colder rather than warmer will sustain more damage, because the cold will make the shingles more brittle.

Exterior Siding Too

Exterior siding can also get destroyed by hail. If your home looks like this, we can help!

hail damage to exterior siding of home in minnesota
Hail damage to the exterior siding of a home in MN.

Wrapping up

As you can see, there are a lot of factors that govern how much damage a residential roof will receive in a hail storm. The size of hail, the density of the hail, the velocity and angle angle of impact, age of materials, type of roofing materials, and temperature all combine into a cocktail of physics. Math equations aside, if your roof is victim in the next hail storm to sweep through minnesota, call All Sons Exteriors for a free hail damage inspection.  952-469-5221