Most roofs in the Midwest are covered with asphalt shingles, as they are relatively inexpensive, easy to install, and most importantly – durable. Asphalt shingles typically consist of a fiber glass mat that serves as the structural base of the shingle, asphalt coating that serves as a water proofing agent, and a granule top coat. The granule top coat, which consists of crushed rock particles pressed into the asphalt, blocks UV rays from breaking down the asphalt and thus, extends the life of your shingles. In addition, these granules are what add the color to your shingles and provide fire resistance.
The granules on your shingles are exposed to the elements 100% of the time, including times of high wind and heavy rain. Because of this, your shingles will experience granule loss, as there is no way to adhere every granule to the shingle and insure that it stays on when exposed to weather. Some granule loss is normal and does not mean you need to replace your roof.
The most determinantal elements to the granules on your roof include hail storms and extremely heavy rain. Your roof can sustain quite the beating though – a typical roof contains around 2,580 pounds of granules and a typical hail storm causes your roof to lose about 30 pounds of granules. It would take quite a few years to lose all the granules at this rate. Moreover, shingles are manufactured to have 40% more granules than they need to handle the inevitable granule loss that will occur, and 12-15% of granules are not securely attached to the shingle. This design is on purpose to help your shingles weather. As such, if you have a new roof and there are heavy rains or hail, you may notice a lot more granules in your gutters than someone with an older roof, simply because these excess granules are sloughed off.
Long story short, if you have a lot of granules in your gutters, do not worry too much. This is just your shingle weathering appropriately and does not automatically mean that you have hail damage. If you are concerned, have a trusted roofing professional inspect your roof for impact points, crushed granules and exposed substrate, all of which are actual indicators of hail damage.