There could be several reasons behind this, and these are the most common ones:
The loss of granules is an entirely normal process in the aging and weathering of all asphalt shingles. The most common causes behind the granule loss are these:
Nevertheless, most of the shingles are made with excess granules on their surfaces, as the granule loss is something to be expected.
Dimensional shingles (30 years)
Dimensional shingles (40 years)
Generally speaking, the chemical melting compounds won’t reduce the expected life span of the asphalt shingles. However, staining will likely occur until the residue is washed away. Magnesium or calcium chloride pellets aren’t as harsh as the sodium chloride ones are, and won’t make as many stains.
The roof should be replaced if you’re starting to have serious problems with the leaking, or if you notice any other trouble signs. Remember that the western and southern exposures usually age a lot quicker, and the same could be said for the lower pitched roof areas.