I have a skylight on a shingles roof. I don't know much about the skylight as it was installed by a previous owner, but every year, at the beginning of the rainy season, the skylight leaks. And every year, when that happens, I go on the roof to add some sealant, which usually gets me through the rainy season.
In the first picture below (click to enlarge), you can see the skylight, the shingles, and the sealant. The second picture zooms on the sealant, which starts cracking. Finally, the 3rd picture shows the sealant I used. I am thinking that the sealant cracks during the summer, explaining the leak when we get our first good rain of the year.
What would you recommend to fix this problem in a more permanent way?