Rocks on roof

Have you ever wondered why rocks are placed on a large flat roof? I learned this earlier this week in my Structures class. I thought they were there to help drain water or something. Their purpose is actually to keep the tar paper from blowing off. With the wind blowing over the roof, the wind movement creates a negative pressure, making the entire roof want to rise. The tar paper isn't hooked as well to the structure as the huge roof, so a simple, cheap, and effective solution is to add rocks to act as ballast.

And I think the simple, cheap, and effective solutions are always the best.