 dry rot, deteriorate, and then it'll begin to leak around that screw. But it'll leak just enough to soften the wood up there where the screw's at. Maybe not enough that you'd ever notice it leaking in your home, but it softens that wood up just enough to where the next big windstorm comes and your metal roof is laying over in your neighbor's yard. Yeah, we've all seen that where it peeled up and, yeah, it's all in a mess.