In my house, some parts of the roof appears to be sagging. I have brought in a few roofers to look into it and they seem to think that it is because some of the panels, which are in good shape, do not have adequate support and/or bad nailing job. And all of them suggested that I change the entire roof, which would cost around $15K to $18K. I was wondering if there is any inexpensive solution to this problem. Also, what kind of skilled professionals would be appropriate for this job? Please help.