I have a porch that runs 3/4 the length of my house. It was added to the original structure with the roof resting at a reduced angle on the original roof. I had to replace the center supporting beam on the porch which was rotting. I have noticed for the last two years that the center of the roof over the porch was sagging. How can I repair this without doing a complete roof repair? I am trying to sell my house and cannot afford a complete roof job. The roof has not had any recorded leaks.