It is probably feasible to straighten up the roof line a bit when the metal roof is installed.
Roof line not straight.
At other times the gutters may need to stay where they are so gravity can do its work.
Screw a 2x4 about 12 long with a small nail sticking out on the 1 1 2 side.
Repeat this on both sides of the roof.
After your floor is straight it is time to concentrate on the next floor s ceiling.
Every 5 inches make a new mark until you get to the rop of the roof.
Use your chalk and tape measure to mark about 12 inches up on the roof for the first set of shingles.
It might be something basic you can handle yourself but.
Roof lines should be arrow straight.
This will be the bottom of your board.
It also may just be due to structural age or possibly a defect in the original structure.
Screw it into the wall and repeat this on the opposite wall.
Sometimes an uneven roof is simply a sign of minor errors or aberrations in construction.
This droopiness could be caused by moisture and ventilation problems.
Run your string line from the nails and measure up from the string.
The end of the line is 4 inches from the lower edge of the roof which is standard exposure.
Ask an assistant to hold the end of a chalk line on the first shingle at the longest edge of the roof.
Sometimes you can adjust gutters and vents for a more uniform appearance.
Use the chalk line to snap a straight line vertically every 6 inches.