Roof Construction exposed

Attacked the roof with a cutting wheel to expose how its put together 

Rail separating

Roof attaches to strip of steel that is then spot welded
 to the 3rd layer that forms the lower return on the drip rail

3rd layer is also rotted out and will need removing

End of drip rail

This may be as far back as i need to cut, it appears to be only
light surface rust along the back edge of the roof - here's hoping!

The plan of attack is to fabricate the roof and 2nd layer while the 3rd layer is still in place, once that fits remove and replace the 3rd layer - this wont be a quick fix