Roof valley:
When placing a new valley simply bend 2-inch (50 mm) galv nails over on the top edges of the very outside of the valley coming directly out of the valley board timber or out of the valley purlin timbers. Thus keeping the entire valley nail hole free. Bend the top most section of the valley over at the top to shape two 45-degree angles as a water-proofing fold. Seal this very inside corner with silicone sealant. And give the valley a turn down into the spouting. Now make marks 2-inches (50 mm) outward from the center of the valley and flick a chalk line between them from top to bottom of the valley. This is the line where the roof sheets will finish. With these chalk lines you will be able to measure and mark out chalk lines on the ground based sheets laid out and ready lapped to fill in these valley sections of the roof. Remember measure twice and cut once.
Note: A trade secret or simply common sense demands making a neat drawing of the roof to be so formed before you make the transfer of these measurements onto the ground based sheets and onwards to cutting them up ready to be fixed into place.
If working with corrugated steel roof use a shifting spanner to turn up the ends where they go up under the ridge covering. Likewise for roof profiles with large pan widths in-between ribs. If you cannot borrow a turn-up tool have a simple one made at a local engineering shop.