After framing and sheathing the new addition s roof snap a line over the existing roof shingles.
How to tie existing roof shingles to shingles.
You want to be about two inches above the very center of the valley itself.
I will be creating a new valley on one side and tieing into the existing shingles on the other.
Now it is time to use a cut valley in order to tie the new shingles in with the old roofing system.
The easiest way to tie in the new shingles with the old is with a cut valley see four ways to shingle a valley.
After the addition roof is framed and sheathed snap a line on the existing roof shingles 2 in.
Up from the valley center and cut through the shingles and underlayment.
Other than that you would just follow the pattern that is already there.
To tie into the side of an existing roof put a new roof truss on the walls of the addition brace it upright then snap chalklines on the shingles to mark the center line of the new roof valleys where old and new roofs will connect.