After the addition roof is framed and sheathed snap a line on the existing roof shingles 2 in.
How to tie existing roof shingles to shingles.
The easiest way to tie in the new shingles with the old is with a cut valley see four ways to shingle a valley.
Up from the valley center and cut through the shingles and underlayment.
After framing and sheathing the new addition s roof snap a line over the existing roof shingles.
Other than that you would just follow the pattern that is already there.
Now it is time to use a cut valley in order to tie the new shingles in with the old roofing system.
I will be creating a new valley on one side and tieing into the existing shingles on the other.
I am about to put an addition on my house and the only thing that is keeping me from doing the shingles myself is the tie in to the existing.
It really just depends on how the existing roof shingles ended.