To tie into existing roof shingles use a cut valley. Install trusses to meet the eave of the house and nail 2 valley boards into place, securing with roofing cement. Frame and sheathe the addition roof, and snap a line on the existing shingles 2 inches above the valley.

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.