is the easiest and cheapest route to go. Get a 2x wolmanized material and lag screw into each wall stud all the way down the ledger. Then determine the pitch of the roof and make the cuts to achieve that pitch, then attach those roof joists to the ledger with Simpson Ties. Will work and will last and allot better than opening up the roof and tieing in that way. Although that way is the most professional looking approach.