Changing Pitch over roof. tying into house rafters?
If I understand your question, you want to tie the porch rafters to the existing house rafters? You don't have a snow load to be concerned about, but anywhere a roof pitch changes as in your case from 6/12 to 3/12 there is the potiental for a leak, at the pitch transitition I would use weather guard, if the main house has shingles I would remove several courses put down the weather guard from there to the edge of your porch, then reshingle. As far as tieing in your prch rafters, remove the facial and soffit boards exposing the house rafter tails, fit the porch rafters so that they rest on the house plate, (where your house rafters set) and along side each house rafter, nail porch rafters to plate and to the side of the house rafters. You might want to check your building codes, areas of high wind may have different requirements.