Agreed, the porch roof will start to hang down too low. I.E. if you have a 9/12 pitch on the house, and the headroom from your porch to the end of the roof tail is 10'(extremely high) and your porch extends 4' from the house, you'll have a 3' drop from where the roof was to where it would be at the end of the porch, which would be 7'. Most houses are at 8' though, then you'd have to duck to make it out from under the roof over your porch. If you don't want to have 2 different angles, you can overframe your roof right now, and put new rafters in all the way to the peak, that will extend out over the porch, and can end up at the same height as the rest of the roof. This is obviously more expensive, but probably looks better than any other method.