in plain english. no, no, no. put up 4x4's, band w/ treated 2x6, if you're using tin & you have no snow load, run your rafters the length of the porch rather than the convental way, when you put up your band, be sure to have put a drop to it for water run of. five important things.  12" galvinized flashing under the dw roof shingles & over the porch roof, some kind of insulation under tin to prevent moisture dripping on you. use good tin.treated lumber. if on the other hand it is going to be a composition roof, forget everything here.