Roofing half a roof plain / seperation between roof line


We have a duplex where the two units are divided on one roof plane. Only one party wants their roof replaced. The roof currently has 3-tab asphalt shingle. The party upgrading their shingle does not want 3-tab, they want to upgrade to a dimensional shingle. What is the proper way to divide the roof plane?
I have been told to cut a line down the shingles, insert a piece of L flashing then step flash the flashing on both sides, then cap the step flashing on both sides of the L flashing. I can provide pictures to make it easier.! What good does the step flashing do beyond what the piece of L flashing sealed to the deck do?

*** Revised, We used two pieces of 2 x 4 L flashing butted against each other and sealed in between them.

The inspector said it requires step flashing. I cannot find anywhere in any publication where this issue is addressed. If someone can point out to where this issue is addressed, I would greatly appreciate the advise. I do not see how step flashing on top of this L flashing then capped would help in any way. If you can explain how step flashing would work in this situation, please advise.


They also have what they call an expansion joint they put on buildings that are slightly seperated or they would shift differently. They usually use them on commercial buildings. The flashing in the picture is it one piece ? If it is to me it looks similar to a splash valley, if it is it will do as long as the roofers sealed under the shingles with roof cement on both sides


No matter what you do, you do not want that downspout on the upper wall dumping directly over the seam as it currently does, that is just asking for problems.