I'm rebuilding my patio cover that has a lattice roof. I want to put on sheating and rolled roofing to tie into the existing flat roof. Right now all the lumber used is 2" x 8" and I need to replace it with 2" x 6" lumber so the sheating will be level with the existing roof. It's 10' from the roof to the posts. I know I'm going undersize with 2" x 6" but to make it work, I think I need too. However, my biggest question: I have 3 4" x 4" posts holding up the structure 83" apart on center. What size header do I need.

Thanks, Dave

That question will be determined when you pull a building permit. This type of job requires one in almost every city in the USA. The building inspector will review your plans and help determine what you need. We have no idea where you live or which code your city follows. Do you have snow in the winter, snow loads increase header sizing

Bob is right. When u file for permit the building inspector will determine if lumber is of correct size. This will also include the size of rafters.

Chuck

Also span between joists and thickness of decking.
You can find charts online for size of deck/flat roof, post sizes, spans of posts and joists, and size of girders/beams.
Your property insurance will go up when you have a real roof over a deck or patio. At least where I live.

I mean property TAX