Most manufacturters do not warrant their products when installed over old roofing, they require a sound smooth surface.

But roof-overs are routinely done in some areas of the country. The original must be secure and not cupping or curlingor so old that it has let leaks weaken the sheathing that holds th enails.

usually a tear-off re-roof is more expensive due to laboutr for removal and the dump fees for debris

Excellence is its own reward!

I am doing mine over, its 6000 SQFT of shingles. I contemplated a tear off and redo, but the cost at this time is prohibitive, plus I have to do it myself. The existing shigles are OK except a few that broke from wind damage, and the main reason is hail damage, the granules are disappearing. I am also removing an old chimney where the furnace and water heater used to exit, along with a few vents that are no longer in use. Aside from the broken ones, which I understand need to be patched so everthing is even, is there anything else I shoul ddo before reshingling? Should I apply a layer of felt first oover the existing shingles?

plan new flashings at penetrations, walls, and valleys

Excellence is its own reward!

what about the felt paper though, should I or does it really matter?

your existing roof shingles function as an underlayment

Excellence is its own reward!