I'm sorry, I worded the question badly.
Is it O.K. if I not worry about a little material left by the, probably 30 year old, underlayment (which seems to have bonded to the concrete in spots) even after strenously scraping, as long as I make it flat?
I've been told that roofing shingles are good for that. ???
If I can figure out how to upload pictures, I will.
Thanks for replying