Bringing unused chimney down below roof line?
For what it''s worth, I did exactly this as part of the process of replacing my shingles (which were nearing end-of-life). That had the advantage of letting the roofers completely open the patient, remove the top courses until it was safely below roof level, install a nice strong patch over the hole (in fact they were putting plywood over the old king-pine boards anyway, so this was almost automatic), then shingle right across where the chimney had been.

At some point I'll take a hammer to what's left and recover the space the chimney occupies. I know the ground-floor main beam no longer rests on the chimney (that had crumbled and later been fixed with columns); we'll have to see whether it supports anything higher up. Hope not...

(This wasn't a gut rehab, so there really wasn't much advantage to removing the whole thing in one swell foop.)