You cut a patch that will cover the tear and extend around it
You paint the patch, and the back of the old felt with one of those black rubbery roofing paints (?Isoflex, is it?) then you press the tacky patch onto the tacky old felt and hold it while it sets.
I would be supicious that water might get into the gap between the patch and the old felt of you hadn't glued the tear enough to seal it.
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