I hate to say it, but if you have hair snapping at the scalp, trying to do your roots and not get a lot more breakage at that line is gojng to be impossible. Possibly a really terrible result, it’s one thing to do your roots with hair that’s breaking further down the hair, but right at the area the bleach is going to be is asking for trouble. I’m afraid my advice is to never bleach that part of the hair that’s snapping unless your ok with getting it cut off to that line. I wouldn’t normally sound so dramatic, but it’s a very likely outcome I’m afraid.