I have read through numerous posts about using excel flashing around the chimney,so if it slips underneath roof at the top it means I have to cut open my roof more to slip it under.guy that installed my roof butchered it terribly and because my chimney is 30 years old I'm changing it out.i have had a rubber boot on there for 10 years with no leaks but from discussions on this forum excel flashing is the way to go.my concern is what if the flashing edge ends up on a roofing rib then what.just not sure or confident to go about this way.can I get a little guidance. Thanks.a video would surely help us