Not always. I have worked on many homes that have Celotex fiberboard sheathing and asbestos shakes. I think the paint bond is all that holds them up.:blink:Fanfold and be sure to nail across the bottom first and keep working up, this keeps any broken pieces in place. Since asbestos is always installed to a solid surface hitting studs is not an issue. Most always you will find ship lapped 1x dimension behind it.