While patching my kitchen ceiling, I cut the hole larger to get to the joists and install my drywall "normally". On one of my edges the old drywall keeps crumbling away. My plan is to make the hole bigger still and toenail in some furring (a board to screw the patch to). However, every time I breathe near this edge it seems to crumble more. The original drywall is probably 30 years old. How do I keep this patch from growing?
The edge on the left is what keeps crumbling away.