In the middle of my kitchen reno and while working in my attic, I
discovered that 1 or 2 full sheets of drywall in my living room are
sagging badly. Apparently the construction adhesive used 30 yrs ago
simply dried up and lost its hold. Since they used nails not screws
in the 1970s to attach the drywall to the rafters, there wasn't much
to keep it up. At the lowest spot, the gap between the drywall and
rafter was about 3/4"!! From inside the attic, I could slide 1/2 my
hand into the gap! I figured the whole thing would collapse from its
own weight any minute. I raced to HD and propped it up with a bunch
of 2x4s. My living room looks like a circus tent.

Getting it supported was step 1. I now need to figure out step
2...repair. Replacing all of that ceiling drywall is NOT the
preferred option. My attic has about 18" of loose, powdery, blown-in
insulation. Moving that stuff around is a nasty job. I don't believe
the drywall is cracked anywhere. It simply sagged as a full sheet (or
2 sheets). I'm hoping that getting it flush to the rafters with the
2x4s and then using LOTS of drywall screws will keep it up there for
another 20 years. With the loose insulation, getting adhesive in the
gap before screwing up will be difficult if not impossible.

Anyone have experience dealing with this problem?

--Jeff