On Feb 13, 9:07 am, Jeff B wrote:
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?

You are in a tough situation. You have several factors adding up to a
practical impossibility.

Newer drywall panels intended for ceiling use are more sag resistant -
the stuff made 30 years ago is more prone to sagging without any
additional load being placed upon it.

You do have a load - that 18" of blown insulation can easily exceed
the sag resistant drywall rating of 1.3 PSF. Blown insulation can
vary a great deal, but 1 to 2 PCF is not unusual. The conversion does
not work in your favor.

USG specifies ceiling screws to be on 12" centers, and that's with the
newer sag resistant stuff. The older spec was screws on 8" centers.
Again, that's based on a 1.3 PSF maximum surplus loading.

The insulation that has worked its way between the drywall and the
ceiling framing isn't going anywhere without help. Buildings,
particularly wood framed buildings, move with changes in temperature
and humidity and with loads applied from above, such as your crawling
around in the attic. If you screw the panel back up as best you can
and there's a gap, more insulation will squeeze its way into the gap
as the building moves. This will be slow to happen, but it will
happen and it will create a fair amount of force and the screw heads
will start to pull through the face paper.

I see zero chance of a long term solution if you don't remove the
insulation from the area in which you are working. You could start at
one end, push the loose insulation to the far end, vacuum out the
crevice under the joist, and then jack up that area and screw the
board up tight. Then you'd have to move the insulation back into the
area you worked and start on the adjacent section.

In your situation you will definitely want the screws more closely
spaced - say 8" centers. Adhesive or expanding foam would help seal
the drywall/joist gap and keep more insulation from working its way
between the two. I would also consider filling the joist bay with
loose fill insulation, then stapling Tyvek on top of the joists so
it's fairly taught, then putting the rest of the loose insulation on
top of the Tyvek. That would be one way, off the top of my head, of
keeping the insulation and taking some of the load off of the drywall
ceiling.

Another option - and it's really easy for me to spend your money for
you - is to remove all of the loose fill insulation and use spray foam
over the entire drywall ceiling. If you put six or more inches in
place in place it would bond to the back of the drywall ceiling and to
the joists and make the whole system more of a diaphragm/beam.

Basically, this is the situation: rock you hard place. Sorry.

R