Sagging Drywalll In Ceiling

[RossAnabel]

I have just purchased a 70s Ranch style house with saggy drywall in the LR and Dining Area. Rafter construction, joists are 2x6x12', 16 OC, 10" cellulose insulation.
I've read lots about pillowing, sagging between 24 OC joists, but this sagging runs perpendicular to the joists and 4' OC. My theory is that original construction was glued board with screws only at the edges, the glue failing at some point during the past 40 years.
Is the anyone who has experience with this or suggestions?

 

[nealtw]

Welcome to the site.
In the 70s there were still nailing drywall up and you can be sure the centers were nailed. They often put 2 nails 2" apart to prevent pull thru.
I suspect you have pull thru. You could prop it up with a 2x4 on flat and an upright under it but if insulation has worked into the space between the sheet and the joist, well

 

[RossAnabel]

Thanks for the info! I figure that there is likely insulation in the gap. I don't know how much it will compress, but one way or another I need to make sure it doesn't come crashing down some day.

 

[nealtw]

I would wedge it back up and see if it will go up and then add some screws.
It will be a lot of work if you have to clear the gap.

 

[bud16415]

I have made a tool to help doing something like this and I will see if I can describe it.

It is a strip of wood, something tough hardwood works. 1X3 x 48 to 72 inches long depending on how you are going to use it. down the length of it I drilled holes that are a slip fit with deck screws 4 or 5 holes. In between those holes I drilled larger holes that will clear the head of a drywall screw.

I find the joists snap a chalk line and then use a few of these boards to draw the drywall up tight by running the deck screws into the joist and the board spreads out the force once it is drawn up I run drywall screws thru the larger holes to hold it up. If you try and just do one screw at a time they will pull thru before the pull it tight, but when you get a row or even 2 rows in they will hold when you take the boards down. Then what I do is run screws in the holes the deck screws were in.

I have found this method pulls up with a lot more force than you can get with a floor prop.

 

[nealtw]

[ame]https://www.youtube.com/watch?v=FkVA9ZoI11Q[/ame]

 

[nealtw]

If it doesn't pull up tight and you still want to screw it up you may want to use a longer drywall screw, like 1 5/8"

 

[RossAnabel]

Thanks for the suggestions. What I had envisioned was using 1x4's perpendicular to the joists every 16", drawing them up with 1/4 x 3 lag screws and washers.

 

[joecaption]

It's been so long since I've personaly used one, and not sure if it would work in your case, but think of this idea.
Use a drywall lift to do the lifting.
That way you could lift and hold the whole sheet while doing the screwing.