I have a unheated garage with a drywall ceiling on 24" centers... over the years the drywall is buckling between joists... can this be remedied by placing furring strips perpendicular to the joists... and if so... what size furring strips... THANX... Chevtri

I doubt that would work, the drywall is most likely ruined at this point. I would guess you have a moisture problem in the attic space above the drywall. Removing the drywall, adding better ventilation to your attic and replacing the insulation and drywall with a vapor barrier on the warm side of the insulation is probably the way to go. Let's hear what others think.

How old is the house and has any drywall work been done to the garage ceiling that you know of? Also, what state are you located in? Is the joint tape showing anywhere? How about around the perimeter of the ceiling? Usually when the sheetrock begins to sag, you'll see tape showing somewhere. Popcorn texture or knockdown? (I'll guess popcorn.)

Moisture and age are the most common reasons for sagging drywall between joists as gravity finally wins over longevity. This normally happens in garages and outside porch ceilings (and inside ceilings with water damage). Furring strips will be a bandage AND a big hassle to install properly. Plus, I doubt they'll hold the weight of the drywall that already has a new "form" and may be quite brittle from age. I've encountered many similar situations and advised the homeowner to demo and replace the entire ceiling. Amricca is correct. It appears your garage ceiling has lived its life and needs to be replaced, completely. I'm talkin' a thousand dollars, minimum. Hope this helps.

Sounds like an old DIY project gone bad. The drywall aws too thin for 24" centers and sagged from its own weight and humidity. Its a lost cause, you won't get a flat surface back even with furring stripsIf you tried 24" 2x4 blocking between the joists you might get a better surface but that would be hundreds of blocks. I'd tear it down and go back with 5/8" dry wall of better yet, no ceiling at all if this is just a garage.