Garage Ceiling Sheetrock Wavy/\/\/\/\/\/\/\/

I actually started my first reply this morning before all other posts & your replies.....I should checked for posts before I continued with my tome.
wrt 5/8 vs 1/2 and 16" o/c vs 24" oc..........
1/2" is fine for 16" o/c, however, the jump to 24" increases the deflection by 3.4 times
24" is even a bit of stretch for 5/8" based on span & drywall thickness (stiffness)...5/8" drywall at 24" o/c would delfect 1.7 times as much as 1/2" at 16"..... the 24" span is the killer.
the debris I speak of is usually (in my attics) windborne debris and wasp nests that fall from the rafters...if anything bigger than 1/8" get between the bottom of the joists & the drywall...the drywall will suffer upon re-seating.
I'm a little stumped when you call the members "joists" .......are they really the bottom chords of roof trusses?
Those 1x4's that you plan to attach flat to the ceiling aren't going to do much at all........ imo, a waste of time & material plus they would look rather odd & complicate the painting process forever. I think you'd be disappointed with the results
Blocking at 16" o/c between the trusses(?) would help a whole lot more than 1x4's...but more work since you really should "take out the sag" locally prior to screwing the drywall to the blocks. Blocking would be much less effective than some sort of continuous member (which is even harder to implement).
It seems that attic access is much easier than garage flor space.
Its a question of "cost of quality vs cost of non- conformance" important is a flat / un-wavy ceiling in the garage, how bad is it really? I might just re-attach, mud, prime & paint...forget the structural mods.
$150 worth of drywall plus a lot hanging, taping, mudding & sanding...priming & paint to R & R it all is a lot work.
cheers Bob
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