How to prevent sagging ceilings when joists are spaced 24"

Question

I recently had a ceiling removed as part of asbestos abatement. The ceiling joists are 24" apart. They run about 20' long and the area is about 8 1/2' wide. What's the best way to avoid the ceiling drywall from sagging and creating a rippled effect?

Would 5/8" drywall be sufficient? If so, should I get 4x10 and run the long side perpendicular to the joists? If I do that, I may not be able to get the ceiling drywall above the in-place wall drywall (you know, the 1/2" or so the wall's drywall sticks out). Or should I do 4x8 with the long side following the joists? That makes it feasible to slide the ceiling pieces above the existing wall drywall.

Or, should I add more joists between each pair of joists? There is a beam about in the middle (which I'm going to raise into the attic and attach the joists to it with joist hangers), so these new joists would only be 10-12' long each. If I do that, would 1/2" 4x8 drywall be sufficient?

Answer 1

Millions of homes have engineered truss roof systems (and floor systems, for that matter) on 24" centers. 5/8" or 1/2" "no-sag" drywall have done fine for almost half a century.

If you're concerned about waste, drywall is cheap (in most locations). You'll only need 5 or 6 sheets anyway. If you got creative you'd maybe save one sheet, but you'd create a bunch of extra taping (including some less desirable butt joints). 5/8" isn't commonly available in 10' lengths. You may need to by 12' sheets.

Also, there's no reason that ceiling drywall has to be above that on the wall. It's a myth that the walls hold up the ceiling. (What holds up the sheets in the middle of the room?) It's done that way only for reasons of convenience during installation--rooms tend to be slightly out of square for one, leaving larger tapered gaps around the ceiling sheets. The wall sheets cover these.

Answer 2

The simplest thing to do is install strapping - 1" x 3" furring strips that run perpendicular to the joists. This makes drywall installation a lot easier, gives you a wider surface for joints, and lets you use thinner cheaper easier-to-handle drywall without it sagging.

Strapping is used a lot in some areas of the US and seldom seen in others. I was skeptical about it, seemed like just wasted material, but when you consider the advantages and the low cost and effort needed to install it, it seems like a pretty good buy.

Answer 3

One stock approach is to use hat channel AKA resilient channel, at right angles to the joists, 16" on center. Of course, that drops the ceiling an inch or so depending on the channel type (channel screws to the joists on the flange with screws for wood, drywall screws to the channel with screws for metal. It provides some sound isolation as well as a more closely spaced attachment structure.