Walls and Ceilings - Drywall 24" on center question?

I have a detached garage that was built from a kit. It has ceiling joists spaced 24" on center. I want to insulate and drywall the garage and was told by a prospective drywall installer that he recommends putting 1/4 inch plywood under the drywall to keep the ceiling from sagging in the future. He said the weight of any insulation and the 24" spacing would cause the ceiling to sag noticeably and I would be calling him back within a year. Anyone heard of this before and are there any other recommendations? Thank you in advance for any responses.

Use 5/8" sheetrock on ceilings.

If a contractor told you the ceiling would sag perhaps the ceiling joists are undersized. The height of the joist is related the span and weight it must bear. Tell us the free span.

I've never heard of anyone using 1/4" ply under drywall. I agree with using 5/8" Most ceilings are hung with 1/2" but it isn't uncommon for the rock to sag over time - even immediately if the rafters are uneven. For the most part 5/8" rock eliminates that.

Are the rafters part of a truss system or do they stand alone?

Thanks to all who replied. My garage is 24 ft. by 24 ft. and the roofing is made up of framed 2x4 trusses. I have lined up two other drywall contractors to get additional quotes. I'll be interested in what they have to say. Thank you all again.

The spacing won't be a problem, when I frame suspended drywall systems I always space the tee's 2' apart. 2' spacing is stantard when hanging trusses, I only do commercial work so 5/8 drywall is what I use but 1/2 drywall will also work. If you insist on 16" spacing screw a furring strip to the truss, wood or metal can be used both are sold at HD. Obviously this will lower the ceiling height an extra 3/4". In the end the only real problem you have is the truss design, most trusses are only designed to support top loads think snow, shingles, etc. the added weight of drywall on the bottom chord could sag the trusses over the years.

I found a drywall installer who sounds like he knows what he's doing (good reputation)! Going to go with the 5/8" (no plywood) and I'll have to watch for any sagging down the road. Sounds like I'll be okay. Thanks again.