36" span to cover w/ 5/8" drywall
5/8" is pretty solid. It is more likely to sag if there is a lot of humidity/dampness in the room. Do you open the windows for natural ventilation, possibly occurring during rainy/cool evenings to save on the AC bill?? If you keep a constant "conditioned" atmosphere your chances are better of not having an issue with sagging. I'm guessing you didn't use adhesive when you were hanging, so if the board DOES begin to sag the screws in the joists on either side will stand a better chance of pulling through the face paper and detaching. Once the integrity of the face paper on the drywall is compromised, especially overhead, you could have a problem. Personally wouldn't take the chance of having to deal with a repair later on down the line when everything is finished/painted/carpeted when I could fix the problem now. If you do the fix properly, DON'T run your screws back in the same holes...
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