because roof rafters are normally 24" on center this prevents sagging.
Drywall has to bear the weight of insulation above–if blown, and wall texture below, also higher R value, with 2’ OC, less chance to sag than 1/2".
If there is living space above code states you should use fire resistant sheetrock, 2 pieces with unaligned seams. This has to be done in garage ceilings for fire reasons. This is New Jerseys requirements but safety first, everywhere! http://njsprayfoaminsulation.com
Great question, Wynn’s Construction Services LLC! The explanations provided below are absolutely correct. For any functional and logical reason, 5/8" drywall is better equipped than 1/2" drywall to remain rigid while weight, gravity and moisture attempt to affect its performance, which would result in sagging and deterioration. But, the market has changed over the last few years, and now most manufacturers are offering a 1/2" drywall board that is ultralight but still holds up to the sag resistance requirements. The basis of advertising for these companies is two-fold: 1. The drywall can remain consistent from the wall to the ceiling, and 2. The drywall is easier to carry and, thus install. The ultralight 1/2" (4x8) drywall board is approximately 40 lbs. But, 5/8" is still required for ceilings - why?
The reason is fire-resistance. 1/2" (and thinner) drywall boards are being revamped with the purpose to create a more lightweight board for ease of install. This is great! But, a 1/2" board exposed to fire will only last for 10 minutes. Typical solid flames burn around 1600 F degrees. The 1/2" drywall board tested at 1850 F degrees was breached within 10 minutes. This is 1/2".
Because of updates in ICC’s book, the more recent codes are now being adopted, so by way of necessity, 5/8" drywall now commonly contains fire-resistant advantages, such as a glass fiber reinforced core - this is considered Type X drywall. There are 3 types: regular, X, and C, which can be considered good, better and best, respectively, with respect to fire resistance.
A Type X 5/8" sheet of drywall tested under the same conditions as the 1/2" sheet of drywall, lasted 57 minutes before being breached by fire. This is approximately a 45-minute difference between 1/2" sheet of drywall and 5/8" sheet of drywall. And, with the ultimate goal of containing a fire until assistance arrives to extinguish the flame, 5/8" provides the better opportunity to achieve that goal, and, at the very least, prevent further damage to the home.
So, in the Chicago market, the codes are being changed to require 5/8" on the ceiling as well as the wall. Hopefully this helps to clarify the reasoning behind the requirement for 5/8" drywall on the ceiling.
P.S.
If you are building a new home or finishing a basement, consider using mineral wool insulation such as Roxul. Roxul burns at 2150 F; it is moisture-resistant; and, it provides good sound insulation. But, mainly, mineral wool insulation increases the whole system’s ability to contain a fire. Just an additional thought on the topic.
5/8 DW is stronger than 1/2 and can support it weigh better along with attic insulation