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I am getting ready to start all the drywall processes. Starting with the ceiling, I have trusses 24" on center. The code here requires type x (so 5/8") firerock on the ceiling. It is also a common issue where you can't install 1/2" drywall on 24" centers and have to use 5/8" just for that reason.

Well, this is all fine and dandy, except for my bathrooms I planned on 5/8" Moisture Resistant rock for the ceilings and it appears to be unavailble here and only sold in 1/2" thickness. I will have exhaust fans in the bathroom, but I am not sure if that would warrant just standard 5/8" rock for the ceilings in the bathrooms. I may be able to find some 5/8 M.R. rock but I'll drive 70 miles for it. Any thoughts from the other DIY'ers and pros?

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  • Where is the structure located (approximately)? (To determine how practical going to Home Depot or Lowe's is.)
    – wallyk
    Commented Dec 5, 2015 at 4:35
  • Moisture-resistant board isn't necessary unless you have severe moisture problems. Modern primer and paint is more than adequate to keep moisture on the surface, where your ventilation system should dry it up in short order.
    – isherwood
    Commented Dec 6, 2015 at 3:54

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Densarmor paperless board comes in 5/8" thicknesses.

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I prefer densarmor for moist areas as there's no paper for any mold or mildew to grow on.

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To minimize the risk of sagging, I'd consider installing strapping perpendicular to the joists, and attach your moisture resistant drywall to that. This will let you get the spacing down to 16" OC (19.2" would be standard for a joist, but with two layers, strapping, and a small space, you may as well get more points to screw).

To get a longer fire rating from the 1/2", you can use 2 layers, but be sure to offset your joints. Start with a half sheet against the wall on your first layer, and then you can do a full sheet on the next layer. Make sure to use a longer screw on the second layer to reach the strapping, 2" would be plenty, 1.5" may be too short.

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Just use 2 layers of 1/2" WR. Its code allowable here, but do check with your AHJ!

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  • that might work but at 24OC you may get some sag. Granted, probably minimal and not that noticeable in a bath.
    – DA01
    Commented Dec 5, 2015 at 5:18
  • hanging 1/2" drywall is SOP here in ontario. nobody lays trusses at less than that unless some hyper specialized installation.
    – personal privacy advocate
    Commented Dec 5, 2015 at 14:46
  • I have fixtures and vent fans set already for only one sheet. I also don't know how stacking it would prevent the second piece from sagging? Looks like there is primer now and that standard rock is okay.
    – CCCBuilder
    Commented Dec 7, 2015 at 2:45
  • you dont want standard drywall in a bathroom. its not even allowed here in ontario.
    – personal privacy advocate
    Commented Dec 8, 2015 at 0:55

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