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The drywall on my garage ceiling is sagging badly. Is this because it's too thin (I think it's 1/2" and I guess it's supposed to be 5/8") or is it because of humidity or something else? If I re-drywall the ceiling with 5/8" fire resistant (I think that's the code requirement) should I use some kind of sealer on it?

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What is the spacing on the joists? –  bib Sep 17 '12 at 22:21
    
What is the span? Is it the sheetrock or the joists that sag, any chance you have a leak? –  mikes Sep 17 '12 at 23:40
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Is it sagging in waves or is the entire sheet dropping in the center in one smooth arc? –  The Evil Greebo Sep 18 '12 at 0:34
    
Is there any indication of water damage? Wet/damp feeling areas, water spots, etc? –  Steven Sep 18 '12 at 2:21
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Thickness per span is not really the right question - drywall is generally to be mounted on studs/joists that are 16" apart on center. Even 1/8" drywall can hold up on the ceiling if it's properly attached. –  The Evil Greebo Sep 18 '12 at 10:46
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4 Answers

up vote 4 down vote accepted

The problem is because sheetrock does weaken a little over time, and with moisture, but the reason you are seeing it sounds like you just haven't used enough fixing points.

Usually you would fix to joists no more than 24" apart, and separate your screws on each joist by 16", which gives you a structure which can even cope with small amounts of water leakage without damage .

If you have joists already present, then you may be able to push the sheetrock back up and nail it correctly, however it may already be permanently warped - this will depend on just how far the bowing is.

I would guess though, that you don't have joists at that spacing - otherwise the sheetrock would probably be fixed to it already - so you will want to remove it all, fix joists and use new sheetrock. (This will be a fun job - especially if the existing stuff is a bit damp - bring friends to help bring it down otherwise you will have an exceedingly messy afternoon)

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Almost gave you a -1 on two points. 1) Don't nail, use screws, and 2) I'd never go more than 16" apart on the screws. Also pretty unlikely IMO that a garage ceiling was built without properly spaced studs. However - your first point - not enough fixing points being used - is likely correct. Someone just got lazy and/or cheap installing this sheetrock. –  The Evil Greebo Sep 18 '12 at 10:45
    
Interesting point on the screws - when I worked in the building trade we never used screws for sheetrock/drywall - only sheetrock nails (wide headed nails). I'll update on the distance though - 24" is the maximum we used to use, but closer is obviously better –  Rory Alsop Sep 18 '12 at 10:48
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Two reasons I don't like nails. 1) Expansion/contraction of the metal can cause them to loosen themselves. 2) every time I've removed sheetrock where nails have been used they've been rusted - but that could be that someone used the wrong nails I suppose. I also can't imagine it being very easy to flush sink the nail heads with a hammer. OTOH I've never seen proper drywall screws rust, with the proper bit they flush sink perfectly for mudding a smooth surface, and they can't work themselves out via expansion and contraction. –  The Evil Greebo Sep 18 '12 at 10:56
    
Updated - it has been 20 years since I did it, so I'm quite prepared to believe things have moved on. Thanks –  Rory Alsop Sep 18 '12 at 11:02
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If the entire 4'x8' sheet is sagging, then I'd suggest that either the fasteners were sunk too deep and they pulled through the back, or that the installers forget to install fasteners in the field of the drywall (I've seen installers that start with the edges and return to add fasteners to the field later). Either way, your advice is correct, to add more fasteners, before the sheet pulls loose and falls. –  BMitch Sep 18 '12 at 11:22
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All garage ceilings face a 9 out 10 chance of distortion or failing completely if there aren't ceiling battens fitted to the joists, as this creates a braced grid type situation. Screws won't hold the ceiling for much and can be no help as stud glue gives a better strength hold.

I guess you can put in a ton of screws, but glue in my opinion would be best applied prior to any rescrewing of plasterboard. Garages are cold and damp and don't often have carpet or heating so an interior plasterboard can suffer more than a interior of a home ceiling with insulation also. The dampness is the problem.

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You could put some filler boards between the joists if they are on 24 centers. If they are on 16 in. centers, you should be able to just screw the drywall back up. If you do new, I would paint it with a primer / sealer, then a coat of paint to look nice and keep the paper dry.

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That sounds like a good idea, to try on a couple sheets at least. –  Dave Sep 18 '12 at 18:15
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It sounds like whomever framed your garage might have been extraordinarily cheap and inept by not spacing your ceiling joists 16" apart on center. A full sheet of drywall is hung securely either horizontally or vertically on 16" centers.

If it is not this then the people who did your drywall were extraordinarily lazy by not hanging it correctly. They might have just nailed or screwed in the sides and corners and neglected or ignored securing to joists running through the middle.

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In some jurisdicitons, 24" spacing is allowed on non-loadbearing walls using thicker drywall, though most builders/carpenters prefer to use 16", even iwhen not required. –  bib Sep 18 '12 at 16:30
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