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Garage Ceiling Puzzler

Months ago, I noticed something odd on the ceiling of our (attached) garage. The sheetrock appeared to be separating slightly and there was a stain indicative of water damage. I didn't think much about it at the time as other things were going on and because, well, I was just optimistically hoping that I was just seeing a shadow or something.

Several weeks ago I saw a similar stain elsewhere in the garage ceiling. The first crack also appeared to be discolored more and longer.

I went into the attic to investigate, but found nothing.

I called our roofer (we had the house done in late Sept.) and told him I might have a problem. He was here within an hour and looked both in the attic and on the roof to see if he could find evidence of what might be wrong. His search was in vain and he said that we'd need a good rain to see what might be happening.

We had 0.8" of rain Thursday morning and it wasn't until this pm when I thought, "Hey, I better go up and check to see what I can learn." Still, I find no evidence of water -- but now I see two additional cracks along the sheetrock seams. I don't see signs of staining (in either one yet). Part of me wonders if maybe these new cracks are stress fractures I caused when I added my weight to that of storage up there, right in the center of the beams spanning the width of the garage. (Ceiling joists are 2x10's on 16" OC.)

Other facts:

- All cracks are along sheetrock seams and perpendicular to the joists.

- The center points of each crack are almost all in a N/S line (our home faces east).

- The southern most crack appeared, followed by the northern one, then the two intermediate ones. (Garage door faces due east.)

- No leaks are apparent where the ceiling and wall sheetrock meet.

- There is no plumbing in the garage save for the water heater. The input water line comes up through the wall by the heater (not over the garage), and the heater itself is in the far SE corner of the garage -- away from all the leaks. Examination of the water heater (about 1 yr old) and plumbing shows no evidence of leaks.

- Other than the water heater, there's no plumbing or venting (other than the roof's ridge vent) in the garage.

- There's no obvious sign of water leaking on the roof's decking, rafters, or floor joists.

- There's no obvious trace of water or water damage to anything in the attic or on the attic side of the sheetrock.

At this point, my strategy is to empty the garage attic entirely, pull up what little sheathing I layed to store things on, put down some plastic, dust the plastic with something (so I can see any traces of water flow), and hope for rain.

By way of background, we bought the home new and it's 11yo. Location is in central Texas.

Any advice on this puzzler would be VERY MUCH appreciated.


Re: Garage Ceiling Puzzler

If I had to venture a guess, I would be inclined to blame it on bad tape joints and/or joint compound that is expanding and contracting to much. A ceiling in an unheated garage is prone to extreme temperature changes. Moisture will often condense on either side of the ceiling drywall, due to temperature differences between the attic space and the interior space. To exacerbate the problem, there may not have been a vapor barrier (plastic) installed under the drywall.

Hmmmm...you may very well have something there, but I wonder - is the fact that this is showing up for the first time 11 years after construction reasonable?

Along with the 0.8" rain I mentioned, we also had a temperature drop of about 40 degrees from the previous day (high 70's to mid 40's). The two new cracks appeared afterward and while I thought it was rain, maybe it was the temp. As for condensation, there's nothing that I can see which'd draw it to those specific spots. There's only drywall and wood.

Oh, also there's no vapor barrier or insulation in the garage ceiling at all. You look straight down between the joists directly onto the sheetrock.

If you are sure that have no roof leaks, there are two things that may solve your problem. First, insulate above the drywall ceiling to diminish the temperature extremes. Second, re-tape the bad joints with a sticky-back fiberglass mesh joint tape, instead of paper joint tape.

I'm not quite sure I understand why adding insulation would work as the garage and attic space above the garage are unheated. Well, perhaps I should qualify that by saying whatever heat comes into the garage is from the cars cooling down and in the attic from the gas water heater. Could you explain a bit more please about why the insulation would help?

While you are at it, make sure the sheet-rock was installed with the right number of screws. If you are storing things above, use large sheets of 3/4" plywood and screw it down to the bottom cord of the truss. this will more evenly distribute the weight and not cause specific trusses to bow down.

Good idea on the screws; I can easily take care of that. I had some 3/8" plywood sheets up there so I'll take your advice and upgrade that as well.

Thank you again for the excellent ideas; I'll do my homework and post with more information.


Re: Garage Ceiling Puzzler


I think you've got me on the right track. :)

The reason this may have just started appearing is that our home was reroofed in September (hail damage) and in the process of doing so our roofer added a ridge vent to the garage attic. (He said there should have been one there originally, but that the builder probably skimped or that the roofer then didn't know what he was doing.) With this venting, the garage attic space is much more like that of the outside rather than being at (or hotter) than the temperature of the garage itself.

Since the garage attic space is uninsulated and there's no vapor barrier in place, the drywall seams are where (in wintertime) the warmer garage air most closely meets the colder attic space. This explains why the stains and slight drywall separation occur.

With this in mind, it also helps explain why the stains are in the peculiar locations and have an odd order of appearance. My wife teaches night classes at a university and typically parks in an open area where dew accumulates on the car. She drives home and parks in the garage, and the warm, moist air rises from her car and hits the garage ceiling. The area where the first stain appeared is nearly above her exhaust pipe.

The second stain appears almost above my exhaust pipe. The reason it was delayed may have to do with the fact that I frequently arrive home during the late afternoon/evening and leave the garage door open. My car has a chance to cool down and benefits from the circulation of having the garage door open. Over the last few weeks, though, we've had some extreme temperature changes and I've been going out in the evenings. This means I'm more closely mirroring what my wife is doing and thus I have the same stain recently showing up.

The two newer stains are smaller and closer to the center of the garage. This I suspect may be due to engine cooling and the rising moisture from there.

To recap the layout, the garage ceiling joists are 2x10's, 16" OC, with no insulation or vapor barrier installed. The problem we see along the seams is more cosmetic than anything else; there is no indication the drywall is detaching, although we do see some cracking along the seams as described above.

We are getting radiant barrier put in (hence my rush to resolve this problem now) and with it they include 5" of blow-in insulation. You mentioned a vapor barrier, but I'm not sure how to proceed with respect to adding that barrier (or even if it's needed). Do you have any recommendations?

I briefly thought about just venting the garage with an exhaust fan, but with a brick exterior going through a wall would be more of a hassle than I'm willing to deal with. Ditto for venting through the ceiling and attic to the outside.


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