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Question DetailsAsked on 1/25/2015

I just noticed today there are two long straight hairline cracks in my unheated garage ceiling.

I just noticed today there are two long straight hairline cracks in my unheated garage ceiling. They ran almost entire length of garage towards the door. We are in New England, so we can have 100f in summer and below 0 in winter. Above garage is a bedroom. Not sure if this is normal or something I should worry about?

Thanks friends.

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1 Answer


I would strongly suspect if you look closely (or hold a straight-edge across the cracks or use metal stud detector to track nail lines) that you will find the cracks are forming down the edges of drywall sheets - though if adjacent sheets were offset from each other half a sheeet like they should have been, the crack would typically be quite straight along the abuting joints, then fairly straight overall (within 1/2 inch or so typically) but locally quite jagged where it crosses through an intact sheet as it can't follow a joint. The thermal effect between the under and oversides of the floor being conditioned on one side and unconditioned on the other can easily cause smalll cracks like this - typically parallel to walls and down the middle of the room. The natural sagging of wood joists can also induce tension cracking on the underside of the joists - the ceiling drywall in this case. Also, in a garage, it is quite possible the joints were originally just mudded with joint compound but not taped, which allows for easier crack formation and propogation.

Case 1) I am presuming in your case that your overhead room floor joists (which comprise the garage ceiling joists) run crosswise to the garage (from side to side), so the crack is perpendicular to the joists. In that case, the natural sag of the joists over time would induce cracking first at the center of the joists - so down the middle of the garage. you commonly get a centerline crack - in much older homes with several inches of ceiling sag, sometimes a secondary or even tertiary pair of cracks spaced a few feet apart from the centerline.

Case 2) If your joists instead run from front to back of the garage (usually done only if there is a large cross-beam midway to cut their span in half - usually half or full depth protruding down into the garage ceiling), then a crack paralleling them could be indicative of sheets that were not properly nailed into the joists - too few nails along the edges, or missed the joists with the nails/screws. If that is the case, the edge(s) of the sheet(s) at the crack will start drooping or pulling away as the fasteners gradually pull through the material, indicating a need for more nailing along the edge. If that is the case, if you push up on the edge of the sagging sheet near the edge it will move up a bit and usually cause popouts of the joint compound over the fasteners - in other words, it will not be tight to the joist.

If a ceiling sag becomes pronounced AND the sheet edge is tight to the joist, then it might indicate a problem with a joist in that area. Ditto if you start seeing brownish or orange staining, indicating water on top of the drywall.

Either way, if just a hairline crack I would not be real concerned initially - fill it in with drywall patching compound or spackle (and paint if desired) so it is easy to see how much it is growing and to restore the fire resistance of the ceiling, then keep an eye on it - it may recrack again several times over the years, but should not become "wide" - certainly not wide enough to put the full metal tip (not just the ball) of a ballpoint pen refill or bic pen into, for instance. Also, if you get differential settlement rather than just a garage-long gradual sag AND the sag is in the joists and not just drywall sagging down due to inadequate fasteners, then you might start thinking of looking at what is going on in the subfloor through an inspection hole or using a fiberoptic inspection camera.

Here are a few prior responses to similar question that might help too, explaining hw and why such cracks can form, and when they are typically indicative of trouble and when not - of course, some of the issues addressed here are specific to those cases and are indicative of what you might be seeing. However, the issue of attic joists/trusses lifting is similar to your case as your joists see widely varying temperature and moisture over the course of the year.

Answered 5 years ago by LCD

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