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I just moved to a new house, and noticed those white marks on my garage. Does anyone know what's behind them?

I would like to install a ceiling rack in my garage. Should I install it on the white marks?

Edit: I'm in North America(Canada) and the house was built in 1994

Garage interior with white joint compound on walls and ceiling

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The white marks are joint compound (aka "drywall mud") - kind of like plaster. The stripes are where separate sheets of drywall are joined with tape and mud and the little splotches are where screw holes have been covered over with mud.

Those actually are good places to install your storage rack because they indicate the locations of the lumber framing that the drywall is hung on. Basically the garage structure is a framework of wooden 2x4 wall studs and 2x6 or larger ceiling joists spaced 16" apart. That gives the garage it's shape and rigidity. The drywall is the finished interior surface that you see, and it's screwed or nailed into the studs.

If you're installing a storage rack, it's really important that you fasten it to the studs, because the drywall itself is only about 1/2" thick and has relatively little strength to hold any heavy load. In a painted wall, it's common to locate studs by knocking to feel solid spots, or using a stud finder tool. In your case though it's easy, because you can see the stud locations by where the drywall is fastened.

There is a stud or joist running behind each seam between drywall panels (long white stripes) and behind each of the parallel lines of screws (little white splotches).

As pointed out in the comments below, the actual studs are only 1 1/2" wide (45mm in metric), and the splotches are a bit wider than that, but you can find the exact location of the studs by looking for little circle marks where the head of the drywall screws are buried. Trace a line through these to find the rough center of the stud. Just center your fasteners for the lumber rack on these lines and you should be good to go.

If you're in doubt about whether you're lined up over a stud, drill a small pilot hole - either you'll break through the drywall into a cavity, or you'll continue drilling into the stud. If you hit a stud, you'll feel continued resistance on the drill and you'll see bits of wood and sawdust come out on the drill bit.

That said, there are exceptions. In my house (1950s era) the ceiling drywall is installed over relatively thin strapping that runs perpendicular to the joists, and it would be unwise to try and support a heavy weight from those straps. If you're hanging something heavy, take precautions to check that the fasteners are secure and the brackets can support the weight. Also, in North America a wooden structure is typical, but in other places the studs might be steel. If anything seems weird, take a pause until you can figure out how the structure works and how to safely fasten to it.

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    To be clear: don't try to mount anything right into the small bright white marks, because you'll hit the screw instead of the stud. – Timbo Nov 12 at 2:16
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    As to finding the actual location of the studs (the patches of drywall mud are relatively large): the picture clearly shows small circular areas which are more intensely white within many of the drywall mud splotches. Those are the actual locations of the screws/nails that were used to secure the drywall. Where those are is the actual location of the studs. Those should be in quite straight lines down the length of the stud, with only a little variation. Along those lines is where the OP would want to attach the ceiling rack supports, verifying that they hit the stud each time. – Makyen Nov 12 at 4:37
  • Ideally you trace & mark all studs, in a way that you can find back after painting; with the help of some rules of thumb (like all except possibly first and/or last should be parallel, they should be evenly spaced except possibly first and/or last, ... ). That plus connecting multiple marks along a single stud (with a long straightedge e.g. a straight 2x2) gives you a better chance of solidly hitting the stud when drilling --- instead of accidentally screwing near the edge. That, plus knocking to hear/feel where. BUT if you see other things off-kilter, expect stud placement to be off too. – user3445853 Nov 12 at 11:43
  • While generally correct your answer is very American, inches and wood might be centimeters and steel in other countries. – Rsf Nov 12 at 14:25
  • Is it common for a joist to be a 2x4 instead of something stronger like a 2x8? – Brad Nov 12 at 16:39
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This is what an unpainted surface looks like. Those white marks are joint/drywall compound and under them are screws holding the drywall to the joists. The long white lines are the same thing except on a junction between two sheets of drywall. So there are screws and a seam under them.

You can install a rack along the white marks but not directly in the middle of them because you'll hit the drywall screw. Having the joint compound exposed like this makes is easy to find the joists though it's still probably a good idea to use a stud finder right where you plan to put a screw just to be safe.

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    The chances of hitting a drywall screw is low, but checking the seam with a magnet will show you exactly where the drywall screws are hidden. – JPhi1618 Nov 11 at 16:44
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    @JPhi1618 definitely low, I agree. I was specifically responding to this part of the question: "should I install it on the white marks" – Brad Nov 11 at 16:47
  • Just a slight correction. Those are almost certainly not plaster but drywall compound. Plaster is a very different substance and is not generally used with drywall. – jwh20 Nov 11 at 16:47
  • @jwh20 thanks! corrected – Brad Nov 11 at 16:49
  • @JPhi1618, you can literally see exactly where those screw heads are. Each white patch has a distinctive circle the size of a screw head. – Octopus Nov 12 at 22:38
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FWIW, I believe your ceiling joists actually run perpendicular to the long mud seams.The give-away for me is the screw pattern near the light and the orientation of the electrical outlet.


To answer your actual question:

The white stuff is drywall compound which covers the drywall seams and screw holes.

The screw holes are a good indicator of where you can find joists to securely install your ceiling rack.

enter image description here

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    I don't know if the orientation of an outlet would provide more evidence of the joists running that direction that the drywall seam and the garage door. If they run that way that would mean the garage door wall is a load bearing wall which is very unlikely. Why the drywall be installed with the long seam perpendicular to a joist? – Brad Nov 12 at 16:38
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    Attaching drywall to lightweight strapping is not unusual. Use caution when judging what you're drilling into based on drywall screw patterns, and confirm it is up to the task you're asking of it before hanging weight off anything. – TheLuckless Nov 12 at 16:59
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If you want to run your rack/whatever in the other direction or in a specific location, you can screw 2x4s as needed flat against the ceiling and perpendicular to the red lines in MonkeyZeus' photo. Add a screw into every joist (green dots) (and there should be one right at the wall), and then you can screw your rack to those 2x4s (magenta oval). So you can add racking next to the garage door in a place where there are no joists right above. Example

The main idea being the 2x4s get attached really well even if they ran at an angle, as long as you get screws into them and the joists above. I'm not saying hang 2,000 pounds, but anyway...

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