View Full Version : Hairline cracks in joint compound
01-09-2009, 08:22 AM
I used some joint compound over a crack in my ceiling. I went over it twice and lightly sanded in between. I notice that when it is somewhat colder, something that looks like a hairline crack similar to the one I just covered appears on the joint compound but then it disappears. Has anyone ever experienced this? The compound that I used is the one that is already premixed.
01-09-2009, 10:12 AM
Sounds like temperature or humidity changes (or maybe even the wind) is causing movement and re-cracking it. Could be deflection as well. Is it for chance underneath a bathtub? Filling one of those can increase the weight on that floor by maybe 1000 pounds if it is a deep soaker tub.
01-09-2009, 10:53 AM
Is the ceiling accessable from the attic? If so sister up the joist with some 2x4 blocking and screw around the cracks into the blocking. Then just drywall. Problem gone.
If no access you'll need to take down some drywall on either side of the crack, back to the closest stud on each side. Put in blocking, redrywall and mud and tape. Done.
01-09-2009, 11:18 AM
It is not underneath a bathtub. The house is one story. The weird thing is that the hairline crack appears and disappears. It is very light and does not go through the other layer....just the top layer. I will check the attic and try using the screws.
Thanks for the info. Always great input and advice on this forum!
01-10-2009, 10:00 AM
Did you open it up into a V & clean out any loose dust & fill it and tape it?
Or just cover the old crack with compound?
If you just go over it... whatever caused the crack in the first place, will cause it to re-occur
01-10-2009, 03:37 PM
If you just put mud over the crack, the crack will never go away. I have had good success with using a piece of nylon stocking over the crack and under new mud. Much the same as tape but thinner.
01-10-2009, 04:31 PM
That's a nifty trick. Same a bunch of time on feathering it out...
Wouldn't shift & distort a lot, when you're embedding it, though?
01-10-2009, 08:01 PM
Frenchie is right on.....the magic of tape. How does a piece of paper stop cracks? Only the shadow knows!
And thanks Gary for the great tip on the nylons!
01-11-2009, 07:03 AM
I did not use tape since it was a hairline but plan on redoing it and using the tape this time. I am new to the DIY but we learn as we go. I also noticed other hairline cracks appearing in toher areas of the ceiling. I plan on taking some pics to post.
If you deepen the crack so the compound has something to adhere to, that might stop the crack from reappearing, but spanning the crack with some material, paper tape, self adhesive open grid tape, or the stocking, is also effective. Paper does not stop the crack it just covers it.
01-11-2009, 07:52 AM
How does a piece of paper stop cracks? Only the shadow knows!
Pretty basic physics, actually.
Plaster has pretty good compressive strength, but little-to-no tensile or sheer strength.
Paper tape or fiberglass tape (or nylons :)) have good tensile strenth; and if they're properly embedded, that means they also have good sheer strength.
Same logic as why you nail sheathing to a stick-framed wall, and it prevents racking.