Plaster Repair

Yes, You Can Fix that Cracked and Loose Plaster!

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I know just how tempting it is to get out the BFH (Big Fat Hammer) when you  see a chunk of plaster about to fall off the wall or ceiling.  But before you get crazy, you might want to consider what you're really getting into.
How about wheelbarrows and wheelbarrows of heavy stuff that's the consistency of concrete?  The trash guys really kinda frown on bags of concrete.  Also, if you rip a wall down to the lathe or studs, you're required to bring all the electrical wiring up to code.  Is that something you really want to get into?  Maybe so, but I would dare say that most people don't want to turn an hour long project into an entire summer of hard, nasty work.
I'll admit, sometimes it's easier, especially if half the wall has already come down, to just put up a piece of drywall.  If you haven't reached that point yet though, it will be incredibly easier to follow this simple procedure to save the wall or ceiling, and save yourself a lot of work and waste.  You'll also keep an original feature of the house that is difficult to reproduce.
This is the curved underside of a staircase that we wanted to save because it would be difficult to apply drywall in such a twisted manner.

Curved Plaster Ceiling
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The first step is to determine where reattachment would be the most beneficial.  Gently press the loose area and see if it goes back into position against the wood lathe.  While you're holding it in position, use a 3/8ths inch bit to drill a VERY shallow depression, just the depth of a drywall screw head. 

Drilling Countersink
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You're only drilling through the surface coat of plaster, not the harder base coat.  When you're finished drilling the countersink, you'll see a little of the base coat in the center of the depression.  If you go too far, the screw will have nothing to grab a hold of, and will go right through the plaster.  Not to worry though, just drill a new countersink nearby.

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A drywall screw is used to reattach the plaster to the lathe.  1-1/4 or 1-5/8 screws work well.  Longer screws can be used if the plaster has moved really far away from the wall.

Drywall Screw

Insert the screw into the countersink depression, and twist it back and forth with a hand screw driver until it drills its own hole through the base coat of plaster.  Don't use a power driver to do this..   

Self Tapping Screw
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If the screw just pushes right through the plaster and doesn't seem to tap into anything solid behind, simply move over an inch or so, drill a new countersink, and try again.  Lathe are spaced with gaps between the boards, so about 15% of the time, you'll miss the lathe.

Lathe Spacing
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When finished, the screw head should be flush with the surface of the plaster, and the plaster will not move when gently pressed.

Screw Head Flush with Surface
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If the plaster is really loose, and the crack spans a long distance, it's a good idea to drill multiple countersinks along the crack and partially insert a screw in each one.  Make sure there are screws on both sides of the crack, all along the crack, spaced about six or eight inches apart.  It's not necessary to do this in sections that are not loose.  Insert each screw so that the screw heads just barely touch the surface of the plaster.  Then gradually tighten the screws, going from screw to screw along the crack, and back and forth from one side of the crack to the other.  The likelihood of the screw breaking through the plaster is reduced by gently pushing the area around the screw with your hand, and then tightening the screw until it meets the surface of the plaster.  The plaster rarely breaks if it's done this way.  Don't force the plaster to go farther than it wants to; just go on to the next screw. 
Usually, after tightening all the screws a little bit, you can start over with the first screw and tighten them all a little more.  When they're tightened in sequence a little at a time, the end result is a perfectly flat surface with a tiny crack that can be easily filled in with joint compound for a flawless repair.

Flat Surface

The nice thing about this method is that it's a permanent repair.  Most plaster repair methods involve only digging around and patching the crack, with the possibility of the crack opening up again later.  The method shown here fixes the root of the problem, which is plaster separating from the lathe.  After these cracks are smoothed with joint compound, they won't reappear.