Patch Those Plaster Cracks
Tips for lath-and-plaster repairs
by Bill and Kevin Burnett, Inman News
Q: We are getting ready to repaint our home office. The walls and ceiling were cracked during an earthquake and poorly patched before we moved in. We had the roof redone last summer, and the banging and hammering 
seriously re-cracked the plaster. In two areas there is a large “plate” of plaster that is loose. You can press it and it moves closer to the wall.
My questions are: Can this be glued back into place, or must we remove all the loose stuff and replaster? How does one handle the bubbles (like an air pocket) that occur in the plaster? Any suggestions for a proper repair would be greatly appreciated. I can handle small holes and cracks pretty well.
A: Old plaster is the bane of old houses. Most often when we’ve been presented with old plaster walls we’ve opted to strip them and apply drywall rather than engage in the seemingly constant battle of repairing them. We also had an ulterior motive.
With the stud bays open, it is an opportune time to reconfigure lighting and electrical outlets to our liking and to add insulation to exterior walls.
But tearing out the plaster and drywalling a room is a ton of work, and we can appreciate your wanting to avoid it.
You’re on the right track when you ask if you can simply reglue the detached plaster. To understand how, it helps to understand the guts of lath-and-plaster walls. The 1 1/4-by-1-inch-thick wood, called lath, is nailed horizontally to the 2-by-4-inch wall studs. The lath is installed with 1/4-inch spaces between each piece. Plaster is applied in three coats and built up to a thickness of 3/4-inch to make the finished wall.
The plasterer forces the first coat of plaster — the brown coat — through the grooves between the laths. The plaster droops on the backside of the laths and dries, forming “keys.” These keys keep the plaster tight to the wall. Over time, with movement the keys break and the plaster becomes loose, forming what you call bubbles.
Reattaching loose plaster bubbles is a multi-step process. You’ll need a cordless drill, a 3/16-inch masonry drill bit, a wet/dry vacuum, adhesive and some drywall screws with plastic washers.
The first order of business is to protect the floors of the work area with drop cloths. Old plaster is composed in part of coarse sand and will destroy the finish of a wood floor or bore deeply into carpet.
Start by using the 3/16-inch carbide-tipped masonry drill bit to bore holes through the plaster. The masonry bit does well moving through the plaster and will not perforate the wooden lath. Drill evenly spaced holes — about every 3 inches — around the damaged wall area.
If you happen to hit one of the gaps between the lath, mark it with a pencil. Remember lath runs horizontally, so when you hit a gap, drill the next hole a little higher or lower. Clean dust from the holes with a wet/dry vacuum.
Be sure to purchase adhesive that will bond the plaster to the lath. Adhesive should come in a tube to be applied with a caulking gun. Check out the local paint store for their recommendation and follow the manufacturer’s instructions.
Trim the adhesive tube’s nozzle to the size of the holes in the plaster with a utility knife. Then inject the adhesive into each hole by giving the caulking gun’s trigger one full squeeze.
Immediately after squeezing adhesive into the holes, use the drill equipped with a Phillips head bit to screw drywall screws with a plastic washer into as many holes as necessary to pull the plaster tight against the lath. Wipe away any adhesive that oozes out of the holes.
Allow the adhesive to dry and remove all the screws and plastic washers. If necessary, scrape the rings from the wall with a putty knife. Scrape off any high points of adhesive off the wall with the 6-inch putty knife.
Apply a thin coat of joint compound to fill the holes. Let the compound dry overnight and sand the surface lightly with 120-grit sandpaper. Apply a second compound coat, let dry and sand. Now you have a solid wall, ready for paint or paper.
By the way, this is a topic that we saw Tom Silva cover years ago on the “This Old House” franchise. We nosed around the Web for a while and were able to find this video: http://www.thisoldhouse.com/toh/video/0,,20210037,00.html.
Copyright 2009 Bill and Kevin Burnett
See Bill and Kevin Burnett’s feature Do-it-Yourself Fix for Squeaky Floors.
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