Patch those plaster cracks

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. ...CONTINUED

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