DIY Repairing Cracks and Loose Lath Plaster
Loose plaster is often a problem in walls, and especially ceilings with wood lath. This type of plaster will usually be found in houses built before the 1920's. After that time gypsum board was developed for use as lathing material which greatly improved on plaster durability.
This older type of plaster can become loose over many years, as the nails in the wood are corroded by the lime in the scratch coat. The result is loose lath strips that move and vibrate due to normal foot traffic, etc. These vibrations cause the plaster key to break free from the wood and the surface of the plaster to crack or crumble as it moves.
To stabilize the plaster and prevent further cracking, the techniques on this page can be used to reattach the wood lath and the plaster coats to the framing. The cracks in the plaster surface may then be repaired as illustrated here. The method on this page will work nicely in cases where the plaster key is still generally intact, but extremely loose ceilings may be impossible to repair using this method. In those cases it is advisable to cover the ceiling with new drywall instead.
Reattaching the Wood Lath
Locate the wood lath and the studs or joists behind the loose plaster. Use a drill and an eighth inch masonry bit to bore holes at one inch intervals along the ceiling or wall until you locate the framing.
Mark the location of the framing and bore a one eighth inch pilot hole through the lath strip and into the framing behind. Do this at each stud or joist. Next bore ½in. countersink holes in the plaster coats at each pilot hole. The portland cement in the coating will dull drill bits in short order so use old bits or masonry bits for drilling and countersinking. Use a wood bit to bore pilot holes through the wood lath and into the framing. Use 2 to 3 inch drywall or wood screws and ½in. washers to reattach the wood lath to the framing.
Press the plaster back into place before driving the screws. To hold the it in place on the wall use a small piece of plywood (about 2X6 feet) and a 2x4 brace at regular intervals. Press the surface as flat as possible before driving the screws. Tighten the screws down slowly to avoid sudden movement that could further crack the plaster coats. Space the anchor screws at about 8 to 12 inches being sure to hit a lath strip and framing member at each point. Repair the holes using the repair methods on the Drywall Repair Nail Pop page. Repair cracks using this crack repair process.
Reattaching the Plaster Coats
Reattach loose plaster coats where the key has broken using setting-type joint compound. In this process new wall compound is "injected" at 8 to 12 inch intervals across the wall. Start by marking the wall at the ceiling and along corners. Use a chalked string to snap a line between corresponding marks forming a grid on the wall. With a masonry drill bit, bore ¾inch holes in the plaster at each point on the grid. Check to be sure that the lath is exposed in the holes.
As an alternate method, the point of a screwdriver can be used to make an opening in the plaster putty and brown coats. This is usually possible with very soft, loose walls.
Mix setting-type joint compound and press the mud into each hole using a drywall joint knife. Fill the holes until mud over flows. Scrape off the excess with an upward stroke of the knife blade before moving to the next hole. Let the mud set for about an hour or until the wall feels solid when pressing against it. Use the joint knife to scrape any excess mud off the wall and sand to smooth. Mix and apply a skim coat of mud over each patch to finish the repair.