How to Repair 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.
The older wood-lath 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 members and bore a one eighth inch pilot hole through the lath strip and into the framing behind. Do this at each stud or joist along the same lath strip.
Next bore ½ inch countersink holes in the plaster coats at each pilot hole. The portland cement in the plaster will dull drill bits in short order so use masonry bits for countersinking.
Before reattaching the plaster with the screws, press it as tightly as possible to the framing using a sheet of plywood. Hold the plywood tight using 2x4's wedged between the floor and wall. Press the plaster as flat as possible before driving the screws.
Use 2 to 3 inch drywall or wood screws and ½ inch washers to reattach the wood lath to the framing as illustrated here. Tighten the screws down slowly to avoid sudden movement that could further crack the plaster coats. Space the anchor screws to attach a lath strip to framing every 8 to 12 inches. 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, or mud. 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.