Old plaster walls and ceilings are made from two layers of plaster. The brown coat is applied over wood lath, strips that are nailed to the studs and spaced about ¼-inch apart to create keyways for the brown coat to grip.
The finish coat is applied after the brown coat has hardened. The integrity of a plaster surface depends on the bond between the plaster and its wood lath. When the bond breaks, the plaster cracks.
How to Fix Loose or Cracked Plaster
Fixing loose or cracked plaster is usually quicker, easier, and cheaper than the alternatives of ripping out the old plaster and hanging new drywall or blueboard.
Simply filling a crack with joint compound is pointless; the crack just comes back. The key is to fix the failed bond between lath and plaster. For years, homeowners and pros have done just that with metal plaster washers. These are fastened to the lath with drywall screws, pulling the plaster tight. But the washers and tape protrude from the plaster and covering them with layers of tape and joint compound requires a fair amount of skill and patience.
Steps to Repairing Plaster Walls
- Protect walls and floors in the repair area with plastic drop cloths; use painter’s tape to hold the drop cloths in place.
- Use a 3/16-inch carbide-tipped masonry drill bit to bore holes through the plaster, but not through the wooden lath. Drill evenly spaced holes—about every 3 inches—around damaged wall area.
- Clean dust from the holes with a wet/dry vacuum.
- Spray liquid conditioner into each hole; remove any conditioner that runs down the wall with a sponge.
- Trim the adhesive tube’s nozzle with a utility knife. Then, inject adhesive into each hole by giving the caulking gun’s trigger one full squeeze.
- Immediately after squeezing adhesive into the holes, use a drill/driver to screw a plaster ring into as many holes as necessary to pull the plaster tight against the lath.
- Allow the adhesive to dry, then use the drill to remove all the screws and plastic rings. If necessary, scrape the rings from the wall with a putty knife.
- Scrape off any high points of adhesive with the 6-in. putty knife
- Apply a thin coat of joint compound to the wall using the putty knife.
- Let the compound dry overnight, sand the surface lightly with 120-grit sandpaper, then apply a second, thinner compound coat.
- Prime and paint the wall.
Adhesives Can Also Do the Job Well
Rory Brennan, a Vermont-based plaster restoration expert, developed Big Wally’s Plaster Magic, a two-part adhesive, to reattach plaster without plaster washers.
- Using Big Wally’s is a matter of drilling a series of holes through the plaster on either side of the crack, stopping when you hit the lath.
- Then, vacuum out the dust and squirt the conditioner, a milky liquid, into the same holes. Old plaster will suck this stuff up like a sponge.
- Next, inject one full caulk-gun squeeze of the thicker adhesive into each hole, then temporarily clamp the plaster to the lath with drywall screws and big plastic washers. Neither the conditioner nor glue has any odor to speak of, and both wash up with water. The glue sets in a day or two.
- After it sets, back out the screws, pop off the washers, and fill the holes with joint compound.
- A quick skim coat of joint compound finishes the job.
A kit that includes everything you need to repair loose plaster costs a lot less than how much it would cost to remove that much plaster, haul it away, then hang and tape the wall or ceiling.