Repairing Plaster Over Lathe


COPYRIGHT 1999 G.G. ALONZY

Dear NH,
I need to do some patches on my old plaster walls, and discovered that in some areas the plaster has pulled away from the lathe... the wood strips that it locks into. Do I have to tear the wall down, or is there an alternative repair?
RB

Dear RB,
There are a number of ways to approach this repair, depending on the severity of the problem. One way to repair a small area of plaster that has released from the lathe is to chisel out the loose plaster and replace it with wallboard. It is generally acceptable to leave the lathe in place and screw the wallboard directly to it as long as you can screw the wallboard firmly to it.

If the lathe is damaged so that there is no screwing surface (angry feet through walls can cause this interesting decorative effect), cut the wall open just wide enough to expose a wall stud and bridge the gap with the wallboard.

Instead of cutting the stud-to-stud opening, you could instead attach 1x2 or larger wood strips across the hole, screwing through the plaster into them so that they bridge the opening and give you a strong nailing surface. You may have to use a masonry bit to predrill the holes for the screws and make a slight depression in the surface of the plaster with a larger masonry bit to "countersink" the screw heads. You don't want the screw heads to show! Get the feeling that working with plaster is not as easy as working with wallboard?

Cut the wallboard to match the shape of the plaster hole, leaving about a quarter inch open around the perimeter of the patch. Use wood shims to raise the level of the wallboard so that it is slightly lower than the level of the plaster. If you are lucky a thin pre-made wood product such as lattice might do the job. If not you will have to cut custom wood shims on a table saw.

Then again, shimming might not be necessary depending on the thickness of the plaster. If standard 1/2" wallboard is too thick and produces a level or raised surface, use 3/8" wallboard instead. The reason I suggest having the wallboard slightly beneath the surface of the plaster is because plaster walls are not always of uniform thickness. "Picture perfect" wallboard will almost never make a good patch without some creative filling.

Slightly dampen the plaster on the perimeter of the patch and the wallboard with water. Apply a stiff mix of Plaster of Paris to the 1/4" gap, pressing it deeply into the gap and also pressing it against the old plaster. Plaster of Paris expands as it dries, filling the opening tightly. It will also seal the damaged edges of the plaster so the final finishing will wallboard compound won't dry too quickly. Plaster of Paris sets quickly so don't dilly dally around! This is why I use it instead of slower drying patching plaster. To avoid having to sand the Plaster of Paris (which can be done but is a pain), make sure the level of the plaster is below the level of the finished surface.

Let the plaster set hard... usually less than a half hour. Now you can begin the fun and artistic part of this repair... smoothing the patch. Apply a coat of wallboard compound with a wide wallboard knife over the entire patch, bringing it slightly above the level of the original plaster and "feathering" the repair out beyond the patch. Don't expect a perfectly smooth surface with the first coat. Allow a day for the first coat and the plaster to dry. You will use multiple coats and a final sanding to obtain a perfectly smooth surface that will blend your patch into the rest of the wall.

Sometimes if a plaster surface is fairly solid but has loosened from the lathe, it can be solidified by reattaching it to the lathe, or "keying" it. This is done by drilling a number of holes in the plaster with a masonry bit and finishing them with a cold chisel so that spaces between the lathe are exposed. Space the holes a foot or so apart, vertically and horizontally.

Dampen the old plaster slightly and press Plaster of Paris into the hole so that it pushes through the lathe. You might want to put pressure on the wall while the plaster dries to keep it firmly against the lathe. You should keep the wall absolutely immobile overnight so the plaster becomes extremely hard. Once fully set, it will bind tenaciously to the lathe, holding the wall firmly in place. Finish repairing the holes with wallboard compound.

As an alternative, you could also screw the wall back against the lathe using 2" drywall screws. This method is less sure than the plaster key method because you have no way to know how strongly each screw is holding in the lathe, especially if the lathe is old dry wood. However, locating the wall studs and adding additional screws through the plaster into them can't hurt!!

If the newly firmed wall has developed unsightly "spider web" cracks, you can apply a special wallpaper to restore the surface to pristine smoothness. Check with your local paint/wallpaper store for the correct product. Of course, using standard wallpaper will also mask these imperfections if you prefer to have this big job end a little more quickly... without the finish painting and seam covering!
NH


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