Before the walls tumble down, patch the plaster


June 04, 1994|By Karol V. Menzie and Randy Johnson

You say you've stopped napping on the sofa because of that ominous-looking crack in the ceiling? You can't take down that 20-year-old paint-by-number sunset in the dining room because of the gaping hole behind it? You've learned to close doors quietly while listening for the sound of parting plaster?

Relax, you can fix it. The truth is, plaster repair is fairly simple, especially if you have some experience in finishing drywall, as most rehabbers do.

Recently Randy has been rehabbing an older house not too far from where the Orioles used to play baseball. The project has required, besides 60 sheets of new drywall, a considerable amount of plaster repair. The owners hoped a lot of the old walls could be repaired and preserved, and that saving them rather than rebuilding them would also save money.

It's always nice to be able to keep as much of the original fabric of the house as possible. Though in this case, the repairs have proven so labor-intensive that it's unlikely any money will be saved. But if the homeowners were doing the work themselves, and didn't count the cost of their own labor, plaster repair might be an economical way to save old house parts.

And the procedure's not difficult. The first step is to take a good look at the walls to figure out how much of the plaster is loose. Plaster is applied over lath, narrow wooden strips with gaps between them. As the wet plaster is applied, some of it oozes through the cracks in the boards. When it dries, the oozed bit forms a sort of knob or slub that holds the plaster tight to the wood. The slubs are called keys, and over the years, because the house may be settling, or because there is always vibration from foot traffic, or even from buses running outside, some of the keys break off.

When it loses its keys, plaster is in trouble. There are plaster

washers, small flat metal rings that can be nailed through the plaster and back into the lath, then coated over with a new layer of plaster. But they are most effective in small areas. When plaster loses keys over a wide area, it's probably best to take it down and replace it with drywall. (You can, if you live in an area where there's been a lot of restoration, find good, old-fashioned plasterers, but that kind of craftsmanship doesn't come cheap.)

Ceilings are often the area worst hit, and if you have a lot of cracked, damaged, loose plaster on a ceiling, it's a good candidate for drywall. In addition, if you are doing a major rehab, removing the ceilings makes it easier to replace plumbing, wiring and heating pipes or ducts.

Let's assume that most of your repair work will be on the walls. If you just have a crack, dig the crack out slightly and bridge it with joint compound and drywall tape; finish like a regular drywall joint.

In larger areas of bad plaster that need repair, draw a rectangle or square on the wall that encompasses the bad section. Then use a utility knife to cut all the way through, so you can break the plaster out to the lines, leaving a nice straight patch. On an interior wall with old wooden lath backing, you would now be down to the wood lath. On a brick wall, you would be down to the brick. This is the point to make sure you have solved any problem that caused the plaster to crack in the first place -- be it roof leaks, plumbing problems or settlement.

The next step is to re-nail any loose lath to the framing. You may then be able to take a short cut if the plaster is the exact thickness of a 3/8 -inch or 1/2 -inch sheet of drywall and you have exposed studs on both vertical sides to nail or screw the drywall patch into. If that's the case, cut a piece of drywall to fit, screw the drywall through the lath, and tape and spackle the patch to the old plaster as you would any other drywall joint. (You have to tape between the old work and the new to keep your final finish from cracking.)

In most cases, however, the thickness of the plaster will vary considerably, so you will have to patch it with plaster.

For a good bond, nail a layer of galvanized metal lath to the old wooden lath. It comes in 2-foot by 6-foot sheets, so you can patch a pretty big area with one sheet. Use 1-inch galvanized roofing nails; on a brick wall, drive them into the mortar between the bricks.

Once the lath is nailed securely, start mixing up the plaster. Begin with a coarse plaster (Gypsolite is one brand name) to form what is called the scratch coat. Mix it so it's fairly stiff -- you don't want it running down the walls. Smear it on the lath with a trowel, pressing so it goes through the holes. Build this first layer out so it's almost flush with the old surface. On a larger patch, you may need to use a board that reaches from old surface to old surface to check the level of the patch.

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