DO-IT-YOURSELF : Patching Cracks in the Plaster Does Not Have to Be Unsettling

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?

Repairing plaster walls found in older homes does not need to be intimidating. The truth is, fixing plaster is fairly simple, especially if you have some experience in finishing drywall.

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 is probably best to take it down and replace it with drywall. (You can, if you live in an area where there has been a lot of restoration, find good, old-fashioned plasterers, but that kind of craftsmanship does not come cheap.)

Ceilings are often the area worst hit, and if you have a lot of cracked, damaged, loose plaster on a ceiling, it is 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. 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.

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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-by-6-foot sheets, so you can patch a pretty big area with one sheet. Use one-inch galvanized roofing nails.

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 is fairly stiff--you do not 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 is 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.

Let the Gypsolite dry a day, then skim-coat it--that is, apply a final thin coating of quick-drying finish plaster such as Durabond 90--and then tape between the old surface and the new. Let the tape dry a bit, and skim over it with a light coat of Durabond 90, using a 12-inch drywall knife. It should be a light coat because Durabond 90 does not sand well; you want to leave it as smooth as possible. For the final coat, you may need to mix a little water into the compound to keep it flowing smoothly.

Once the last layer of plaster dries, skim the tape and the patch with ordinary joint compound. Joint compound does sand easily, but if you have been careful, you should not need much sanding.

Keep the compound clean. Always use a small drywall pan or hawk (flat plate with a handle on the bottom) and dip fresh drywall out of the bucket at intervals. Clean tools thoroughly after each use. Keep the bucket covered. Any foreign objects or bits of dried plaster that get into the compound can mar the final finish.

There is a new “light” type of joint compound that is 33% less dense than the traditional variety. It is a little harder to apply, at least until you get the hang of it, but it works well on ceiling cracks and patches because it is not so heavy that it falls out.

There is nothing worse than standing under someone who is doing a sloppy job on a ceiling. Which brings up one more point: It is always a good idea to wear a hat.