Walkabout: Plastered, Part Three

Read Part 1, Part 2 and Part 4 of this story.

So you live in an older home with plaster walls and ceilings, and you need to renovate. You need to run new electric lines, or repair plumbing, or perhaps change the placement of a door in the room, or open up a floor. Do you keep the plaster walls? Or maybe your walls or ceilings are cracked, or the plaster is falling off, exposing lath or brick. Or do you tear the plaster walls down to the studs, or at least the lath, and start over with sheets of drywall? Or do you run your lines, or make your repairs, opening up the walls as needed, and then repair the wall or ceiling? These are decisions old house owners make regarding the question of plaster or drywall, or perhaps a combination of both. The answer has a lot to do with the condition of existing plaster, time, personal preference, and of course, money. Are you a renovator or a restorer at heart?

The use of plaster has an ancient history. That story was told in Part One of this series. A skilled plasterer was a craftsman, worth the time and expense because of the beauty and long lasting nature of his work. The invention and use of drywall, aka sheetrock, in the early 20th century, was seen as a revolution in rapid and inexpensive homebuilding, replacing plaster in new construction during World War II. Part Two of this series explored the early use and later innovations to this increasingly versatile building material.

Cracks in original plaster occur for a number of reasons. Houses settle on their foundations, cracks can occur from nearby construction or road vibrations. Stress fractures can occur. The expansion and contraction of the wooden lath for a hundred years can cause plaster to crack or bow. If the original builder cut corners and used an inferior plaster mix, that can be the reason, after 100 years, the plaster is in bad shape. Water leakage is a biggie. So are opening walls for electrical and plumbing upgrades. There are a lot of reasons why plaster gives out, but when it does there are repairs that can be made if you choose to keep your plaster.

Whether you do this yourself, or hire an expert plasterer, you will be following the same basic steps. Repairing plaster can be more time consuming than drywalling, but it preserves those plaster walls you fell in love with when you bought your house. I’m going to go over some of the steps involved in a traditional repair to plaster. The information is distilled in part from information put out by the Technical Services department of the National Park Service, the National Register of Historic Places and the National Trust for Historic Preservation. You can download this, and other briefs for restoring historic materials and structures here: http://www.nps.gov/tps/how-to-preserve/briefs.htm. There is a wealth of good information.

First of all, if the cracks and damage to your home are caused by structural issues, those issues need to be resolved. For serious and persistent cracks in plaster, you should consult a structural engineer. If plaster damage is caused by leaks, it pays in the long run to have that problem fixed for good, and you might need to have some pipes replaced, not just patched. Cracks are symptoms, not the disease, and the underlying cause should be resolved, or you are just wasting your time and money getting them fixed, otherwise.

Assuming the cracks are from old age and settling, and are not from a larger problem, the cracks can be fixed in the following ways:
1. Hairline cracks can be filled in with patching material, sanded, and then repainted.
2. Larger cracks due to seasonal humidity changes are generally made wider, creating a V shaped furrow. The crack is then filled in with patching material, sanded and repainted. Woven fiberglass tape is often used across the crack to further stabilize and cover the repair. This tape is completely covered with patching material, and should not be visible.

Holes are much more than simple cracks, and can be much more complicated, depending on the conditions of the surrounding plaster, and the size of the hole. Small holes (less than 4” in diameter) that go down to the lath are filled in with a base coat of plaster, which is allowed to set before the top coat is troweled on and smoothed.

Many times, when people try to repair small holes with joint compound, they fill the hole in one application. This always results in drying and cracking, and a convex hole that looks as bad as the original damage. The hole should always be patched in stages, like the original plaster. A base coat, which is less than the depth of the hole, followed by a top coat. Yeah, it takes longer, but it works, and doesn’t sink or crack. Remember, it is almost always necessary to make a hole bigger before you can repair it.

Larger holes take special preparation. The first step is to expand the hole by removing the loose plaster from around it. Sometimes that can make your repair twice as big as it originally was, but it is necessary. If your plaster is falling away from the lath, the part of the wall (or ceiling) that is still on the wall can be reinforced. There are two ways of doing this. One method is to gently drill holes through the existing plaster around the hole, and using a caulk gun, squeeze in plaster cement. This cement will bond the plaster to the lath and stabilize the sides of the hole.

Another method of reattaching the plaster to the lath is to use flat head screws threaded through wide plastic washers. The screws and washers hold up the old plaster, screwing it to the lath. This works on both walls and ceilings. If you are determined to save your plaster, especially on a ceiling, this method works. A ceiling or wall that looks pretty much intact, but has lost its keys and is sagging away from the lath can be successfully saved through the use of these screws/washers. It may take an awful lot of them, but it can be done. The washers are covered by your new skim coat at the end of the repair.

Following the stabilization, the next step is rebuilding. The right materials need to be used. For the repair of large holes, joint compound is not the right material. That’s not what it is made for. You need a plaster mixture similar to the original. Lime based plaster compounds designed for wall repair are available, and should be used, not straight plaster, and not joint compound. Joint compound sets up too quickly, and is harder than the original lime plaster mix. That’s why it cracks.

The exposed lath needs to be checked for repair. If necessary, it may need some boards screwed back to the studs. Loose lathing will prevent your repair from being successful. The new plaster needs something to form keys with. Most often in large repairs, a metal diamond grid sheet is attached to the lath, and the base coat of plaster is applied to the surface. If you don’t use a grid, new plaster is applied directly to the old lath, so the lath needs to be wetted down first, as the plaster will suck the moisture out of the wood, causing the boards to twist and separate from the studs. Your repair will be wasted, as it just won’t hold up.

After the lath or metal grid is ready, it’s time for the base, or scratch coat. This is a thick mixture of lime plaster the consistency of peanut butter. It is pressed into the lath or grid so that it oozes behind the materials, and wraps around it, forming keys. This is the structure of the plaster coating. Without good keys, your plaster will not last. The scratch coat is laid until it is a bit less than half the thickness of the total repair. It is then scarified, or roughened up and scored, and then allowed to set and dry. The scoring gives the second layer a textured surface to grab on to.

The second coat of plaster is then applied. It is a bit wetter than the first, with the consistency of cake frosting. The extra moisture allows it to form an adhesion with the scratch coat, binding it with a vacuum seal, as the water is sucked into the scratch coat. This second coat is laid on so that it is a bit less than the level of the rest of the wall. You need to leave room for the top coat of fine plaster. This layer needs at least an hour to set up, and dry.

Finally, the top coat, or skim coat is applied. This is your surface layer, so it is mixed up to be smooth and silky. “This Old House” recommends using joint compound for this layer. Their master plasterer used three very thin layers of joint compound applied one at a time, over the course of three days. This allowed the plaster to further dry and cure before the next layer was applied. He smoothed out the final layer with a damp sponge, feathering the edges with the original plaster wall. By building up thin layers of joint compound, he was able to construct a smooth wall that did not need any sanding. The completed repair was then painted. You couldn’t even tell it was there. Which is the whole point.

Next time: Covering an entire wall with a new skim coat, using fiberglass mesh, repairing with dry wall, and tying up loose ends.

(Photo:ask-the-rehabber.com)