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Plaster... An Irreplaceable Treasure

Have you ever run your hand along the wall of an old house on a hot summer day and been surprised by its coolness? Flat plaster walls that were installed in homes until as late as the 1950s are an integral part of both the historic and environmental value of older homes. Yet plaster is the first element that a contractor will tell you “has to go.” Actually, more often than not, it will be removed because either the contractor does not have the knowledge necessary to make repairs and/or the homeowner does not realize what will be lost along with its removal.

Homes built of wood and plaster are actually a superb, natural-functioning system.  While humidity passes through the materials to the outdoors, heat and cold are not transferred. In fact, plaster and wood are very poor conductors. The chilly interior drafts that you feel each winter are pouring through joints, cracks and crevices and are not coming through the walls themselves. The most efficient and economical solution to cold air leaks is to maintain and replace caulk around windows and along all gaps and cracks. Unfortunately, well-intentioned improvements, like blown-in insulation, vinyl replacement siding and vapor barriers actually defeat the old, efficient system of moisture exchange and add little to improve on the insulating air pocket between the plaster lath and the exterior walls. Worse yet, these materials greatly increase the chance for damaging moisture build-up and harmful mold growth, as well as, trapping the toxic fumes from out-gassing paint finishes and petroleum-based furnishings.

Although the environmental concerns are significant, it is the rich, textural beauty of plaster that is its greatest asset and the most compelling reason to preserve and restore this valuable material. The alternative, modern drywall, has a flat cardboard-like appearance and lacks the sound-insulating quality and fire resistance of plaster. The transmission of voices, footsteps, music and plumbing sounds is particularly annoying. In addition, replacing plaster with standard drywall has the effect of reducing the historic integrity of the building and, therefore, impacts the monetary value of the home within the historic home market.

Living with Plaster Walls
Many homeowners’ first experience with the unique properties of plaster is while trying to hang a picture. Because of the hardness and multi-layered construction of a plaster wall, it is not easy to pound in a nail. The result may be cracked plaster and a hammer that almost seems to bounce off the nail, as the underlying wood lath repels the blow. If your home was built with traditional plaster walls, it has a system of wood strips, or lath, that were nailed across the studs with enough space between to allow the wet material to be forced through, creating an overhanging lip of plaster known as a key. The keys are what give plaster its structural strength.

plaster lath detail

picture hook

If your home was built up through the mid 1920s it is very likely that it was built with lime plaster. Applied in three coats, it was a mixture of quick lime, water, sand and animal hair. The first coat was known as the scratch coat and it was topped with a thinner layer, the brown coat, both adding up to about 3/8 of an inch. The final coat, or finish coat, had a higher amount of lime and did not include animal hair. It was this coat, only about 1/8 inch thick, that created the beautiful, hard, smooth surface. In the 1920s, plasterers began using gypsum plaster. Unlike lime, which could take many months to cure, gypsum begins to set up immediately and cures in a couple of weeks. The down side of gypsum is that it is somewhat more susceptible to water damage. Gypsum plaster is still being installed in high-end construction projects. There is another type of plaster wall that was occasionally used, starting in the mid-1930s, and that is known as rock lathe. In those instances, gypsum plaster was applied to perforated gypsum board. A similar system, known as veneer plaster, is now used in some new construction.

So, this isn’t like tapping a nail into soft, chalky drywall. You can see that your plaster walls are a unique asset that should be preserved. Luckily there is an easy solution to hanging a picture without damaging the wall. A pilot hole is made with a drill bit slightly smaller in diameter than the picture nail.

Once the nail has gone through the lath, and possibly entered a stud, it will be firmly in place, providing a much more secure support than when nailing into drywall. However, an even better remedy is to install a picture rail, which is molding that runs just below the ceiling, leaving a narrow gap. Pictures and mirrors can then be suspended on picture wire or decorative cord supported by old-fashioned picture hooks. Many homes, some as late as the 1940s, were built with these rails, which allowed pictures to be moved around or changed seasonally without marring the walls.

Repairing Plaster
Often much of the visible damage is actually quite superficial. The most common problems are hairline cracks from settling, humidity changes, and traffic vibrations, and small holes from picture nails and minor collisions. These usually can be repaired fairly easily by the homeowner or by a plasterer. (Yes, there are still many professional plasterers in the Chicago area.) Small cracks can simply be filled with an all-purpose joint compound. If the crack is wider and deeper, or if it reopens seasonally, use fiberglass tape and a quick-set joint compound, which is less likely to fail when the opening varies with humidity changes. 

plaster washers Larger damage first requires identification of the cause. If the source of the damage is water or structural flaws, these must be discovered and repaired first. The direction of cracks can be a tip as to what is wrong. For instance, diagonal cracks starting at a door or window may be caused by stress from the weight of the upper floor and/or roof. Other types of cracks may simply be from settling or from the effect of variations in humidity over the years. Larger holes, which sometimes have edges that have separated from the lath, need to first be re-affixed to the wood with plaster washers and drywall screws. After that is accomplished, the opening needs to be correctly re-plastered. (See how-to sources below.) Almost any damage can be repaired, but depending on the size of the job, you may want to consult a contractor who has expertise in working on older homes.

Even ceiling plaster that has begun to separate from the lath can be salvaged by experts. An excellent source for the care and repair of historic homes, including plaster, windows, woodwork and more, is The Preservation of Historic Architecture, put out by The Department of the Interior. Articles from this collection can be found online at The September/October 1995 issue of This Old House magazine has a good article on how to patch with lime plaster. The article can be found online, including step-by step pictures, at,,1172537-2,00.html.

More Sources: for an article on patching with joint compound.  See Document #10.
About Your House, by Bob Yapp & Richard Binsacca, Bay Books, 1997

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