Gypsum begins to cure as soon as it is mixed with water. It sets in minutes and completely dries in two to three weeks. Historically, gypsum made a more rigid plaster and did not require a fibrous binder. However it is difficult to tell the difference between lime and gypsum plaster once the plaster has cured.
Despite these desirable working characteristics, gypsum plaster was more vulnerable to water damage than lime. Lime plasters had often been applied directly to masonry walls (without lathing), forming a suction bond. They could survive occasional wind-driven moisture or water winking up from the ground. Gypsum plaster needed protection from water. Furring strips had to be used against masonry walls to create a dead air space. This prevented moisture transfer.
In rehabilitation and restoration projects, one should rely on the plasterer’s judgment about whether to use lime or gypsum plaster. In general, gypsum plaster is the material plasterers use today. Different types of aggregate may be specified by the architect such as clean river sand, perlite, pumice, or vermiculite; however, if historic finishes and textures are being replicated, sand should be used as the base-coat aggregate. Today, if fiber is required in a base coat, a special gypsum is available which includes wood fibers. Lime putty, mixed with about 35 percent gypsum (gauging plaster) to help it harden, is still used as the finish coat.
Lath provided a means of holding the plaster in place. Wooden lath was nailed at right angles directly to the structural members of the buildings (the joists and studs), or it was fastened to nonstructural spaced strips known as furring strips. Three types of lath can be found on historic buildings.
Wood Lath. Wood lath is usually made up of narrow, thin strips of wood with spaces in between. The plasterer applies a slight pressure to push the wet plaster through the spaces. The plaster slumps down on the inside of the wall, forming plaster “keys.” These keys hold the plaster in place.
Metal Lath. Metal lath, patented in England in 1797, began to be used in parts of the United States toward the end of the 19th century. The steel making up the metal lath contained many more spaces than wood lath had contained. These spaces increased the number of keys; metal lath was better able to hold plaster than wood lath had been.
Rock Lath. A third lath system commonly used was rock lath (also called plaster board or gypsum-board lath). In use as early as 1900, rock lath was made up of compressed gypsum covered by a paper facing. Some rock lath was textured or perforated to provide a key for wet plaster. A special paper with gypsum crystals in it provides the key for rock lath used today; when wet plaster is applied to the surface, a crystalline bond is achieved.
Rock lath was the most economical of the three lathing systems. Lathers or carpenters could prepare a room more quickly. By the late 1930s, rock lath was used almost exclusively in residential plastering.
When plaster dries, it is a relatively rigid material which should last almost indefinitely. However, there are conditions that cause plaster to crack, effloresce, separate, or become detached from its lath framework. These include:
Overloading. Stresses within a wall, or acting on the house as a whole, can create stress cracks. Appearing as diagonal lines in a wall, stress cracks usually start at a door or window frame, but they can appear anywhere in the wall, with seemingly random starting points .
Builders of now-historic houses had no codes to help them size the structural members of buildings. The weight of the roof, the second and third stories, the furniture, and the occupants could impose a heavy burden on beams, joists, and studs. Even when houses were built properly, later remodeling efforts may have cut in a doorway or window without adding a structural beam or “header” across the top of the opening. Occasionally, load-bearing members were simply too small to carry the loads above them. Deflection or wood “creep” (deflection that occurs over time) can create cracks in plaster.
Overloading and structural movement (especially when combined with rotting lath, rusted nails, or poor quality plaster) can cause plaster to detach from the lath. The plaster loses its key. When the mechanical bond with the lath is broken, plaster becomes loose or bowed. If repairs are not made, especially to ceilings, gravity will simply cause chunks of plaster to fall to the floor.
Settlement/Vibration. Cracks in walls can also result when houses settle. Houses built on clay soils are especially vulnerable. Many types of clay (such as montmorillonite) are highly expansive. In the dry season, water evaporates from the clay particles, causing them to contract. During the rainy season, the clay swells. Thus, a building can be riding on an unstable footing. Diagonal cracks running in opposite directions suggest that house settling and soil conditions may be at fault. Similar symptoms occur when there is a nearby source of vibration-blasting, a train line, busy highway, or repeated sonic booms.
Lath movement. Horizontal cracks are often caused by lath movement. Because it absorbs moisture from the air, wood lath expands and contracts as humidity rises and falls. This can cause cracks to appear year after year. Cracks can also appear between rock lath panels. A nail holding the edge of a piece of lath may rust or loosen, or structural movement in the wood framing behind the lath may cause a seam to open. Heavy loads in a storage area above a rock-lath ceiling can also cause ceiling cracks.
Errors in initial building construction such as improper bracing, poor corner construction, faulty framing of doors and windows, and undersized beams and floor joists eventually “telegraph” through to the plaster surface.
In addition to problems caused by movement or weakness in the structural framework, plaster durability can be affected by poor materials or workmanship.
Poorly proportioned mix. The proper proportioning and mixing of materials are vital to the quality of the plaster job. A bad mix can cause problems that appear years later in a plaster wall. Until recently, proportions of aggregate and lime were mixed on the job. A plasterer may have skimped on the amount of cementing material (lime or gypsum) because sand was the cheaper material. Over sanding can cause the plaster to weaken or crumble. Plaster made from a poorly proportioned mix may be more difficult to repair.
Incompatible base coats and finish coats. Use of perlite as an aggregate also presented problems. Perlite is a lightweight aggregate used in the base coat instead of sand. It performs well in cold weather and has a slightly better insulating value. But if a smooth lime finish coat was applied over perlited base coats on wood or rock lath, cracks would appear in the finish coat and the entire job would have to be redone. To prevent this, a plasterer had to add fine silica sand or finely crushed perlite to the finish coat to compensate for the dramatically differing shrinkage rates between the base coat and the finish coat.
Improper plaster application. The finish coat is subject to “chip cracking” if it was applied over an excessively dry base coat, or was insufficiently troweled, or if too little gauging plaster was used. Chip cracking looks very much like an alligatored paint surface. Another common problem is called map cracking–fine, irregular cracks that occur when the finish coat has been applied to an over sanded base coat or a very thin base coat.
Too much retardant. Retarding agents are added to slow down the rate at which plaster sets, and thus inhibit hardening. They have traditionally included ammonia, glue, gelatin, starch, molasses, or vegetable oil. If the plasterer has used too much retardant, however, a gypsum plaster will not set within a normal 20 to 30 minute time period. As a result, the surface becomes soft and powdery.
Inadequate plaster thickness. Plaster is applied in three coats over wood lath and metal lath–the scratch, brown, and finish coats. In three-coat work, the scratch coat and brown coat were sometimes applied on successive days to make up the required wall thickness. Using rock lath allowed the plasterer to apply one base coat and the finish coat–a two-coat job.
If a plasterer skimped on materials, the wall may not have sufficient plaster thickness to withstand the normal stresses within a building. The minimum total thickness for plaster on gypsum board (rock lath) is Æ inch. On metal lath the minimum thickness is 5/8 inch; and for wood lath it is about 3/4 to 7/8 inch. This minimum plaster thickness may affect the thickness of trim projecting from the wall’s plane.
Proper temperature and air circulation during curing are key factors in a durable plaster job. The ideal temperature for plaster to cure is between 5570 degrees Fahrenheit. However, historic houses were sometimes plastered before window sashes were put in. There was no way to control temperature and humidity.
Dry outs, freezing, and sweat-routs. When temperatures were too hot, the plaster would return to its original condition before it was mixed with water, that is, calcined gypsum. A plasterer would have to spray the wall with alum water to reset the plaster. If freezing occurred before the plaster had set, the job would simply have to be redone. If the windows were shut so that air could not circulate, the plaster was subject to sweat-out or rot. Since there is no cure for rotted plaster, the affected area had to be removed and replastered.
Plaster applied to a masonry wall is vulnerable to water damage if the wall is constantly wet. When salts from the masonry substrate come in contact with water, they migrate to the surface of the plaster, appearing as dry bubbles or efflorescence. The source of the moisture must be eliminated before replastering the damaged area.
Sources of Water Damage. Moisture problems occur for several reasons. Interior plumbing leaks in older houses are common. Roofs may leak, causing ceiling damage. Gutters and downspouts may also leak, pouring rain water next to the building foundation. In brick buildings, dampness at the foundation level can wick up into the above-grade walls. Another common source of moisture is splashback. When there is a paved area next to a masonry building, rainwater splashing up from the paving can dampen masonry walls. In both cases water travels through the masonry and damages interior plaster. Coatings applied to the interior are not effective over the long run. The moisture problem must be stopped on the outside of the wall.
Repairing Historic Plaster
Many of the problems described above may not be easy to remedy. If major structural problems are found to be the source of the plaster problem, the structural problem should be corrected. Some repairs can be made by removing only small sections of plaster to gain access. Minor structural problems that will not endanger the building can generally be ignored. Cosmetic damages from minor building movement, holes, or bowed areas can be repaired without the need for wholesale demolition. However, it may be necessary to remove deteriorated plaster caused by rising damp in order for masonry walls to dry out. Repairs made to a wet base will fail again.
Canvassing Uneven Wall Surfaces
Uneven wall surfaces, caused by previous patching or by partial wallpaper removal, are common in old houses. As long as the plaster is generally sound, cosmetically unattractive plaster walls can be “wallpapered” with strips of a canvas or fabric-like material. Historically, canvassing covered imperfections in the plaster and provided a stable base for decorative painting or wallpaper.
Hairline cracks in wall and ceiling plaster are not a serious cause for concern as long as the underlying plaster is in good condition. They may be filled easily with a patching material (see Patching Materials, page 13). For cracks that reopen with seasonal humidity change, a slightly different method is used. First the crack is widened slightly with a sharp, pointed tool such as a crack widener or a triangular can opener. Then the crack is filled. For more persistent cracks, it may be necessary to bridge the crack with tape. In this instance, a fiberglass mesh tape is pressed into the patching material. After the first application of a quick setting joint compound dries, a second coat is used to cover the tape, feathering it at the edges. A third coat is applied to even out the surface, followed by light sanding. The area is cleaned off with a damp sponge, then dried to remove any leftover plaster residue or dust.
When cracks are larger and due to structural movement, repairs need to be made to the structural system before repairing the plaster. Then, the plaster on each side of the crack should be removed to a width of about 6 inches down to the lath. The debris is cleaned out, and metal lath applied to the cleared area, leaving the existing wood lath in place. The metal lath usually prevents further cracking. The crack is patched with an appropriate plaster in three layers (i.e., base coats and finish coat). If a crack seems to be expanding, a structural engineer should be consulted.
Replacing Delaminated Areas of the Finish Coat
Sometimes the finish coat of plaster comes loose from the base coat. In making this type of repair, the plasterer paints a liquid plaster-bonding agent onto the areas of base-coat plaster that will be replastered with a new lime finish coat. A homeowner wishing to repair small areas of delaminated finish coat can use the methods described in “Patching Materials.”
Patching Holes in Walls
For small holes (less than 4 inches in diameter) that involve loss of the brown and finish coats, the repair is made in two applications. First, a layer of base coat plaster is troweled in place and scraped back below the level of the existing plaster. When the base coat has set but not dried, more plaster is applied to create a smooth, level surface. One-coat patching is not generally recommended by plasterers because it tends to produce concave surfaces that show up when the work is painted. Of course, if the lath only had one coat of plaster originally, then a one-coat patch is appropriate.
For larger holes where all three coats of plaster are damaged or missing down to the wood lath, plasterers generally proceed along these lines. First, all the old plaster is cleaned out and any loose lath is re-nailed. Next, a water mist is sprayed on the old lath to keep it from twisting when the new, wet plaster is applied, or better still, a bonding agent is used. To provide more reliable keying and to strengthen the patch, expanded metal lath (diamond mesh) should be attached to the wood lath with tie wires or nailed over the wood lath with lath nails. The plaster is then applied in three layers over the metal lath, lapping each new layer of plaster over the old plaster so that old and new are evenly joined. This stepping is recommended to produce a strong, invisible patch. Also, if a patch is made in a plaster wall that is slightly wavy, the contour of the patch should be made to conform to the irregularities of the existing work. A flat patch will stand out from the rest of the wall.
Patching Holes in Ceilings
Hairline cracks and holes may be unsightly, but when portions of the ceiling come loose, a more serious problem exists. The keys holding the plaster to the ceiling have probably broken. First, the plaster around the loose plaster should be examined. Keys may have deteriorated because of a localized moisture problem, poor quality plaster, or structural overloading; yet, the surrounding system may be intact. If the areas surrounding the loose area are in reasonably good condition, the loose plaster can be reattached to the lath using flathead wood screws and plaster washers. To patch a hole in the ceiling plaster, metal lath is fastened over the wood lath; then the hole is filled with successive layers of plaster, as described above.
Establishing New Plaster Keys
If the back of the ceiling lath is accessible (usually from the attic or after removing floor boards), small areas of bowed-out plaster can be pushed back against the lath. A padded piece of plywood and braces are used to secure the loose plaster. After dampening the old lath and coating the damaged area with a bonding agent, a fairly liquid plaster mix (with a glue size retardant added) is applied to the backs of the lath, and worked into the voids between the faces of the lath and the back of the plaster. While this first layer is still damp, plaster-soaked strips of jute scrim are laid across the backs of the lath and pressed firmly into the first layer as reinforcement. The original lath must be secure, otherwise the weight of the patching plaster may loosen it.
Loose, damaged plaster can also be re-keyed when the goal is to conserve decorative surfaces or wallpaper. Large areas of ceilings and walls can be saved. This method requires the assistance of a skilled conservator–it is not a repair technique used by most plasterers. The conservator injects an acrylic adhesive mixture through holes drilled in the face of the plaster (or through the lath from behind, when accessible). The loose plaster is held firm with plywood bracing until the adhesive bonding mixture sets. When complete, gaps between the plaster and lath are filled, and the loose plaster is secure.
Can you fix old plaster walls? ›
Steps for Repairing Plaster Walls
Look for compound, paint flakes, and other debris in the crack and scrape it out. Using the putty knife, apply a thin coat of joint compound to the crack. Start at the top of the crack and drag the compound down, pushing it into the crack as you go.
Repairs and restoration work on a lath and plaster ceiling should only be undertaken by a contractor or plaster craftsman with specialist knowledge of the subject. This ensures that the work is completed to the highest of standards, and prevents possible damage or injury to you or to others attempting the work.How can you tell if a ceiling is lath or plaster? ›
One of the best ways of checking is to get up in your loft and take a look underneath the insulation. You could also lift a floorboard in one of your upstairs bedrooms to check. It isn't too hard to identify a lath and plaster ceiling — there will be many narrow strips of timber coated in a creamy coloured plaster.When did they stop using lath and plaster ceiling? ›
The lath and plaster technique was generally used to finish interior walls and ceilings from the 1700s to the early-to-mid 1900s before it was superseded by modern gypsum plaster and plasterboard.When was asbestos used in plaster? ›
Houses built between 1930 and 1950 may have asbestos as insulation. Asbestos may be present in textured paint and patching compounds used on wall and ceiling joints. Their use was banned in 1977.Is it cheaper to repair plaster or replace with drywall? ›
When building a new house, real plaster walls are more expensive than drywall. But in an old home that already contains plaster, it is almost always more cost effective — and more attractive — for the homeowner to keep the plaster than replace it.How do you fix crumbling plaster walls? ›
However, for larger cracks or cracks where the plaster has been crumbling, dilute some PVA glue in water, then paint the solution into the crack with a brush. The PVA seeps into the plaster making it more solid and more reliable when adding the filler in the next step. Apply some filler with a filling knife.What is behind lath and plaster walls? ›
What Is a Lath and Plaster Wall? Lath and plaster refer to an interior wall construction technique that typically predates the 1940s. Four-foot-long strips of wood lath, typically 1-inch wide, are nailed directly to the open wall studs. The lath is then embedded with three layers of wet plaster.Why do lath and plaster ceilings collapse? ›
Plaster is brittle by nature and will crack at its weakest point under vibration or through water ingress. This weak point is usually at the lugs that wrap around the laths. If this spreads across the ceiling even the horsehair can't support the weight and the ceiling, or sections of it, will sag and then may collapse.What are old plaster ceilings made of? ›
Historical Background. Plasterers in North America have relied on two materials to create their handiwork–lime and gypsum. Until the end of the 19th century, plasterers used lime plaster. Lime plaster was made from four ingredients: lime, aggregate, fiber, and water.
Should I board over a lath and plaster ceiling? ›
To be done properly the lath and plaster ceiling really needs to be pulled down and re-boarded directly on to the ceiling joists with 12.5mm plasterboards.What is under a plaster ceiling? ›
Today most projects will be finished with drywall and then 'skimmed' with a thin layer of plaster to create a totally smooth surface. You can plaster a ceiling in the traditional way or you can short-cut it by applying plaster to existing drywall (as above).Is it worth removing lath and plaster? ›
Taking them out isn't going to add to the cost of your project. And when it's time to reinstall the walls, it's much cheaper to go with drywall rather than plaster. In fact, choosing to reinstall plaster walls would up the cost of your project significantly.How thick is the plaster on lath and plaster walls? ›
A traditional 3-coat plaster is typically 7/8″ thick and when you add in the 1/4″ wood lath that supports the plaster wall, you have a wall that is more than 1″ thick! Compared to today's most common drywall thickness of only 1/2″, that is a difference worth noting.Are lath and plaster walls load bearing? ›
A stud or partition wall, built with either plasterboard, or lath and plaster, is rarely constructed as a load-bearing structure. There are however exceptions to this – a stud wall may still help strengthen the structure of a building even though it may not technically be load-bearing (particularly in older homes).How can you tell if a wall is lath or plaster? ›
If the pin pokes into the wall easily, that's drywall. If it doesn't, then that's plaster. A pushpin can penetrate drywalls easily because they're softer compared to plaster. Meanwhile, lath and plaster walls won't even budge with a thumbtack unless you use a hammer.Is it OK to drywall over lath? ›
Lath and plaster walls can be updated to drywall by covering them, or replacing them altogether. You can cover the old plaster by installing drywall panels on top. This is the simplest and least messy way to go, but not necessarily the most efficient.What is a concern about lath and plaster? ›
Plaster can break away or crack along the walls because it is coated on top of lath, instead of directly to studs inside walls. Sometimes due to poor workmanship and inappropriate hand application of plaster through the spaces inside the wall, the plaster does not stay in place.How do I know if my plaster walls have asbestos? ›
The misuse and application of older, banned plaster building materials can also create risk for asbestos exposure. Hiring a professional to inspect the property is the only way to be 100% sure if asbestos is present or not.How do I know if I have asbestos in my walls? ›
It's extremely difficult to identify asbestos just by looking at it, so you need to send samples to a lab for testing. Homeowners can collect samples and have them tested, but it's much safer for you and your family to hire a trained asbestos professional for the job.
Does asbestos degrade after 80 years? ›
Asbestos usually degrades after 80 years.Do plaster walls increase home value? ›
Plaster dries to a very hard finish that is difficult to damage. This makes the home more durable and reduces how much maintenance homeowners must do – which always increases the value.How much does it cost to Replaster a small part of a wall? ›
Plasterwork usually costs between $2 and $10 per square foot. The cost depends on whether you want resurfacing work or not. Expect to pay at least $10 per square foot with resurfacing; otherwise, plastering a wall usually costs between $2 and $5 per square foot.Why does old plaster crumble? ›
Moisture is the single greatest cause of plaster crumbling in the home. You will first notice a fine white powder on the surface of the plaster that is caused when soluble salts move to the surface. Assuming the moisture issue is not dealt with the plaster is, at this point, doomed to crumble.How long does plaster last before cracking? ›
It may come as a surprise, but it can take approximately one to three years for a building to stabilise. During this time, the building or extensions' weight will cause the plaster to move and, in some places, crack.Why does plaster crack in old houses? ›
Cracking plaster is a common problem with older homes. There are several reasons why this happens. Cracks can be the result of thermal movement, the settling of a foundation or sill work, or even exposure to moisture for an extended period of time.What is the wood behind plaster called? ›
plaster on ceilings and walls.Do stud finders work on lath and plaster walls? ›
Lath and plaster walls have an irregular surface that are a problem for stud finders. As a result, your stud finder can show a false positive (a stud indication when it is actually not a stud) when it finds an increase in density, even if it is just a glob of plaster.How much weight can lath and plaster walls hold? ›
Luckily, items can be hung from plaster walls, and in some cases, it's easier to hang things from plaster walls than it is drywall. The wood lath behind the plaster is strong and can support up to 25 lbs of weight, given the screw is driven directly into the wood.How do you fix a pillowing ceiling? ›
- Define the Damaged Area. Draw a rectangle around a patch of sagging drywall with a yardstick and a pencil. ...
- Cut Out the Damaged Drywall. ...
- Cut New Drywall. ...
- Attach the Drywall. ...
- Tape the Seams. ...
- Locate the Joists. ...
- Screw in Furring Strips. ...
- Hang the New Drywall.
Can you skim over lath plaster ceiling? ›
Unless it is of particular merit, taking down a lath and plaster ceiling is often the best way. You can board over it, but you should be aware that there is a lot of weight in these ceilings and the plaster can be up to an inch thick. You really don't want to be underneath it if a chunk of that comes down.Can plaster ceiling be repaired? ›
The good news is, it's possible to repair and rescue plaster ceilings from further damage. We have successfully reattached many old ceilings by injecting adhesive between the plaster and lath where the keys (anchors) have broken away over time.Can you Replaster a ceiling? ›
We all know that plastering can be a tricky job that takes time, effort and lots of practice – and plastering a ceiling can be one of the trickiest jobs to do. However, with the right know-how and tools in place, plastering a ceiling can be completed by plasterers and DIY enthusiasts alike.How do you fix textured plaster ceilings? ›
- Clean the existing textured ceiling.
- Apply a coat of PVA per traditional plastering techniques.
- Apply a thin layer of a lightweight plaster composite to a smooth finish.
Since plaster is considered a higher quality material than drywall anyway, it should not be replaced with drywall in most situations. The one exception is if you're pulling down the walls to replace the plumbing and electrical systems anyway. In that case, it makes sense to replace with drywall.Can I Replaster a wall myself? ›
It is a skill, it takes practice, care, precision and a lot of experience to get it 100% right. That is, however, not to say you can't DIY plastering. Of course, you can, but you should know it's going to take some time to get used to learn the process and get to know what you're doing.What is the cheapest way to redo a ceiling? ›
- Paint. It's a great way to change the perception of your space. ...
- Tin Tiles. Hide damage and uneven surfaces with tin ceiling tiles that introduce a vintage vibe. ...
- Moldings and More. ...
- Tiles or Planks.
Plastering is a job which many people usually leave to the experts. But by preparing well and carefully following a practised method, it's perfectly possible for you to plaster a wall. The finished result will be a nice smooth wall which is ready to accept paint or wallpaper.How much does it cost to smooth a textured ceiling? ›
According to painting experts, popcorn ceiling removal costs range from $1,923 to $3,876 with an average price of $2,899 to hire a professional. The average sqft cost to remove popcorn ceilings ranges from $5 to $9 per square foot. This includes all labor, preparation work, and materials to do the job right.What is behind plaster walls? ›
The wooden lath layer is attached to the studs, and the hardened goop seeping through the lath is the plaster which makes up the wall on the other side of the lath. This is what's behind your plaster wall.
Should I remove old plaster? ›
As long as they're in decent condition (in other words, not falling away from the lath in chunks), you may opt to leave them as they are. For many, plaster walls are a big part of an older home's historical charm, and they're well worth keeping intact.