.~~.~- ~~~ PRESERVATION 

BRIEFS 

I . . " . 

Repairing Historic Flat PlasterWalls and Ceilings 

Marylee MacDonald 

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u.s. Department of the Interior National Park Service 

Preservation Assistance Division Technical Preservation Services 

Plaster in a historic building is like a family album. The 

handwriting of the artisans, the taste of the original 

occupants, and the evolving styles of decoration are 

embodied in the fabric of the building. From modest 

farmhouses to great buildings, regardless of the ethnic 

origins of the occupants, plaster has traditionally been 

used to finish interior walls. 

A versatile material, plaster could be applied over 

brick, stone, half-timber, or frame construction. It provided a durable surface that was easy to clean and that 

could be applied to flat or curved walls and ceilings. 

Plaster could be treated in any number of ways: it 

could receive stenciling, decorative painting, wallpaper, 

or whitewash. This variety and the adaptability of the 

material to nearly any building size, shape, or configuration meant that plaster was the wall surface chosen 

for nearly all buildings until the 1930s or 40s (Fig. 1). 

Historic plaster may first appear so fraught with problems that its total removal seems the only alternative. 

But there are practical and historical reasons for saving 

it. First, three-coat plaster is unmatched in strength 

Fig. 1. Left: Schifferstadt, Frederick, Maryland, 1756. Right: First Christian Church, Eugene, Oregon, 1911. Although these two structures are 

separated in history /Jy over 150 years and differences in size, ethnic origin, geography, construction techniques, and architectural character, their 

builders both used plaster as the interior surface coating for flat and curved walls. Photo left: Kay Weeks. Photo right: Kaye Ellen Simonson. 

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and durability. It resists fire and reduces sound transmission. Next, replacing plaster is expensive. A building owner needs to think carefully about the condition 

of the plaster that remains; plaster is often not as badly 

damaged as it first appears. Of more concern to preservationists, however, original lime and gypsum plaster 

is part of the building's historic fabric-its smoothtroweled or textured surfaces and subtle contours 

evoke the presence of America's earlier craftsmen. Plaster can also serve as a plain surface for irreplaceable 

decorative finishes. For both reasons, plaster walls and 

ceilings contribute to the historic character of the interior and should be left in place and repaired if at all 

possible (Fig. 2). 

Fig. 2. A hole in the wall of a 1760s Custom House in Chestertown, 

Maryland illustrates the evolution of the room. (a) The original 

plaster was applied directly to an exterior masonry wall and the 

chairrail (missing here, see arrow) was in place before the wet pIaster was applied to the wall. Sometime later when the interior was 

modified, the masonry was furred out. Machine-sawn wood lath (b) 

was nailed to the furring strips and (c) new three-coat plaster was 

applied. Photo: Maryland Historical Trust. 

The approaches described in this Brief stress repairs 

using wet plaster, and traditional materials and techniques that will best assist the preservation of historic 

plaster walls and ceilings-and their appearance. Dry 

wall repairs are not included here, but have been written about extensively in other contexts. Finally, this 

Brief describes a replacement option when historic 

plaster cannot be repaired. Thus, a veneer plaster system is discussed rather than dry wall. Veneer systems 

include a coat or coats of wet plaster-although thinly 

applied-which can, to a greater extent, simulate traditional hand-troweled or textured finish coats. This system is generally better suited to historic preservation 

projects than dry wall. 

To repair plaster, a building owner must often enlist the 

help of a plasterer. Plastering is a skilled craft, requiring years of training and special tools (Fig. 3). While 

minor repairs can be undertaken by building owners, 

most repairs will require the assistance of a plasterer. 

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Fig. 3. Many of these traditional plastering tools are still used 

today: (a) screen to separate coarse sand from fine sand; (b) lime 

screen to remove unslaked particles of lime; (c) hoe; (d) shovel; (e) 

hawk to hold small amounts of plaster; (f) angle float to apply finishes to inside angles; (g), (h), (i) assorted trowels to apply basecoats and finish coat; (j) padded float to level off humps and fill in 

hollows caused by other tools; (k) a two-handled float or "darby" to 

float larger surfaces; (I) a simple straight edge; (m) a square to test 

the trueness of angles; (n) plumb to check verticality of plastered 

surfaces; (0), (p), (q), (r) jointing and mitering tools to pick out 

angles in decorative moldings; (s) comb made of sharpened lath 

pieces to scratch the basecoat of plaster; (t) brush to dampen plaster 

surfaces while they are worked smooth; (u) template made of wood 

and metal to cut a required outline for a fancy mold. 
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. The lime came from 

ground-and-heated limestone or oyster shells; the aggregate from sand; and the fiber from cattle or hog 

hair. Manufacturing changes at the end of the 19th 

century made it possible to use gypsum as a plastering 

material. Gypsum and lime plasters were used in combination for the base and finish coats during the early 

part of the 20th century; gypsum was eventually favored because it set more rapidly and, initially, had a 

harder finish. 

Not only did the basic plastering material change, but 

the method of application changed also. In early America, the windows, doors, and all other trim were installed before the plaster was applied to the wall (Fig. 

4). Generally the woodwork was prime-painted before 

plastering. Obtaining a plumb, level wall, while working against built-up mouldings, must have been difficult. But sometime in the first half of the 19th century, 

builders began installing wooden plaster "grounds" 

around windows and doors and at the base of the wall. 

Installing these grounds so that they were level and 

plumb made the job much easier because the plasterer 

could work from a level, plumb, straight surface. 

Woodwork was then nailed to the "grounds" after the 

walls were plastered (Fig. 5). Evidence of plaster behind trim is often an aid to dating historic houses, or to 

discerning their physical evolution. 

Fig. 4. The builders of this mid-18th century house installed the 

baseboard moulding first, then applied a mud and horse hair plaster 

(called paling) to the masonry wall. Lime was used for the finish 

plaster. Also shown are the hacking marks which prepared the wall 

for a subsequent layer of plaster. Photo: Kay Weeks. 

Fig. 5 (a). The photo above shows the use of wooden plaster 

"grounds" nailed to the wall studs of the mid-19th century Lockwood House in Harpers Ferry, West Virginia. This allowed the 

plasterer to work flush with the surface of the grounds. Afterwards, 

the carpenter could nail the finish woodwork to the ground, effectively hiding the joint between the plaster and the ground. The trim 

was painted after its installation, leaving a paint outline on the 

plaster. Fig. 5 (b). The photo below shows door trim and mouldings 

in place after the plastering was complete. Photos: Kaye Ellen 

Simonson. 

Lime Plaster 

When building a house, plasterers traditionally mixed 

bags of quick lime with water to "hydrate" or "slake" 

the lime. As the lime absorbed the water, heat was 

given off. When the heat diminished, and the lime and 

water were thoroughly mixed, the lime putty that resulted was used to make plaster. 

When lime putty, sand, water, and animal hair were 

mixed, the mixture provided the plasterer with "coarse 

stuff." This mixture was applied in one or two layers to 

build up the wall thickness. But the best plaster was 

done with three coats. The first two coats made up the 

coarse stuff; they were the scratch coat and the brown 

coat. The finish plaster, called "setting stuff" contained 

a much higher proportion of lime putty, little aggregate, and no fiber, and gave the wall a smooth white 

surface finish. 

Compared to the 3/8-inch-thick layers of the scratch 

and brown coats, the finish coat was a mere 1/8-inch 

thick. Additives were used for various finish qualities. 

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For example, fine white sand was mixed in for a "float 

finish." This finish was popular in the early 1900s. (If 

the plasterer raked the sand with a broom, the plaster 

wall would retain swirl marks or stipples.) Or marble 

dust was added to create a hard-finish white coat 

which could be smoothed and polished with a steel 

trowel. Finally, a little plaster of Paris, or "gauged 

stuf£;' was often added to the finish plaster to accelerate the setting time. 

Although lime plaster was used in this country until 

the early 1900s, it had certain disadvantages. A plastered wall could take more than a year to dry; this delayed painting or papering. In addition, bagged quick 

lime had to be carefully protected from contact with air, 

or it became inert because it reacted with ambient 

moisture and carbon dioxide . Around 1900, gypsum 

began to be used as a plastering material. 

Gypsum Plaster 

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 wicking 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 

Lath provi8.ed 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 non-structural spaced 

strips known as furring strips. Three types of lath can 

be found on historic buildings (Fig. 6). 

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. 

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Fig. 6. Top to bottom: Hand-riven lath, machine-sawn wood lath, 

expanded metal (diamond mesh) lath, and perforated gypsum board 

lath. Profile views of their keying characteristics are shown to the 

right. For plaster repairs or replastering, galvanized metal lath is 

the most reliable in terms of longevity, stability, and proper keying. 

Drawing: Kaye Ellen Simonson. 
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. 

Common Plaster Problems 

When plaster dries, it is a rehl.tively 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 

(Fig. 7). These include: 

• Structural Problems 

• Poor Workmanship 

• Improper Curing 

• Moisture 

Structural Problems 

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. 

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Fig. 7 (a) to (d). A series of photographs taken in different rooms of an early 20th century house in West Virginia reveal a variety of plaster wall 

surface problems, most of which can easily be remedied through sensitive repair: Hairline cracks (a) in an otherwise sound wall can be filled with joint 

compound or patching plaster. The wall can also be canvassed or wallpapered. Stress cracks (b) in plaster over a kitchen door frame can be repaired 

using fiberglass mesh tape and joint compound. Settlement cracks (c) in a bedroom can be similarly repaired. The dark crack at the ;uncture between 

walls, however, may be a structural crack and should be investigated for its underlyinR cause. Moisture damaRe (d) from leakinR plumbin!{ 

on the second floor has damaged both wallpaper and plaster in the dining room. After fixing the leaking pipes, the wall covering and rotted plaster 

will need to be replaced and any holes repaired. Photos: Kay Weeks. 

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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. 

SettlementNibration. 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. 

Poor Workmanship 

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 

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cheaper material. Oversanding can cause the plaster to 

weaken or crumble (Fig. 8). Plaster made from a poorly 

proportioned mix may be more difficult to repair. 

Fig. 8. Too much aggregate (sand) and not enough cementing material (lime or gypsum) in the base coat has made this plaster surface 

weak and crumbly. Besides losing its key with the lath, the layers 

are disintegrating. It will most likely need to be totally removed and 

replaced with all new plaster. Photo: M"rylt't' MllcOo/wld . 

Incompatible basecoats 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 re-done. 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 oversanded 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 ~wo-coat job. 

If a plasterer skimped on materials, the wall may not 

have sufficient plaster thickness to withstand the normal stresses withb a building. The minimum total 

thickness for plaster on gypsum board (rock lath) is 1/2 

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. 

Improper Curing 

Proper temperature and air circulation during curing 

are key factors in a durable plaster job. The ideal temperature for plaster to cure is between 55-70 degrees 

Fahrenheit. However, historic houses were sometimes 

plastered before window sashes were put in. There 

was no way to control temperature and humidity. 

Dryouts, freezing, and sweat~outs. 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 re-set the plaster. If freezing occurred before the plaster had set, the job would simply have to be 

re-done. 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. 

Moisture 

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 splash-back. 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. 

Filling Cracks 

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 re-open 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 quicksetting 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., basecoats and 

finish coat). If a crack seems to be expanding, a structural engineer should be consulted. 

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Replacing Delaminated Areas of the Finish Coat 

Sometimes the finish coat of plaster comes loose from 

the base coat (Fig. 9). 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. 

Fig. 9. The smooth-troweled lime finish coat has delaminated from 

the brown coat underneath. This is another repair that can be undertaken without further loss of the historic plaster. Photo: Marylee 

MacDonald. 

I 

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 basecoat 

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 

(Fig. 10). 

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 (Fig. 11). 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 (Fig. 12). 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. 

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Fig. 10 (a) and (b). In this New Hampshire residence dating from the 1790s, the original plaster was a single coat of lime, sand, and horsehair applied over split lath. A one-coat repair, in this case, is appropriate. To the left: a fiat sheet of galvanized expanded metal lath is placed over the patch 

area and an outline marked with a large soft lumber crayon. The metal lath is then cut to fit the hole and nailed to the lath. To the right: the edges of 

the original plaster and wood lath beneath have been thoroughly soaked with water. A steel trowel is used to apply the plaster in large, rough strokes. 

Finally, it will be scraped and smoothed off. Because only one coat of plaster is used, without a finish coat, a clean butt-joint is made with the original 

plaster. Photos: John Leeke. 

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Fig. 11. Repairs are being made to the historic plaster in an early 

20th century residence in Tennessee. A fairly sizeable hole in threecoat plaster extends to the wood lath. Expanded metal lath has been 

cut to fit the hole, then attached to the wood lath with a tie-wire. 

Two ready-mix gypsum base coats are in the process of being applied. After they set, the finish coat will be smooth-troweled gauged 

lime to match the existing wall. Photo: Walter Jowers. 

The patch is stepped 

so that each new coat 

of plaster laps over 

the others 

2 x 4 studs 

I 

Fig. 12. This explains how a hole in historic plaster is repaired over 

the existing wood lath. First, metal lath ;s secured over the wood 

lath with a tie wire, then the new plaster is applied in three layers, 

"stepped" so that each new coat overlaps the old plaster to create a 

good adhesive bond. Drawing: Kaye Ellen Simonson. 

Patching Holes in Ceilings 

Hairline cracks and holes may be unsightly, but when 

portions of the ceiling come loose, a more serious problem exists (Fig. 13). 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 flat-head wood screws and plaster washers 

(Fig. 14). 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. 

Fig. 13. This beaded ceiling in one of the bedrooms of the 1847 

Lockwood House, Harpers Ferry, West Virginia, is missing portions 

of plaster due to broken keys. This is attributable, in part, to deterioration of the wood lath. Photo: Kaye Ellen Simonson. 

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Fig. 14. In a late 18th century house in Massachusetts, fiat-head 

wood screws and plaster washers were used to reattach loose ceiling 

plaster to the wood lath. After the crack is covered with fiberglass 

mesh tape, both the taped crack and the plaster washers will be 

skim-coated with a patching material. Photo: John Obed Curtis. 

9 
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 (Fig. 15). 

Fig. 15. When ceiling repairs are made with wet plaster or with an 

injected adhesive mixture, the old loose plaster must be supported 

with a plywood brace until re-keying is complete. Photo: John Leeke. 

10 

Replastering Over the Old Ceiling 

If a historic ceiling is too cracked to patch or is sagging 

(but not damaged from moisture), plasterers routinely 

keep the old ceiling and simply relath and rep laster 

over it. This repair technique can be used if lowering 

the ceiling slightly does not affect other ornamental 

features. The existing ceiling is covered with lx3-inch 

wood furring strips, one to each joist, and fastened 

completely through the old lath and plaster using a 

screw gun. Expanded metal lath or gypsum board lath 

is nailed over the furring strips. Finally, two or three 

coats are applied according to traditional methods. 

Replastering over the old ceiling saves time, creates 

much less dust than demolition, and gives added fire 

protection. 

When Damaged Plaster Cannot be 

Repaired-Replacement Options 

Partial or complete removal may be necessary if plaster 

is badly damaged, particularly if the damage was 

caused by long-term moisture problems. Workers undertaking demolition should wear OSHA-approved 

masks because the plaster dust that flies into the air 

may contain decades of coal soot. Lead, from leadbased paint, is another danger. Long-sleeved clothing 

and head-and-eye protection should be worn. Asbestos, used in the mid-twentieth century as an insulating 

and fireproofing additive, may also be present and 

OSHA-recommended precautions should be taken. If 

plaster in adjacent rooms is still in good condition, 

walls should not be pounded-a small trowel or pry 

bar is worked behind the plaster carefully in order to 

pry loose pieces off the wall. 

When the damaged plaster has been removed, the 

owner must decide whether to replaster over the existing lath or use a different system. This decision should 

be based in part on the thickness of the original plaster 

and the condition of the original lath. Economy and 

time are also valid considerations. It is important to 

ensure that the wood trim around the windows and 

doors will have the same "reveal" as before. (The "reveal" is the projection of the wood trim from the surface of the plastered wall). A lath and plaster system 

that will give this required depth should be selected. 

Replastering-Alternative Lath Systems for New 

Plaster 

Replastering old wood lath. When plasterers work with 

old lath, each lath strip is re-nailed and the chunks of 

old plaster are cleaned out. Because the old lath is dry, 

it must be thoroughly soaked before applying the base 

coats of plaster, or it will warp and buckle; furthermore, because the water is drawn out, the plaster will 

fail to set properly. As noted earlier, if new metal lath is 

installed over old wood lath as the base for new plaster, 
many of these problems can be avoided and the histo!ic lath can be retained (Fig. 16). The ceiling should 

still be sprayed unless a vapor barrier is placed behind 

the metal lath. 

Replastering over new metal lath. An alternative to reusing the old w.ood lath is to i~stall a different lathing 

system. Galvamzed metal lath IS the most expensive, 

~)Ut also the most reliable in terms of longevity, stabilIty, and proper keying. When lathing over open joists, 

the plasterer should cover the joists with kraft paper or 

a polyet~ylene vapo~ barrier. Three coats of wet plaster 

are apphed consecutively to form a solid monolithic 

unit with the lath. The scratch coat keys into the metal 

lath; the second, or brown, coat bonds to the scratch 

coat ~nd bu~ds the thickness; the third, or finish coat, 

consIsts of hme putty and gauging plaster. 

Replastering over new rock lath. It is also possible to 

use rock lath ~s ~ plaster base. Plasterers may need to 

remove the eXIstmg wood lath to maintain the woodwork's reveal. Rock lath is a 16x36-inch, 1/2-inch thick, 

gypsum-core pa~el covered with absorbent paper with 

gypsum crystals m the paper. The crystals in the paper 

bond the wet plaster and anchor it securely. This type 

of lath requires two coats of new plaster-the brown 

coat and the finish coat. The gypsum lath itself takes 

the place of the first, or scratch, coat of plaster. 

Painting New Plaster 

The key to a successful paint job is proper drying of 

the plaster. Historically, lime plasters were allowed to 

cure for at l~ast a year before the walls were painted or 

papered .. WIth modern ventilation, plaster cures in a 

s.hort~r ~Ime; however, fresh gypsum plaster with a 

h~e f~Ish c<;>at should still be perfectly dry before 

pamt IS apphed-or the paint may peel. (Plasterers 

traditionally used the "match test" on new plaster. If a 

match would light by striking it on the new plaster 

surface, the plaster was considered dry.) Today it is 

best to allow new plaster to cure two to three weeks. A 

good alkaline-resistant primer, specifically formulated 

for new plaster, should then be used. A compatible 

latex or oil-based paint can be used for the final coat. 

A Modern Replacement System 

Veneer Plaster. Using one of the traditional lath and 

plaster sys~ems provides the highest quality plaster job. 

H?wever, m some cases, budget and time considerations may lead the owner to consider a less expensive 

replacement alternative. Designed to reduce the cost of 

materi~s, a more recent lath and plaster system is less 

e~pensive than a tw~-or-three coat plaster job, but only 

shghtly more expensIve than drywall. This plaster system is called veneer plaster. 

Fig. 16. In the rest?ration of a ca. 1830s house in Maine, split-board lath has been covered with expanded metal lath in re aration for new 

cNoatt~ oflPplastkerS' Th~s mfi~lthod permits the early lath to be saved while the metal lath, with its superior keying serves as !i/fiorcement Photo' a IOna ar ervlce es. ,. . 

11 
many of these problems can be avoided and the histo!ic lath can be retained (Fig. 16). The ceiling should 

still be sprayed unless a vapor barrier is placed behind 

the metal lath. 

Replastering over new metal lath. An alternative to reusing the old w.ood lath is to i~stall a different lathing 

system. Galvamzed metal lath IS the most expensive, 

~)Ut also the most reliable in terms of longevity, stabilIty, and proper keying. When lathing over open joists, 

the plasterer should cover the joists with kraft paper or 

a polyet~ylene vapo~ barrier. Three coats of wet plaster 

are apphed consecutively to form a solid monolithic 

unit with the lath. The scratch coat keys into the metal 

lath; the second, or brown, coat bonds to the scratch 

coat ~nd bu~ds the thickness; the third, or finish coat, 

consIsts of hme putty and gauging plaster. 

Replastering over new rock lath. It is also possible to 

use rock lath ~s ~ plaster base. Plasterers may need to 

remove the eXIstmg wood lath to maintain the woodwork's reveal. Rock lath is a 16x36-inch, 1/2-inch thick, 

gypsum-core pa~el covered with absorbent paper with 

gypsum crystals m the paper. The crystals in the paper 

bond the wet plaster and anchor it securely. This type 

of lath requires two coats of new plaster-the brown 

coat and the finish coat. The gypsum lath itself takes 

the place of the first, or scratch, coat of plaster. 

Painting New Plaster 

The key to a successful paint job is proper drying of 

the plaster. Historically, lime plasters were allowed to 

cure for at l~ast a year before the walls were painted or 

papered .. WIth modern ventilation, plaster cures in a 

s.hort~r ~Ime; however, fresh gypsum plaster with a 

h~e f~Ish c<;>at should still be perfectly dry before 

pamt IS apphed-or the paint may peel. (Plasterers 

traditionally used the "match test" on new plaster. If a 

match would light by striking it on the new plaster 

surface, the plaster was considered dry.) Today it is 

best to allow new plaster to cure two to three weeks. A 

good alkaline-resistant primer, specifically formulated 

for new plaster, should then be used. A compatible 

latex or oil-based paint can be used for the final coat. 

A Modern Replacement System 

Veneer Plaster. Using one of the traditional lath and 

plaster sys~ems provides the highest quality plaster job. 

H?wever, m some cases, budget and time considerations may lead the owner to consider a less expensive 

replacement alternative. Designed to reduce the cost of 

materi~s, a more recent lath and plaster system is less 

e~pensive than a tw~-or-three coat plaster job, but only 

shghtly more expensIve than drywall. This plaster system is called veneer plaster. 

Fig. 16. In the rest?ration of a ca. 1830s house in Maine, split-board lath has been covered with expanded metal lath in re aration for new 

cNoatt~ oflPplastkerS' Th~s mfi~lthod permits the early lath to be saved while the metal lath, with its superior keying serves as !i/fiorcement Photo' a IOna ar ervlce es. ,. . 

11 
ftl&il1IIIII ....... 

PSllteu!1s 

~ 

=~ use ready-mix base-coat plaster for 

. y where large holes need to be filled. 

'n1e reaay-mix plaSter contains gypsum and aggregate =per proportions. The plasterer only needs to-add 

~0th4!!r mix plasterers use to patch cracks or small 

for finish-coat repair, is a "high gauge" lime 

peJrterlltWlJlei 50 pen:ent gauging plaster). 

,~=;-:: will a white, sJIiOOth patch. It is 

t suitable surface repairs. 

AJthough property owners cannot duplicate the years aI accumulateQ knowledge and craft sldlls of a pfofes- aiDnal plasterer, there are materials that can be used for 

-yourself repairs. For example, fine cracks can be 

~ 

with an arr-p~se drf.!vaIl joint compound. For 

larger crilckS using fi~ass tape, a homeawner can use a "quick-~" Joint compound. This can~und has a fast ~ time-60, 90, or 120 minutes. Quick-setting joint compound dries because of a 

chemical reaction, not because of water evap()ration. It 

shrinks less than all-purpose joint com~und and has 

much the same worICabillty as ready-mIX base-coat 

plaster. However, because quick-set joint comROunds 

are hard to sand, they shoUld only be used to bed tape 

or to fill large holes. A1l-p~se Joint compound 

should be used as the fuial coat prior to sanding. 

Homeowners may also want to try using a ready-mix 

perlited base-coat plaster for scratch and brown coat 

repair. The plaster can be hand-mixed in small quantities, but ba~ed ready-mix should be protected from 

ambient moisture. A "mill-mixed pre-gauged" lime 

finish coat plaster can also be used by nomeowners. A 

base coat utilizing perlite or other lightweistht aggregates should only De used for makiIig small repatrs 

(less than 4 ft. patches). For large-scare repairs and 

entire room re-plastering, see tile precautions in Table 1 

for using perlite. 

Homeowners may see a material sold as "patching 

plaster" or "plaster of Paris" in hardware stores. This ~ powder cannot be used by itself for plaster repairs. 

It must be combined with lime to create a successful 

patching mixture. 

When using a lime finish coat for any repair, wait 

longer to paint, or use an alkaline-resistant primer. 

TABLE 1 

REPLASTERING 

Selected Plaster Bases/Compatible Basecoats and Finish Coats 

Traditional Plaster Bases Compatible Basecoats Compatible Finish Coats 

OLD WOOD LATH gypsum/sand plaster lime putty/gauging plaster 

gypsum/perlite plaster 2 lime putty/gauging plaster 

METAL LATH ~rsum sand flaster 

hIgh strength 

lime putty/gauging plaster 

gypsum/perlite plaster 2 lime putty/gauging plaster 

GYPSUM (ROCK) LATH PANELS gypsum/sand plaster lime putty/gauging plaster 

gypsum/perlite plaster 2 lime putty/gauging plaster 

UNGLAZED BRICK/CLAY TILE gypsum/perlite plaster 2 lime putty/gauging plaster 

(masonry type) 

Modern Plaster Base Compatible Basecoat Compatible Finish Coat 

GYPSUM CORE VENEER PANELS veneer plaster veneer plaster 

(BLUE BOARD) or 

lime putty/gauging plaster 

I On traditional bases (wood, metal, and rock lath), the thickness of base coat plaster is one of the most important elements of a good plaster job~ Grounds should be set t~ obtain the following minimum 

plaster thicknesses: (1) Over rock lath-1/2" (2) Over brick, clay tile, or other masonry-SIS" (3) Over metal lath, measured from face of lat~ -S S (4) Over wood lath-7/S . In no case should the talh 

laster thickness be less than 1/2". The allowance for the finish coat is approximately 1116" which requires the base coat to be 7/16" for 1/2 ~roun~s. ThlS ~ a trll1l1mum.base coat thlcknes~ on rOCk, at . 

h,e standard for other masonry units and metal lath is 5/8" thick, including the finish. Certain types of construction or fire ratings may req.urre an mcrease m a~ er thlcknes~ (and/or an mcrease ~ the 

psum to aggregate ration) but never a thinner application of plaster than recommended above. Job experience indicates that thin applicahons ~f plaster often evl?en.ce cracking where normal apphca-

~~ns to standard grounds do not. This condition is a direct result of the inability of thin section areas to resist external forces as adequately as thicker, normal applicahons of plaster. 

2 Perlite is a lightweight aggregate often used in gypsum plaster in place of sand. It performs well in cold weather and has a slightly better insulating value. than sand. In a con tr~ction with m~t~ lath, . 

rlite aggregate is not recom.mended in the basecoat except under a sand or "float" finish . ~hen gypsum p:rlit~ basecoats ~ used over .any other base (I.e., w~, rock lat~,' br~c~J ~n~ the ftmsh coat IS 

rae be a "white" finish coat (smooth-troweled gauged lime putty) it is necessary to add fine silica sand or perlIte flOes to the finish coat. This measure prevents cracking of the white finish coat due to 

differential shrinkage. 

13 
ftl&il1IIIII ....... 

PSllteu!1s 

~ 

=~ use ready-mix base-coat plaster for 

. y where large holes need to be filled. 

'n1e reaay-mix plaSter contains gypsum and aggregate =per proportions. The plasterer only needs to-add 

~0th4!!r mix plasterers use to patch cracks or small 

for finish-coat repair, is a "high gauge" lime 

peJrterlltWlJlei 50 pen:ent gauging plaster). 

,~=;-:: will a white, sJIiOOth patch. It is 

t suitable surface repairs. 

AJthough property owners cannot duplicate the years aI accumulateQ knowledge and craft sldlls of a pfofes- aiDnal plasterer, there are materials that can be used for 

-yourself repairs. For example, fine cracks can be 

~ 

with an arr-p~se drf.!vaIl joint compound. For 

larger crilckS using fi~ass tape, a homeawner can use a "quick-~" Joint compound. This can~und has a fast ~ time-60, 90, or 120 minutes. Quick-setting joint compound dries because of a 

chemical reaction, not because of water evap()ration. It 

shrinks less than all-purpose joint com~und and has 

much the same worICabillty as ready-mIX base-coat 

plaster. However, because quick-set joint comROunds 

are hard to sand, they shoUld only be used to bed tape 

or to fill large holes. A1l-p~se Joint compound 

should be used as the fuial coat prior to sanding. 

Homeowners may also want to try using a ready-mix 

perlited base-coat plaster for scratch and brown coat 

repair. The plaster can be hand-mixed in small quantities, but ba~ed ready-mix should be protected from 

ambient moisture. A "mill-mixed pre-gauged" lime 

finish coat plaster can also be used by nomeowners. A 

base coat utilizing perlite or other lightweistht aggregates should only De used for makiIig small repatrs 

(less than 4 ft. patches). For large-scare repairs and 

entire room re-plastering, see tile precautions in Table 1 

for using perlite. 

Homeowners may see a material sold as "patching 

plaster" or "plaster of Paris" in hardware stores. This ~ powder cannot be used by itself for plaster repairs. 

It must be combined with lime to create a successful 

patching mixture. 

When using a lime finish coat for any repair, wait 

longer to paint, or use an alkaline-resistant primer. 

TABLE 1 

REPLASTERING 

Selected Plaster Bases/Compatible Basecoats and Finish Coats 

Traditional Plaster Bases Compatible Basecoats Compatible Finish Coats 

OLD WOOD LATH gypsum/sand plaster lime putty/gauging plaster 

gypsum/perlite plaster 2 lime putty/gauging plaster 

METAL LATH ~rsum sand flaster 

hIgh strength 

lime putty/gauging plaster 

gypsum/perlite plaster 2 lime putty/gauging plaster 

GYPSUM (ROCK) LATH PANELS gypsum/sand plaster lime putty/gauging plaster 

gypsum/perlite plaster 2 lime putty/gauging plaster 

UNGLAZED BRICK/CLAY TILE gypsum/perlite plaster 2 lime putty/gauging plaster 

(masonry type) 

Modern Plaster Base Compatible Basecoat Compatible Finish Coat 

GYPSUM CORE VENEER PANELS veneer plaster veneer plaster 

(BLUE BOARD) or 

lime putty/gauging plaster 

I On traditional bases (wood, metal, and rock lath), the thickness of base coat plaster is one of the most important elements of a good plaster job~ Grounds should be set t~ obtain the following minimu

plaster thicknesses: (1) Over rock lath-1/2" (2) Over brick, clay tile, or other masonry-SIS" (3) Over metal lath, measured from face of lat~ -S S (4) Over wood lath-7/S . In no case should the talh 

laster thickness be less than 1/2". The allowance for the finish coat is approximately 1116" which requires the base coat to be 7/16" for 1/2 ~roun~s. ThlS ~ a trll1l1mum.base coat thlcknes~ on rOCk, at . 

h,e standard for other masonry units and metal lath is 5/8" thick, including the finish. Certain types of construction or fire ratings may req.urre an mcrease m a~ er thlcknes~ (and/or an mcrease ~ the 

psum to aggregate ration) but never a thinner application of plaster than recommended above. Job experience indicates that thin applicahons ~f plaster often evl?en.ce cracking where normal apphca-

~~ns to standard grounds do not. This condition is a direct result of the inability of thin section areas to resist external forces as adequately as thicker, normal applicahons of plaster. 

2 Perlite is a lightweight aggregate often used in gypsum plaster in place of sand. It performs well in cold weather and has a slightly better insulating value. than sand. In a con tr~ction with m~t~ lath, 

rlite aggregate is not recom.mended in the basecoat except under a sand or "float" finish . ~hen gypsum p:rlit~ basecoats ~ used over .any other base (I.e., w~, rock lat~,' br~c~J ~n~ the ftmsh coat 

rae be a "white" finish coat (smooth-troweled gauged lime putty) it is necessary to add fine silica sand or perlIte flOes to the finish coat. This measure prevents cracking of the white finish coat due to 

differential shrinkage. 

13