Repairing Historic Flat Plaster-Walls
and Ceilings
Marylee MacDonald
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 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 smooth troweled 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).
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.
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 moldings, 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.
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. 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 stuff," 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 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 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 (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.
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 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
(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 .
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.
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 cheaper material. Over sanding can cause the plaster to weaken or
crumble (Fig. 8). 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.
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 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.
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 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.
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 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
(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."
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 (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.
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 flathead 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.
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).
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 relate
and replaster 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 1x3-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 lead based
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 historic 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
wood lath is to install a different lathing system. Galvanized metal lath
is the most expensive, but 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 polyethylene vapor barrier.
Three coats of wet plaster are applied 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 and builds the thickness;
the third, or finish coat, consists of lime putty and gauging plaster.
Replastering over new rock lath. It is also possible to use rock lath
as a plaster base. Plasterers may need to remove the existing wood lath
to maintain the woodwork's reveal. Rock lath is a 16x36-inch, 1/2-inch
thick, gypsum-core panel covered with absorbent paper with gypsum crystals
in 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 least a year before the walls
were painted or papered. With modern ventilation, plaster cures in a shorter
time; however, fresh gypsum plaster with a lime finish coat should still
be perfectly dry before paint is applied--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 systems provides
the highest quality plaster job. However, in some cases, budget and time
considerations may lead the owner to consider a less expensive replacement
alternative. Designed to reduce the cost of materials, a more recent lath
and plaster system is less expensive than a two-or-three coat plaster
job, but only slightly more expensive than drywall. This plaster system
is called veneer plaster.
The system uses gypsum-core panels that are the same size as drywall
(4x8 feet), and specially made for veneer plaster. They can be installed
over furring channels to masonry walls or over old wood lath walls and
ceilings. Known most commonly as "blue board," the panels are covered
with a special paper compatible with veneer plaster. Joints between the
4-foot wide sheets are taped with fiberglass mesh, which is bedded in
the veneer plaster. After the tape is bedded, a thin, 1/16-inch coat of
high-strength veneer plaster is applied to the entire wall surface. A
second veneer layer can be used as the "finish" coat, or the veneer plaster
can be covered with a gauged lime finish-coat--the same coat that covers
ordinary plaster (Fig. 17).
Although extremely thin, a two-coat veneer plaster system has a 1,500
psi rating and is thus able to withstand structural movements in a building
or surface abrasion. With either a veneer finish or a gauged lime putty
finish coat, the room will be ready for painting almost immediately. When
complete, the troweled or textured wall surface looks more like traditional
plaster than drywall.
The thin profile of the veneer system has an added benefit, especially
for owners of uninsulated masonry buildings. Insulation can be installed
between the pieces of furring channel used to attach blue board to masonry
walls. This can be done without having to fur out the window and door
jambs. The insulation plus the veneer system will result in the same thickness
as the original plaster. Occupants in the rooms will be more comfortable
because they will not be losing heat to cold wall surfaces.
Patching Materials Plasterers
general use ready-mix base-coat plaster for patching, especially where
large holes need to be filled. The ready-mix plaster contains gypsum and
aggregate in proper proportions. The plasterer only needs to add water.
Another mix plasterers use to patch cracks or small holes, or for finish-coat
repair, is a "high gauge" lime putty (50 percent lime; 50 percent gauging
plaster). This material will produce a white, smooth patch. It is especially
suitable for surface repairs.
Although property owners cannot duplicate the years of accumulated knowledge
and craft skills of a professional plasterer, there are materials that
can be used for do-it-yourself repairs. For example, fine cracks can be
filled with an all-purpose drywall joint compound. For bridging larger
cracks using fiberglass tape, a homeowner can use a "quicksetting" joint
compound. This compound has a fast drying time--60, 90, or 120 minutes.
Quick-setting joint compound dries because of a chemical reaction, not
because of water evaporation. It shrinks less than all-purpose joint compound
and has much the same workability as ready-mix base-coat plaster. However,
because quick-set joint compounds are hard to sand, they should only be
used to bed tape or to fill large holes. All-purpose point compound should
be used as the final 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 bagged ready-mix should be protected from ambient
moisture. A "millmixed pre-gauged" lime finish coat plaster can also be
used by homeowners. A base coat utilizing perlite or other lightweight
aggregates should only be used for making small repairs (less than 4 ft.
patches). For large-scale repairs and entire room replastering, see the
precautions in Table 1 for using perlite.
Homeowners may see a material sold as "patching plaster" or "plaster
of Paris" in hardware stores. This dry 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.
Selected Plaster Bases/Compatible Base-coats and Finish Coats
| Traditional Plaster |
Compatible Base-coats |
Compatible Finish Coats Bases |
| OLD WOOD LATH |
gypsum/sand plaster |
lime putty/gauging plaster |
| |
gypsum/perlite plaster(2) |
lime putty/gauging plaster |
| METAL LATH |
gypsum/sand plaster
(high strength) |
lime putty/gauging plaster |
| |
gypsum/perlite plaster (2) |
lime putty/gauging plaster |
| GYPSUM (ROCK) LATH |
gypsum/sand plaster |
lime putty/gauging plaster |
| PANELS |
gypsum/perlite plaster(2) |
lime putty/gauging plaster |
| UNGLAZED BRICK/CLAY |
gypsum/perlite plaster(2)
(masonry type) |
lime putty/gauging plaster |
| Modern Plaster Base |
Compatible Base-coat |
Compatible Finish Coat |
| GYPSUM CORE VENEER |
veneer plaster |
veneer plaster or |
| PANELS (BLUE BOARD) |
lime putty/gauging plaster |
|
NOTES
(1) 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 to obtain the following minimum plaster thicknesses:
(1) Over rock lath -- ‡" (2) Over brick clay tile or other masonry--5/8"
(3) Over metal lath measured from face of lath--5/8" (4) Over wood lath--7/8".
In no case should the total plaster thickness be less than ‡".
The allowance for the finish coat is approximately 1/16" which requires
the base coat Tom be 7/16" for ‡" grounds. This is a minimum base
coat thickness on rock lath. The standard for other masonry units and
metal lath is 5/8" thick including the finish. Certain types of construction
or fire ratings may require an increase in plaster thickness (and/or an
increase in the gypsum to aggregate ration) but never a thinner application
of plaster than recommended above. Job experience indicates that thin
applications of plaster often evidence cracking where normal applications
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 applications 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 construction with metal lath,
perlite aggregate is not recommended in the base coat excerpt under a
sand or "float" finish. When gypsum/perlite base coats are used over any
other base (i.e., wood, rock lath, brick) and the finish coat is to be
a "white" finish coat (smooth-troweled gauged lime putty), it is necessary
to add fine silica sand or perlite fines to the finish coat. This measure
prevents cracking of the "white" finish coat due to differential shrinkage.
Summary The National Park Service
recommends retaining historic plaster if at all possible. Plaster is a
significant part of the "fabric" of the building. Much of the building's
history is documented in the layers of paint and paper found covering
old plaster. For buildings with decorative painting, conservation of historic
flat plaster is even more important. Consultation with the National Park
Service, with State Historic Preservation Officers, local preservation
organizations, historic preservation consultants, or with the Association
for Preservation Technology is recommended. Where plaster cannot be repaired
or conserved using one of the approaches outlined in this Brief, documentation
of the layers of wallpaper and paint should be undertaken before removing
the historic plaster. This information may be needed to complete a restoration
plan.
Plaster Terms Scratch
coat. The first base coat put on wood or metal lath. The wet plaster
is "scratched" with a scarifier or comb to provide a rough surface so
the next layer of base coat will stick to it.
Brown coat. The brown coat is the second application of
wet, base-coat plaster with wood lath or metal systems. With gypsum board
lath (rock lath, plasterboard), it is the only base coat needed.
Finish coat. Pure lime, mixed with about 35 percent gauging
plaster to help it harden, is used for the very thin surface finish of
the plaster wall. Fine sand can be added for a sanded finish coat.
Casing Bead. Early casing bead was made of wood. In the
19th century, metal casing beads were sometimes used around fireplace
projections, and door and window openings. Like a wood ground, they indicate
the proper thickness for the plaster.
Corner Bead. Wire mesh with a rigid metal spline used
on
Outside corners. Installing the corner bead plumb is important.
Cornerite. Wire mesh used on inside corners of adjoining
walls and ceilings. It keeps corners from cracking.
Ground. Plasterers use metal or wood strips around the
edges of doors and windows and at the bottom of walls. These grounds help
keep the plaster the same thickness and provide a stopping edge for the
plaster. Early plaster work, however, did not use grounds. On early buildings,
the woodwork was installed and primed before plastering began. Some time
in the early 19th century, a transition occurred, and plasterers applied
their wall finish before woodwork was installed.
Gypsum. Once mined from large gypsum quarries near Paris
(thus the name plaster of Paris), gypsum in its natural form is calcium
sulfate. When calcined (or heated), one-and-a-half water molecules are
driven off, leaving a hemi-hydrate of calcium sulfate. When mixed with
water, it becomes calcium sulfate again. While gypsum was used in base-coat
plaster from the 1890s on, it has always been used in finish coat and
decorative plaster. For finish coats, gauging plaster was added to lime
putty; it causes the lime to harden. Gypsum is also the ingredient in
moulding plaster, a finer plaster used to create decorative moldings in
ornamental plaster work.
Lime. Found in limestone formations or shell mounds, naturally
occurring lime is calcium carbonate. When heated, it becomes calcium oxide.
After water has been added, it becomes calcium hydroxide. This calcium
hydroxide reacts with carbon dioxide in the air to recreate the original
calcium carbonate.
Screed. Screeds are strips of plaster run vertically or
horizontally on walls or ceilings. They are used to plumb and straighten
uneven walls and level ceilings. Metal screeds are used to separate different
types of plaster finishes or to separate lime and cement plasters.
Reading List Ashurst, John and
Ashurst, Nicola. Practical Building Conservation, English Heritage Technical
Handbook, Volume 3. Mortars, Plasters and Renders. New York: Halsted Press,
1988.
Gypsum Construction Handbook. Chicago: United States Gypsum Company,
1986.
Hodgson, Frederick Thomas. Plaster and Plastering: Mortars and Cements,
How to Make and How to Use. New York: The Industrial Publication Company,
1901
Jowers, Walter. "Plaster Patching, Part Il." Restoration Primer. New
England Builder, November, 1987, pp. 4143.
Leeke, John. "Problems with Plaster, Part One." Landmarks Observer,
Vol. 12. March/April, 1985., pp. 10,14. Also "Problems with Plaster, Part
Two." Vol. 12., May/June, 1985, p. 12.
Leeke, John. "Saving Irreplaceable Plaster." Old House Journal. Vol.
XV, No. 6, November/December, 1987, pp. 5155.
McKee, Harley J., FAIA. Introduction to Early American MasonryStone,
Brick, Mortar, and Plaster. New York: National Trust for Historic Preservation
and Columbia University. 1973.
Phillips, Morgan. "Adhesives for the Reattachment of Loose Plaster"
A.P.T Bulletin, Vol. XII, No. 2, 1980, pp. 3763.
Poore, Patricia. "The Basics of Plaster Repair." Old House Journal,
Vol. 16, No. 2, March/April, 1988, pp. 2935.
Shivers, Natalie. Walls and Molding: How to Care for Old and Historic
Wood and Plaster. Washington, D.C.: National Trust for Historic Preservation,
1989.
Stagg, W. D. and B. Pegg. Plastering: A Craftsman's Encyclopedia. Woodstock,
New York: Beekman Publishers, 1976.
Van den Branden, F. and Thomas L. Hartsell. Plastering Skills. Homewood,
Illinois: American Technical Publishers, Inc., 1984.
Weaver, Martin. "Nuts and Bolts: Properly Plastered." Canadian Heritage.
Aug./Sept., 1981, pp. 3436. Also "Nuts and Bolts: Fixing Plaster." Oct.,
1981, pp. 3335.
Acknowledgements Preservation Brief
21 was based on an article in Old House Restoration on repairing historic
plaster published by the University of Illinois at UrbanaChampaign, 1984.
Kay D. Weeks, Preservation Assistance Division, Technical Preservation
Services Branch, expanded the article and made substantial contributions
to its development as a Brief. Special thanks go to the technical experts
in the field who reviewed and comment upon the draft manuscript: Andrew
Ladygo (Society for the Preservation of New England Antiquities), David
Flaharty, Gilbert Wolf (National Plastering Industries), Michael Kempster,
and Walter Jowers. Insightful comments were offered by the Technical Preservation
Services Branch which is directed by H. Ward Jandl. Finally, staff member
Karen Kummer, Small Homes CouncilBuilding Research Council, University
of Illinois, provided invaluable production assistance.
| 
