Preservation Briefs: 21 Repairing Historic Flat Plaster - Walls And Ceilings
PRESERVATION BRIEFS: 21
REPAIRING HISTORIC FLAT PLASTER - WALLS AND CEILINGS
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original Preservation Brief developed by the National Park Service.
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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 1930's or 40's.
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.
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 drywall. 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. While minor repairs can be undertaken
by building owners, most repairs will require the assistance of a
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
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. 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. Evidence of plaster
behind trim is often an aid to dating historic houses, or to
discerning their physical evolution.
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
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 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
In rehabilitation and restoration projects, one should rely on the
plasterer's judgment about whether to use lime or gypsum plaster.
In general, gypsum plaster is the material plasterers use today.
Different types of aggregate may be specified by the architect such
as clean river sand, perlite, pumice, or vermiculite; however, if
historic finishes and textures are being replicated, sand should be
used as the base-coat aggregate. Today, if fiber is required in a
base coat, a special gypsum is available which includes wood
fibers. Lime putty, mixed with about 35 percent gypsum (gauging
plaster) to help it harden, is still used as the finish coat.
Lath provided a means of holding the plaster in place. Wooden lath
was nailed at right angles directly to the structural members of
the buildings (the joists and studs), or it was fastened to
non-structural spaced strips known as furring strips. Three types
of lath can be found on historic buildings.
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, 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.
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
***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. These include:
- Structural Problems
- Poor Workmanship
- Improper Curing
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
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.
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
Horizontal cracks are often caused by lath movement. Because it
absorbs moisture from the air, wood lath expands and contracts as
humidity rises and falls. This can cause cracks to appear year
after year. Cracks can also appear between rock lath panels. A
nail holding the edge of a piece of lath may rust or loosen, or
structural movement in the wood framing behind the lath may cause
a seam to open. Heavy loads in a storage area above a rock-lath
ceiling can also cause ceiling cracks.
Errors in initial building construction such as improper bracing,
poor corner construction, faulty framing of doors and windows, and
undersized beams and floor joists eventually "telegraph" through to
the plaster surface.
In addition to problems caused by movement or weakness in the
structural framework, plaster durability can be affected by poor
materials or workmanship.
Poorly proportioned mix:
The proper proportioning and mixing of materials are vital to the
quality of the plaster job. A bad mix can cause problems that
appear years later in a plaster wall. Until recently, proportions
of aggregate and lime were mixed on the job. A plasterer may have
skimped on the amount of cementing material (lime or gypsum)
because sand was the cheaper material. Oversanding can cause the
plaster to weaken or crumble. Plaster made from a poorly
proportioned mix may be more difficult to repair.
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 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 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 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 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.
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 reset 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.
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
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
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
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. 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 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.
REPLACING DELAMINATED AREAS OF THE FINISH COAT:
Sometimes the finish coat of plaster comes loose from the base
coat. In making this type of repair, the plasterer paints a liquid
plaster-bonding agent onto the areas of base-coat plaster that will
be replastered with a new lime finish coat. A homeowner wishing to
repair small areas of delaminated finish coat can use the methods
described in Patching Materials.
PATCHING HOLES IN WALLS:
For small holes (less than 4 inches in diameter) that involve loss
of the brown and finish coats, the repair is made in two
applications. First, a layer of 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.
For larger holes where all three coats of plaster are damaged or
missing down to the wood lath, plasterers generally proceed along
these lines. First, all the old plaster is cleaned out and any
loose lath is re-nailed. Next, a water mist is sprayed on the old
lath to keep it from twisting when the new, wet plaster is applied,
or better still, a bonding agent is used. To provide more reliable
keying and to strengthen the patch, expanded metal lath (diamond
mesh) should be attached to the wood lath with tie wires or nailed
over the wood lath with lath nails. The plaster is then applied in
three layers over the metal lath, lapping each new layer of plaster
over the old plaster so that old and new are evenly joined. This
stepping is recommended to produce a strong, invisible patch.
Also, if a patch is made in a plaster wall that is slightly wavy,
the contour of the patch should be made to conform to the
irregularities of the existing work. A flat patch will stand out
from the rest of the wall.
PATCHING HOLES IN CEILINGS:
Hairline cracks and holes may be unsightly, but when portions of
the ceiling come loose, a more serious problem exists. The keys
holding the plaster to the ceiling have probably broken. First,
the plaster around the loose plaster should be examined. Keys may
have deteriorated because of a localized moisture problem, poor
quality plaster, or structural overloading; yet, the surrounding
system may be intact. If the areas surrounding the loose area are
in reasonably good condition, the loose plaster can be reattached
to the lath using flat-head wood screws and plaster washers. To
patch a hole in the ceiling plaster, metal lath is fastened over
the wood lath; then the hole is filled with successive layers of
plaster, as described above.
ESTABLISHING NEW PLASTER KEYS:
If the back of the ceiling lath is accessible (usually from the
attic or after removing floor boards), small areas of bowed-out
plaster can be pushed back against the lath. A padded piece of
plywood and braces are used to secure the loose plaster. After
dampening the old lath and coating the damaged area with a bonding
agent, a fairly liquid plaster mix (with a glue size retardant
added) is applied to the backs of the lath, and worked into the
voids between the faces of the lath and the back of the plaster.
While this first layer is still damp, plaster-soaked strips of jute
scrim are laid across the backs of the lath and pressed firmly into
the first layer as reinforcement. The original lath must be
secure, otherwise the weight of the patching plaster may loosen it.
Loose, damaged plaster can also be re-keyed when the goal is to
conserve decorative surfaces or wallpaper. Large areas of ceilings
and walls can be saved. This method requires the assistance of a
skilled conservator--it is not a repair technique used by most
plasterers. The conservator injects an acrylic adhesive mixture
through holes drilled in the face of the plaster (or through the
lath from behind, when accessible). The loose plaster is held firm
with plywood bracing until the adhesive bonding mixture sets. When
complete; gaps between the plaster and lath are filled, and the
loose plaster is secure.
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 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 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
***WHEN DAMAGED PLASTER CANNOT BE REPAIRED--REPLACEMENT
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
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. 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:
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 "blueboard,"
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.
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