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The Preservation and Repair
of Historic Stucco
Anne Grimmer
The term "stucco" is used here to describe a type of exterior plaster
applied as a two-or-three part coating directly onto masonry, or applied
over wood or metal lath to a log or wood frame structure.
Stucco is found in many forms on historic structures throughout the
United States. It is so common, in fact, that it frequently goes unnoticed,
and is often disguised or used to imitate another material. Historic
stucco is also sometimes incorrectly viewed as a sacrificial coating,
and consequently removed to reveal stone, brick or logs that historically
were never intended to be exposed. Age and lack of maintenance hasten
the deterioration of many historic stucco buildings. Like most historic
building materials, stucco is at the mercy of the elements, and even
though it is a protective coating, it is particularly susceptible to
water damage.
Stucco is a material
of deceptive simplicity: in most cases its repair should not be undertaken
by a property owner unfamiliar with the art of plastering. Successful
stucco repair requires the skill and experience of a professional plasterer.
Therefore, this Brief has been prepared to provide background information
on the nature and components of traditional stucco, as well as offer
guidance on proper maintenance and repairs. The Brief will outline the
requirements for stucco repair, and, when necessary, replacement. Although
several stucco mixes representative of different periods are provided
here for reference, this Brief does not include specifications for carrying
out repair projects. Each project is unique, with its own set of problems
that require individual solutions.
Historical Background
Stucco has been
used since ancient times. Still widely used throughout the world, it
is one of the most common of traditional building materials (Fig. 1).
Up until the late 1800's, stucco, like mortar, was primarily lime-based,
but the popularization of portland cement changed the composition of
stucco, as well as mortar, to a harder material. Historically, the term
"plaster" has often been interchangeable with "stucco"; the term is
still favored by many, particularly when referring to the traditional
lime-based coating. By the nineteenth century "stucco," although originally
denoting fine interior ornamental plasterwork, had gained wide acceptance
in the United States to describe exterior plastering. "Render" and "rendering"
are also terms used to describe stucco, especially in Great Britain.
Other historic treatments and coatings related to stucco in that they
consist at least in part of a similarly plastic or malleable material
include: parging and pargeting, wattle and daub, "cob" or chalk mud,
pise de terre, rammed earth, briquete entre poteaux or bousillage, half-timbering,
and adobe. All of these are regional variations on traditional mixtures
of mud, clay, lime, chalk, cement, gravel or straw. Many are still used
today.
The Stucco Tradition in the United States
Stucco is primarily
used on residential buildings and relatively small-scale commercial
structures. Some of the earliest stucco buildings in the United States
include examples of the Federal, Greek and Gothic Revival styles of
the eighteenth and the nineteenth centuries that emulated European architectural
fashions. Benjamin Henry Latrobe, appointed by Thomas Jefferson as Surveyor
of Public Buildings of the United States in 1803, was responsible for
the design of a number of important stucco buildings, including St.
John's Church (1816), in Washington, D.C. (Fig. 2). Nearly half a century
later Andrew Jackson Downing also advocated the use of stucco in his
influential book The Architecture of Country Houses, published in 1850.
In Downing's opinion, stucco was superior in many respects to plain
brick or stone because it was cheaper, warmer and dryer, and could be
"agreeably" tinted. As a result of his advice, stuccoed Italianate style
urban and suburban villas proliferated in many parts of the country
during the third quarter of the nineteenth century.
Revival Styles Promote Use of Stucco
The introduction
of the many revival styles of architecture around the turn of the twentieth
century, combined with the improvement and increased availability of
portland cement resulted in a "craze" for stucco as a building material
in the United States. Beginning about 1890 and gaining momentum into
the 1930's and 1940's, stucco was associated with certain historic architectural
styles, including: Prairie; Art Deco, and Art Moderne; Spanish Colonial,
Mission, Pueblo, Mediterranean, English Cotswold Cottage, and Tudor
Revival styles; as well as the ubiquitous bungalow and "four-square"
house (Fig. 3). The fad for Spanish Colonial Revival, and other variations
on this theme, was especially important in furthering stucco as a building
material in the United States during this period, since stucco clearly
looked like adobe (Fig. 4).
Although stucco
buildings were especially prevalent in California, the Southwest and
Florida, ostensibly because of their Spanish heritage, this period also
spawned stucco-coated, revival-style buildings all over the United States
and Canada. The popularity of stucco as a cheap, and readily available
material meant that by the 1920's, it was used for an increasing variety
of building types. Resort hotels, apartment buildings, private mansions
and movie theaters, railroad stations, and even gas stations and tourist
courts took advantage of the "romance" of period styles, and adopted
the stucco construction that had become synonymous with these styles
(Fig. 5).
A Practical Building Material
Stucco has traditionally
been popular for a variety of reasons. It was an inexpensive material
that could simulate finely dressed stonework, especially when "scored"
or "lined" in the European tradition. A stucco coating over a less finished
and less costly substrate such as rubblestone, fieldstone, brick, log
or wood frame, gave the building the appearance of being a more expensive
and important structure. As a weather-repellent coating, stucco protected
the building from wind and rain penetration, and also offered a certain
amount of fire protection. While stucco was usually applied during construction
as part of the building design, particularly over rubblestone or fieldstone,
in some instances it was added later to protect the structure, or when
a rise in the owner's social status demanded a comparable rise in his
standard of living.
Composition of Historic Stucco
Before the mid-to-late
nineteenth century, stucco consisted primarily of hydrated or slaked
lime, water and sand, with straw or animal hair included as a binder.
Natural cements were frequently used in stucco mixes after their discovery
in the United States during the 1820's. Portland cement was first manufactured
in the United States in 1871, and it gradually replaced natural cement.
After about 1900, most stucco was composed primarily of portland cement,
mixed with some lime. With the addition of portland cement, stucco became
even more versatile and durable. No longer used just as a coating for
a substantial material like masonry or log, stucco could now be applied
over wood or metal lath attached to a light wood frame. With this increased
strength, stucco ceased to be just a veneer and became a more integral
part of the building structure.
Today, gypsum,
which is hydrated calcium sulfate or sulfate of lime, has to a great
extent replaced lime Gypsum is preferred because it hardens faster and
has less shrinkage than lime. Lime is generally used only in the finish
coat in contemporary stucco work.
The composition
of stucco depended on local custom and available materials. Stucco often
contained substantial amounts of mud or clay, marble or brick dust,
or even sawdust, and an array of additives ranging from animal blood
or urine, to eggs, keratin or gluesize (animal hooves and horns), varnish,
wheat paste, sugar, salt, sodium silicate, alum, tallow, linseed oil,
beeswax, and wine, beer, or rye whiskey. Waxes, fats and oils were included
to introduce water-repellent properties, sugary materials reduced the
amount of water needed and slowed down the setting time, and alcohol
acted as an air entrainer. All of these additives contributed to the
strength and durability of the stucco.
The appearance
of much stucco was determined by the color of the sand--or sometimes
burnt clay, used in the mix, but often stucco was also tinted with natural
pigments, or the surface whitewashed or color-washed after stuccoing
was completed. Brick dust could provide color, and other coloring materials
that were not affected by lime, mostly mineral pigments, could be added
to the mix for the final finish coat. Stucco was also marbled or marbleized--stained
to look like stone by diluting oil of vitriol (sulfuric acid) with water,
and mixing this with a yellow ochre, or another color (Fig. 6). As the
twentieth century progressed, manufactured or synthetic pigments were
added at the factory to some prepared stucco mixes.
Methods of Application
Stucco is applied
directly, without lath, to masonry substrates such as brick, stone,
concrete or hollow tile (Fig. 7). But on wood structures, stucco, like
its interior counterpart plaster, must be applied over lath in order
to obtain an adequate key to hold the stucco. Thus, when applied over
a log structure, stucco is laid on horizontal wood lath that has been
nailed on vertical wood furring strips attached to the logs (Fig. 8).
If it is applied over a wood frame structure, stucco may be applied
to wood or metal lath nailed directly to the wood frame; it may also
be placed on lath that has been attached to furring strips. The furring
strips are themselves laid over building paper covering the wood sheathing
(Fig. 9). Wood lath was gradually superseded by expanded metal lath
introduced in the late-nineteenth and early-twentieth century. When
stuccoing over a stone or brick substrate, it was customary to cut back
or rake out the mortar joints if they were not already recessed by natural
weathering or erosion, and sometimes the bricks themselves were gouged
to provide a key for the stucco. This helped provide the necessary bond
for the stucco to remain attached to the masonry, much like the key
provided by wood or metal lath on frame buildings.
Like interior
wall plaster, stucco has traditionally been applied as a multiple-layer
process, sometimes consisting of two coats, but more commonly as three.
Whether applied directly to a masonry substrate or onto wood or metal
lath, this consists of a first "scratch" or "pricking-up" coat, followed
by a second scratch coat, sometimes referred to as a "floating" or "brown"
coat, followed finally by the "finishing" coat. Up until the late-nineteenth
century, the first and the second coats were of much the same composition,
generally consisting of lime, or natural cement, sand, perhaps clay,
and one or more of the additives previously mentioned. Straw or animal
hair was usually added to the first coat as a binder. The third, or
finishing coat, consisted primarily of a very fine mesh grade of lime
and sand, and sometimes pigment. As already noted, after the 1820's,
natural cement was also a common ingredient in stucco until it was replaced
by portland cement.
Both masonry and
wood lath must be kept wet or damp to ensure a good bond with the stucco.
Wetting these materials helps to prevent them from pulling moisture
out of the stucco too rapidly, which results in cracking, loss of bond,
and generally poor quality
stuccowork.
Traditional Stucco Finishes
Until the early-twentieth
century when a variety of novelty finishes or textures were introduced,
the last coat of stucco was commonly given a smooth, troweled finish,
and then scored or lined in imitation of ashlar. The illusion of masonry
joints was sometimes enhanced by a thin line of white lime putty, graphite,
or some other pigment. Some nineteenth century buildings feature a water
table or raised foundation of roughcast stucco that differentiates it
from the stucco surface above, which is smooth and scored (Fig. 10).
Other novelty or textured finishes associated with the "period" or revival
styles of the early-twentieth century include: the English cottage finish,
adobe and Spanish, pebble-dashed or dry-dash surface, fan and sponge
texture, reticulated and vermiculated, roughcast (or wet dash), and
sgraffito (Fig. 11).
Repairing Deteriorated
Stucco Regular Maintenance
Although A. J.
Downing alluded to stuccoed houses in Pennsylvania that had survived
for over a century in relatively good condition, historic stucco is
inherently not a particularly permanent or long-lasting building material.
Regular maintenance is required to keep it in good condition. Unfortunately,
many older or historic buildings are not always accorded this kind of
care.
Because building
owners knew stucco to be a protective, but also somewhat fragile coating,
they employed a variety of means to prolong its usefulness. The most
common treatment was to whitewash stucco, often annually. The lime in
the whitewash offered protection and stability and helped to harden
the stucco. Most importantly, it filled hairline cracks before they
could develop into larger cracks and let in moisture. To improve water
repellency, stucco buildings were also sometimes coated with paraffin,
another type of wax, or other stucco-like coatings, such as oil mastics.
Assessing Damage
Most stucco deterioration
is the result of water infiltration into the building structure, either
through the roof, around chimneys, window and door openings, or excessive
ground water or moisture penetrating through, or splashing up from the
foundation. Potential causes of deterioration include: ground settlement
lintel and door frame settlement, inadequate or leaking gutters and
downspouts, intrusive vegetation, moisture migration within walls due
to interior condensation and humidity, vapor drive problems caused by
furnace, bathroom and kitchen vents, and rising damp resulting from
excessive ground water and poor drainage around the foundation. Water
infiltration will cause wood lath to rot, and metal lath and nails to
rust, which eventually will cause stucco to lose its bond and pull away
from its substrate.
After the cause
of deterioration has been identified, any necessary repairs to the building
should be made first before repairing the stucco. Such work is likely
to include repairs designed to keep excessive water away from the stucco,
such as roof, gutter, downspout and flashing repairs, improving drainage,
and redirecting rainwater runoff and splash-back away from the building.
Horizontal areas such as the tops of parapet walls or chimneys are particularly
vulnerable to water infiltration, and may require modifications to their
original design, such as the addition of flashing to correct the problem.
Previous repairs
inexpertly carried out may have caused additional deterioration, particularly
if executed in portland cement, which tends to be very rigid, and therefore
incompatible with early, mostly soft lime-based stucco that is more
"flexible." Incompatible repairs, external vibration caused by traffic
or construction, or building settlement can also result in cracks which
permit the entrance of water and cause the stucco to fail (Fig. 12).
Before beginning
any stucco repair, an assessment of the stucco should be undertaken
to determine the extent of the damage, and how much must be replaced
or repaired. Testing should be carried out systematically on all elevations
of the building to determine the overall condition of the stucco. Some
areas in need of repair will be clearly evidenced by missing sections
of stucco or stucco layers. Bulging or cracked areas are obvious places
to begin. Unsound, punky or soft areas that have lost their key will
echo with a hollow sound when tapped gently with a wooden or acrylic
hammer or mallet.
Identifying the Stucco Type
Analysis of the
historic stucco will provide useful information on its primary ingredients
and their proportions, and will help to ensure that the new replacement
stucco will duplicate the old in strength, composition, color and texture
as closely as possible. However, unless authentic, period restoration
is required, it may not be worthwhile, nor in many instances possible,
to attempt to duplicate all of the ingredients (particularly some of
the additives), in creating the new stucco mortar. Some items are no
longer available, and others, notably sand and lime--the major components
of traditional stucco--have changed radically over time. For example,
most sand used in contemporary masonry work is manufactured sand, because
river sand, which was used historically, is difficult to obtain today
in many parts of the country. The physical and visual qualities of manufactured
sand versus river sand, are quite different, and this affects the way
stucco works, as well as the way it looks. The same is true of lime,
which is frequently replaced by gypsum in modern stucco mixes. And even
if identification of all the items in the historic stucco mix were possible,
the analysis would still not reveal how the original stucco was mixed
and applied.
There are, however,
simple tests that can be carried out on a small piece of stucco to determine
its basic makeup. A dilute solution of hydrochloric (muriatic) acid
will dissolve lime-based stucco, but not portland cement. Although the
use of portland cement became common after 1900, there are no precise
cutoff dates, as stuccoing practices varied among individual plasterers,
and from region to region. Some plasterers began using portland cement
in the 1880's, but others may have continued to favor lime stucco well
into the early twentieth century. While it is safe to assume that a
late-eighteenth or early-nineteenth century stucco is lime-based, late-nineteenth
or early-twentieth century stucco may be based on either lime or portland
cement. Another important factor to take into consideration is that
an early lime-stucco building is likely to have been repaired many times
over the ensuing years, and it is probable that at least some of these
patches consist of portland cement.
Planning the Repair
Once the extent
of damage has been determined, a number of repair options may be considered.
Small hairline cracks usually are not serious and may be sealed with
a thin slurry coat consisting of the finish coat ingredients, or even
with a coat of paint or whitewash.
Commercially available
caulking compounds are not suitable materials for patching hairline
cracks. Because their consistency and texture is unlike that of stucco,
they tend to weather differently, and attract more dirt; as a result,
repairs made with caulking compounds may be highly visible, and unsightly
(Fig. 13). Larger cracks will have to be cut out in preparation for
more extensive repair. Most stucco repairs will require the skill and
expertise of a professional plasterer (Fig. 14).
In the interest
of saving or preserving as much as possible of the historic stucco,
patching rather than wholesale replacement is preferable. When repairing
heavily textured surfaces, it is not usually necessary to replace an
entire wall section, as the textured finish, if well-executed, tends
to conceal patches, and helps them to blend in with the existing stucco.
However, because of the nature of smooth-finished stucco, patching a
number of small areas scattered over one elevation may not be a successful
repair approach unless the stucco has been previously painted, or is
to be painted following the repair work. On unpainted stucco such patches
are hard to conceal, because they may not match exactly or blend in
with the rest of the historic stucco surface. For this reason it is
recommended, if possible, that stucco repair be carried out in a contained
or well-defined area, or if the stucco is scored, the repair patch should
be "squared-off" in such a way as to follow existing scoring. In some
cases, especially in a highly visible location, it may be preferable
to restucco an entire wall section or feature. In this way, any differences
between the patched area and the historic surface will not be so readily
apparent.
Repair of historic
stucco generally follows most of the same principles used in plaster
repair. First, all deteriorated, severely cracked and loose stucco should
be removed down to the lath (assuming that the lath is securely attached
to the substrate), or down to the masonry if the stucco is directly
applied to a masonry substrate. A clean surface is necessary to obtain
a good bond between the stucco and substrate. The areas to be patched
should be cleaned of all debris with a bristle brush, and all plant
growth, dirt, loose paint, oil or grease should be removed (Fig. 15).
If necessary, brick or stone mortar joints should then be raked out
to a depth of approximately 5/8" to ensure a good bond between the substrate
and the new stucco.
To obtain a neat
repair, the area to be patched should be squared-off with a butt joint,
using a cold chisel, a hatchet, a diamond blade saw, or a masonry bit.
Sometimes it may be preferable to leave the area to be patched in an
irregular shape which may result in a less conspicuous patch. Proper
preparation of the area to be patched requires very sharp tools, and
extreme caution on the part of the plasterer not to break keys of surrounding
good stucco by "over-sounding" when removing deteriorated stucco. To
ensure a firm bond, the new patch must not overlap the old stucco. If
the stucco has lost its bond or key from wood lath, or the lath has
deteriorated or come loose from the substrate, a decision must be made
whether to try to reattach the old lath, to replace deteriorated lath
with new wood lath, or to leave the historic wood lath in place and
supplement it with modern expanded metal lath. Unless authenticity is
important, it is generally preferable (and easier) to nail new metal
lath over the old wood lath to support the patch. Metal lath that is
no longer securely fastened to the substrate may be removed and replaced
in kind, or left in place, and supplemented with new wire lath.
When repairing
lime-based stucco applied directly to masonry, the new stucco should
be applied in the same manner, directly onto the stone or brick. The
stucco will bond onto the masonry itself without the addition of lath
because of the irregularities in the masonry or those of its mortar
joints, or because its surface has been scratched, scored or otherwise
roughened to provide an additional key. Cutting out the old stucco at
a diagonal angle may also help secure the bond between the new and the
old stucco. For the most part it is not advisable to insert metal lath
when restuccoing historic masonry in sound condition, as it can hasten
deterioration of the repair work. Not only will attaching the lath damage
the masonry, but the slightest moisture penetration can cause metal
lath to rust. This will cause metal to expand, eventually resulting
in spalling of the stucco, and possibly the masonry substrate too.
If the area to
be patched is properly cleaned and prepared, a bonding agent is usually
not necessary. However, a bonding agent may be useful when repairing
hairline cracks, or when dealing with substrates that do not offer a
good bonding surface. These may include dense stone or brick, previously
painted or stuccoed masonry, or spalling brick substrates. A good mechanical
bond is always preferable to reliance on bonding agents. Bonding agents
should not be used on a wall that is likely to remain damp or where
large amounts of salts are present. Many bonding agents do not survive
well under such conditions, and their use could jeopardize the longevity
of the stucco repair.
A stucco mix compatible
with the historic stucco should be selected after analyzing the existing
stucco. It can be adapted from a standard traditional mix of the period,
or based on one of the mixes included here. Stucco consisting mostly
of portland cement generally will not be physically compatible with
the softer, more flexible lime-rich historic stuccos used throughout
the eighteenth and much of the nineteenth centuries. The differing expansion
and contraction rates of lime stucco and portland cement stucco will
normally cause the stucco to crack. Choosing a stucco mix that is durable
and compatible with the historic stucco on the building is likely to
involve considerable trial and error, and probably will require a number
of test samples, and even more if it is necessary to match the color.
It is best to let the stucco test samples weather as long as possible--ideally
one year, or at least through a change of seasons, in order to study
the durability of the mix and its compatibility with the existing stucco,
as well as the weathering of the tint if the building will not be painted
and color match is an important factor. If the test samples are not
executed on the building, they should be placed next to the stucco remaining
on the building to compare the color, texture and composition of the
samples with the original. The number and thickness of stucco coats
used in the repair should also match the original.
After thoroughly
dampening the masonry or wood lath, the first, scratch coat should be
applied to the masonry substrate, or wood or metal lath, in a thickness
that corresponds to the original if extant, or generally about 1/4"
to 3/8". The scratch coat should be scratched or crosshatched with a
comb to provide a key to hold the second coat. It usually takes 2472
hours, and longer in cold weather, for each coat to dry before the next
coat can be applied. The second coat should be about the same thickness
as the first, and the total thickness of the first two coats should
generally not exceed about 5/8". This second or leveling coat should
be roughened using a wood float with a nail protruding to provide a
key for the final or finish coat. The finish coat, about 1/4" thick,
is applied after the previous coat has initially set. If this is not
feasible, the base coat should be thoroughly dampened when the finish
coat is applied later. The finish coat should be worked to match the
texture of the original stucco (Fig. 16).
Colors and Tints for Historic Stucco Repair
The color of most
early stucco was supplied by the aggregate included in the mix--usually
the sand. Sometimes natural pigments were added to the mix, and eighteenth
and nineteenth-century scored stucco was often marbleized or painted
in imitation of marble or granite. Stucco was also frequently coated
with whitewash or a colorwash. This tradition later evolved into the
use of paint, its popularity depending on the vagaries of fashion as
much as a means of concealing repairs. Because most of the early colors
were derived from nature, the resultant stucco tints tended to ne mostly
earth-toned. This was true until the advent of brightly colored stucco
in the early decades of the twentieth century. This was the so-called
"Jazz Plaster" developed by O.A. Malone, the "man who put color into
California," and who founded the California Stone Products Corporation
in 1927. California Stucco was revolutionary for its time as the first
stucco/plaster to contain colored pigment in its pre-packaged factory
mix.
When patching
or repairing a historic stucco surface known to have been tinted, it
may be possible to determine through visual or microscopic analysis
whether the source of the coloring is sand, cement, or pigment. Although
some pigments or aggregates used traditionally may no longer be available,
a sufficiently close color-match can generally be approximately using
sand, natural or mineral pigments, or a combination of these. Obtaining
such a match will require testing and comparing the color of the dried
test samples with the original. Successfully combining pigments in the
dry stucco mix prepared for the finish coat requires considerable skill.
The amount of pigment must be carefully measured for each batch of stucco.
Overworking the mix can make the pigment separate from the lime. Changing
the amount of water added to the mix, or using water to apply the tinted
finish coat, will also affect the color of the stucco when it dries.
Generally, the
color obtained by hand-mixing these ingredients will provide a sufficiently
close match to cover an entire wall or an area distinct enough from
the rest of the structure that the color differences will not be obvious.
However, it may not work for small patches conspicuously located on
a primary elevation, where color differences will be especially noticeable.
In these instances, it may be necessary to conceal the repairs by painting
the entire patched elevation, or even the whole building.
Many stucco buildings
have been painted over the years and will require repainting after the
stucco repairs have been made. Limewash or cement-based paint, latex
paint, or oil-based paint are appropriate coatings for stucco buildings.
The most important factor to consider when repainting a previously painted
or coated surface is that the new paint be compatible with any coating
already on the surface. In preparation for repainting, all loose or
peeling paint or other coating material not firmly adhered to the stucco
must be removed by hand-scraping or natural bristle brushes. The surface
should then be cleaned.
Cement-based paints,
most of which today contain some portland cement and are really a type
of limewash, have traditionally been used on stucco buildings. The ingredients
were easily obtainable. Furthermore, the lime in such paints actually
bonded or joined with the stucco and provided a very durable coating.
In many regions, whitewash was applied annually during spring cleaning.
Modern, commercially available premixed masonry and mineral-based paints
may also be used on historic stucco buildings.
If the structure
must be painted for the first time to conceal repairs, almost any of
these coatings may be acceptable depending on the situation. Latex paint,
for example, may be applied to slightly damp walls or where there is
an excess of moisture, but latex paint will not stick to chalky or powdery
areas. Oil-based, or alkyd paints must be applied only to dry walls;
new stucco must cure up to a year before it can be painted with oil-based
paint.
Contemporary Stucco Products
There are many
contemporary stucco products on the market today. Many of them are not
compatible, either physically or visually, with historic stucco buildings.
Such products should be considered for use only after consulting with
a historic masonry specialist. However, some of these prepackaged tinted
stucco coatings may be suitable for use on stucco buildings dating from
the late-nineteenth or early-twentieth century, as long as the color
and texture are appropriate for the period and style of the building.
While some masonry contractors may, as a matter of course, suggest that
a water-repellent coating be applied after repairing old stucco, in
most cases this should not be necessary, since colorwashes and paints
serve the same purpose, and stucco itself is a protective coating.
Cleaning Historic Stucco Surfaces
Historic stucco
buildings often exhibit multiple layers of paint or limewash. Although
some stucco surfaces may be cleaned by water washing, the relative success
of this procedure depends on two factors: the surface texture of the
stucco, and the type of dirt to be removed. If simply removing airborne
dirt, smooth unpainted stucco, and heavily-textured painted stucco may
sometimes be cleaned using a low-pressure water wash, supplemented by
scrubbing with soft natural bristle brushes, and possibly non-ionic
detergents. Organic plant material, such as algae and mold, and metallic
stains may be removed from stucco using poultices and appropriate solvents.
Although these same methods may be employed to clean unpainted roughcast,
pebble-dash, or any stucco surface featuring exposed aggregate, due
to the surface irregularities, it may be difficult to remove dirt, without
also removing portions of the decorative textured surface. Difficulty
in cleaning these surfaces may explain why so many of these textured
surfaces have been painted.
When Total Replacement is Necessary
Complete replacement
of the historic stucco with new stucco of either a traditional or modern
mix will probably be necessary only in cases of extreme deterioration--
that is, a loss of bond on over 4050 per cent of the stucco surface.
Another reason for total removal might be that the physical and visual
integrity of the historic stucco has been so compromised by prior incompatible
and ill-conceived repairs that patching would not be successful.
When stucco no
longer exists on a building there is more flexibility in choosing a
suitable mix for the replacement. Since compatibility of old and new
stucco will not be an issue, the most important factors to consider
are durability, color, texture and finish. Depending on the construction
and substrate of the building, in some instances it may be acceptable
to use a relatively strong cement-based stucco mortar. This is certainly
true for many late-nineteenth and early-twentieth century buildings,
and may even be appropriate to use on some stone substrates even if
the original mortar would have been weaker, as long as the historic
visual qualities noted above have been replicated. Generally, the best
principle to follow for a masonry building is that the stucco mix, whether
for repair or replacement of historic stucco, should be somewhat weaker
than the masonry to which it is to be applied in order not to damage
the
substrate.
General Guidance for Historic Stucco Repair
A skilled professional
plasterer will be familiar with the properties of materials involved
in stucco repair and will be able to avoid some of the pitfalls that
would hinder someone less experienced. General suggestions for successful
stucco repair parallel those involving restoration and repair of historic
mortar or plaster. In addition, the following principles are important
to remember:
- Mix only as
much stucco as can be used in one and one-half to two hours. This
will depend on the weather (mortar will harden faster under hot and
dry, or sunny conditions); and experience is likely to be the best
guidance. Any remaining mortar should be discarded; it should not
be retempered.
- Stucco mortar
should not be over-mixed. (Hand mix for 1015 minutes after adding
water, or machine mix for 34 minutes after all ingredients are in
mixer.) Over-mixing can cause crazing and discoloration, especially
in tinted mortars. Over-mixing will also tend to make the mortar set
too fast, which will result in cracking and poor bonding or keying
to the lath or masonry substrate.
- Wood lath or
a masonry substrate, but not metal lath, must be thoroughly wetted
before applying stucco patches so that it does not draw moisture out
of the stucco too rapidly. To a certain extent, bonding agents also
serve this same purpose. Wetting the substrate helps retard drying.
- To prevent
cracking, it is imperative that stucco not dry too fast. Therefore,
the area to be stuccoed should be shaded, or even covered if possible,
particularly in hot weather. It is also a good idea in hot weather
to keep the newly stuccoed area damp, at approximately 90 per cent
humidity, for a period of 48 to 72 hours.
- Stucco repairs,
like most other exterior masonry work, should not be undertaken in
cold weather (below 40 degrees Fahrenheit, and preferably warmer),
or if there is danger of frost.
Historic Stucco Textures
Most of the oldest
stucco in the U.S. dating prior to the late-nineteenth century, will
generally have a smooth, troweled finish (sometimes called a sand or
float finish), possibly scored to resemble ashlar masonry units. Scoring
may be incised to simulate masonry joints, the scored lines may be emphasized
by black or white penciling, or the lines may simply be drawn or painted
on the surface of the stucco. In some regions, at least as early as
the first decades of the nineteenth century, it was not uncommon to
use a roughcast finish on the foundation or base of an otherwise smooth-surfaced
building (Fig. a). Roughcast was also used as ah overall stucco finish
for some outbuildings, and other less important types of structures.
A wide variety
of decorative surface textures may be found on revival style stucco
buildings, particularly residential architecture. These styles evolved
in the late-nineteenth century and peaked in popularity in the early
decades of the twentieth century. Frank Lloyd Wright favored a smooth
finish stucco, which was imitated on much of the Prairie style architecture
inspired by his work. Some of the more picturesque surface textures
include: English Cottage or English Cotswold finish; sponge finish (Fig.
b); fan texture; adobe finish (Fig. c), and Spanish or Italian finish.
Many of these finishes and countless other regional and personalized
variations on them are still in use.
The most common
early-twentieth century stucco finishes are often found on bungalow-style
houses, and include: spatter or spatterdash (sometimes called roughcast,
harling, or wetdash), and pebble-dash or drydash. The spatterdash finish
is applied by throwing the stucco mortar against the wall using a whisk
broom or a stiff fiber brush, and it requires considerable skill on
the part of the plasterer to achieve a consistently rough wall surface.
The mortar used to obtain this texture is usually composed simply of
a regular sand, lime, and cement mortar, although it may sometimes contain
small pebbles or crushed stone aggregate, which replaces one-half the
normal sand content. The pebble-dash or drydash finish is accomplished
manually by the plasterer throwing or "dashing" dry pebbles (about 1/8"
to 1/4" in size), onto a coat of stucco freshly applied by another plasterer.
The pebbles must be thrown at the wall with a scoop with sufficient
force and skill that they will stick to the stuccoed wall. A more even
or uniform surface can be achieved by patting the stones down with a
wooden float. This finish may also be created using a texturing machine
(Figs. df illustrate 3 versions of this finish. Photos: National Park
Service Files).
Summary
Stucco on historic
buildings is especially vulnerable not only to the wear of time and
exposure to the elements, but also at the hands of well-intentioned
"restorers," who may want to remove stucco from eighteenth and nineteenth
century structures, to expose what they believe to be the original or
more "historic" brick, stone or log underneath. Historic stucco is a
character-defining feature and should be considered an important historic
building material, significant in its own right. While many eighteenth
and nineteenth century buildings were stuccoed at the time of construction,
others were stuccoed later for reasons of fashion or practicality. As
such, it is likely that this stucco has acquired significance over time,
as part of the history and evolution of a building. Thus, even later,
non-historic stucco should be retained in most instances; and similar
logic dictates that new stucco should not be applied to a historic building
that was not stuccoed previously. When repairing historic stucco, the
new stucco should duplicate the old as closely as possible in strength,
composition, color and texture.
Mixes for
Repair of Historic Stucco
Historic stucco
mixes varied a great deal regionally, depending as they did on the availability
of local materials. There are probably almost as many mixes that can
be used for repair of historic stucco as there are historic stucco buildings.
For this reason it is recommended that at least a rudimentary analysis
of the existing historic stucco be carried out in order to determine
its general proportions and primary ingredients. However, if this is
not possible, or if test results are inconclusive, the following mixes
are provided as reference. Many of the publications listed under "Selected
Reading" include a variety of stucco mixes and should also be consulted
for additional guidance.
Materials Specifications
should conform to those contained in Preservation Briefs 2: Repointing
Mortar Joints in Historic Brick Buildings, and are as follows:
- Lime should
conform to ASTM C207, Type S, Hydrated Lime for Masonry Purposes.
- Sand should
conform to ASTM C144 to assure proper gradation and freedom from impurities.
Sand, or other type of aggregate, should match the original as closely
as possible.
- Cement should
conform to ASTM C150, Type II (white, nonstaining), portland cement.
- Water should
be fresh, clean and potable.
- If hair or
fiber is used, it should be goat or cattle hair, or pure manilla fiber
of good quality, ‡" to 2" in length, clean, and free of dust,
dirt, oil, grease or other impurities.
- Rules to remember:
More lime will make the mixture more plastic, but stucco mortar with
a very large proportion of lime to sand is more likely to crack because
of greater shrinkage; it is also weaker and slower to set. More sand
or aggregate, will minimize shrinkage, but make the mixture harder
to trowel smooth, and will weaken the mortar.
# # # #
Soft Lime Stucco (suitable for application to buildings dating from 17001850)
A.J. Downing's
Recipe for Soft Lime Stucco
1 part lime
2 parts sand
(A.J. Downing,
"The Architecture of Country Houses," 1850)
Vieux Carre Masonry
Maintenance Guidelines
Base Coats (2):
1 part by volume
hydrated lime
3 parts by volume
aggregate [sand]--size to match original
6 pounds/cubic
yards hair or fiber
Water to form
a workable mix,
Finish Coat:
1 part by volume
hydrated lime
3 parts aggregate
[sand]--size to match original
Water to form
a workable mix.
Note: No portland
cement is recommended in this mix, but if it is needed to increase the
workability of the mix and to decrease the setting time, the amount
of portland cement added should never exceed 1 part to 12 parts lime
and sand.
("Vieux Carre
Masonry Maintenance Guidelines," June, 1980.)
"Materials for
Soft Brick Mortar and for Soft Stucco"
5 gallons hydrated
lime
10 gallons sand
1 quart white,
nonstaining portland cement (1 cup only for pointing)
Water to form
a workable mix.
(Koch and Wilson,
Architects, New Orleans, Louisiana, February, 1980)
Mix for Repair
of Traditional Natural Cement or Hydraulic Lime Stucco
1 part by volume
hydrated lime
2 parts by volume
white portland cement
3 parts by volume
fine mason's sand
If hydraulic lime
is available, it may be used instead of lime-cement blends. ("Conservation
Techniques for the Repair of Historical Ornamental Exterior Stucco,
January, 1990)
Early twentieth century Portland Cement Stucco
1 part portland
cement
2-1/2 parts sand
Hydrated lime
= to not more than 15% of the cement's volume
Water to form
a workable mix.
The same basic
mix was used for all coats, but the finish coat generally contained
more lime than the undercoats. ("Illinois Preservation Series No. 2:
Stucco," January, 1980)
American Portland Cement Stucco Specifications (c. 1929)
Base Coats:
5 pounds, dry,
hydrated lime
1 bag portland
cement (94 lbs.)
Not less than
3 cubic feet (3 bags) sand (passed through a #8 screen)
Water to make
a workable mix.
Finish Coat:
Use WHITE portland
cement in the mix in the same proportions as above.
To color the stucco
add not more than 10 pounds pigment for each bag of cement contained
in the mix.
Selected Reading
Ashurst John,
and Nicola Ashurst. Practical Building Conservation, English Heritage
Technical Handbook, Volume 3. Mortars, Plasters and Renders. New York:
Halsted Press, 1988
Conway, Brian
D. Illinois Preservation Series Number 2: Stucco. Springfield, IL: Illinois
Department of Conservation, Division of Historic Sites, 1980.
Grimmer, Anne
E Keeping it Clean: Removing Exterior Dirt, Paint, Stains and Graffiti
from Historic Masonry Buildings. Washington, D.C.: National Park Service,
U.S. Department of the Interior, 1988.
Hodgson, Frederick
T. Plaster and Plastering. Mortars and Cements, How to Make, and How
to Use . . . with An Illustrated Glossary of Terms. New York: The Industrial
Publication Company, 1901.
Johnson, LeRoy,
Jr. (editor). Handbook of Maintenance Techniques for Building Conservation;
in the Strand Historic District, Galveston, Texas. (Revised edition
originally published in 1980 as Preservation Maintenance Handbook, prepared
by Michael Emrick, AIA, for the Galveston Historical Foundation.) Austin,
TX: Texas Historical Commission, 1984.
Jowers, Walter.
"Bungalow Building Materials: How to Repair Stucco." The Old-House Journal.
Vol. XIII, No. 4 (May 1985), pp. 8083.
MacDonald, Marylee.
Preservation Briefs 21: Repairing Historic Flat Plaster-Walls and Ceilings.
Washington, D.C.: National Park Service, U.S. Department of the Interior,
1989.
Mack, Robert C.,
AIA, de Teel Patterson Tiller, and James S. Askins. Preservation Briefs
2: Repointing Mortar Joints in Historic Brick Buildings. Washington,
D.C.: National Park Service, U.S. Department of the Interior, 1980.
McKee, Harley
J., FAIA. Introduction to Early American Masonry--Stone, Brick, Mortar
and Plaster Washington, D.C.: National Trust for Historic Preservation
and Columbia University, 1973.
Matero, Frank
G., Mary Hardy, Antonio Rava and Joel Snodgrass. Conservation Techniques
for the Repair of Historical Ornamental Exterior Stucco. (With a Case
Study for the Repair of the Cabildo Pedimental Sculpture). Report prepared
for the Division of Historic Preservation, Office of Cultural Development,
Louisiana Department of Culture, Recreation and Development by The Center
for Preservation Research, Columbia University, New York. January 1990.
Portland Cement
Plaster (Stucco) Manual. Skokie, IL: Portland Cement Association, 1980.
Van Den Branden,
F., and Thomas L. Hartsell. Plastering Skills. Second edition. Homewood,
IL: American Technical Publishers, Inc., 1984.
Vieux Carre Masonry
Maintenance Guidelines. Revised from the initial report prepared by
Mary L. Oehrlein in 1977. New Orleans, LA: Vieux Carre Commission, 1980.
Whitewash &
Coldwater Paints. Bulletin No. 304G. Washington, D.C.: National Lime
Association, 1955.
Worsham, Gibson.
"Exterior Plaster Restoration at the Lord Morton House, Lexington, Kentucky."
Association for Preservation Technology Bulletin. Vol. XIII, No. 4 (1981),
pp. 2733.
Acknowledgements
The author gratefully
acknowledges the technical expertise contributed to the preparation
of this publication by Gilbert Wolf, National Plastering Industries;
Walter Jowers; Brian Conway, Michigan Bureau of History; and master
plasterer, Lawrence Ring, Sr. In addition, invaluable comments were
provided by Michael Auer, Charles Fisher, Lauren Meier, Sharon Park,
and Kay Weeks, professional staff of the Technical Preservation Services
Branch,
National Park
Service; professional staff of the Cultural Resources program. Mid-Atlantic
Regional Office, National Park Service; and S. Elizabeth Sasser of the
Williamsport Preservation Training Center, National Park Service.
Washington, D.C.
October 1990
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