The stucco on the early-19th century Richardson-Owens-Thomas House in Savannah,
Georgia, is a type of natural cement.
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Stucco has been used since ancient times. Still widely used throughout the world,
it is one of the most common of traditional building materials. 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 finish on Arlington House, Arlington, Virginia, was marbleized on
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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 1930s and
1940s, 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. 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.
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 1920s, 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.
The damage to this stucco appears to be caused by moisture infiltration. |
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 1820s. 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.
Caulking is not an appropriate method for repairing cracks in historic stucco.
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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. 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. 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. 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.
The dry materials must be mixed thoroughly before adding water to make the stucco.
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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 1820s, 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. 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.
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.
The deteriorated surface of this catch basin is being re-stuccoed.
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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.
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 1880s, 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. 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.
The stucco will be applied to the wire lath laid over the area to be patched.
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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. 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.
The final finish coat will be applied to this scratch coat.
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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 24-72 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.
Colors and Tints for Historic Stucco Repair
The new addition on the right is stucco scored to imitate the limestone of the
historic building on the left.
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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 color washes 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 40-50 percent 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 10-15 minutes after adding
water, or machine mix for 3-4 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. Roughcast was also used as an overall stucco
finish for some outbuildings, and other less important types of structures.
This stucco house has a rough cast finish. |
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; fan texture; adobe finish; 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.
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.
