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The Cleaning and Waterproof Coating
of Masonry Buildings
Robert C. Mack,
AIA The inappropriate cleaning and waterproofing or masonry
buildings is a major cause of deterioration of the Nation's historic resources.
While both treatments may be appropriate in some cases, they may cause
serious deterioration in others. The purpose of this leaflet is to provide
guidance on the techniques of cleaning and waterproofing, and to explain
the consequences of their inappropriate use.
Why Clean?
The reasons for cleaning any building must be considered carefully before
arriving at a decision to clean.
Is the cleaning being done to improve the appearance of the building
or to make it look new? The so called "dirt" actually may be weathered
masonry, not accumulated deposits; a portion of the masonry itself thus
will be removed if a "clean" appearance is desired.
Is there any evidence that dirt and pollutants are having a harmful
effect on the masonry? Improper cleaning can accelerate the deteriorating
effect of pollutants.
Is the cleaning an effort to "get your project started" and improve
public relations? Cleaning may help local groups with short term fund
raising, yet cause long term damage to the building.
These concerns may lead to the conclusion that cleaning is not desirable
at least not until further study is made of the building its environment
and possible cleaning methods.
What Is The Dirt? The general
nature and source of dirt on a building must be determined in order to
remove it in the most effective, yet least harmful, manner. Soot and smoke,
for example, may require a different method of cleaning than oil stains
or bird droppings. The "dirt" also may be a weathered or discolored portion
of the masonry itself rather than extraneous materials. Removal of part
of the masonry thus would be required to obtain a "clean" appearance,
leading to loss of detail and gradual erosion of the masonry. Other common
cleaning problems include metal stains such as rust or copper stains,
and organic matter such as the tendrils left on the masonry after removal
of ivy. The source of dirt, such as coal soot, may no longer be a factor
in planning for longer term maintenance, or it may be a continuing source
of problems. Full evaluation of dirt and its effect on the building may
require one or several kinds of expertise: consultants may include building
conservators, geologists, chemists, and preservation architects. Other
sources of local experience or information may include building owners
in the area, local universities, the State Historic Preservation Officer,
and the AIA State Preservation Coordinator.
If the proposed cleaning is to remove paint, it is important in each
case to learn whether or not exposed brick is historically appropriate.
Many buildings were painted at the time of construction or shortly thereafter;
retention of the paint, therefore, may be more appropriate historically
than exposing the brick, in spite of current attitudes about "natural"
brick. Even in cases where unpainted masonry is appropriate, the retention
of the paint may bc more practical than removal in terms of long range
preservation of the masonry. In some cases, however, removal of the paint
may be desirable. For example, the old paint layers may have built up
to such an extent that removal is necessary prior to repainting. It is
essential, however, that research on the paint type, color, and layering
be completed on the entire building before removal.
What Is The Construction
Of The Building? The construction of the building must be
considered in developing a cleaning program because inappropriate cleaning
can have a corrosive effect on both the masonry and the other building
materials.
Incorrectly chosen cleaning products can cause damaging chemical reactions
with the masonry itself. For example, the effect of acidic cleaners on
marble and limestone generally is recognized. Other masonry products also
are subject to adverse chemical reactions with incompatible cleaning products.
Thorough understanding of the physical and chemical properties of the
masonry can help you avoid the inadvertent selection of damaging cleaning
materials.
Other building materials also may be affected by the cleaning process;
some chemicals, for example, may have a corrosive effect on paint or glass.
The portions of building elements most vulnerable to deterioration may
not be visible, such as embedded ends of iron window bars. Other totally
unseen items, such as iron cramps or ties which hold the masonry to the
structural frame, also may be subject to corrosion from the use of chemicals
or even from plain water (Fig. 1). The only way to prevent problems in
these cases is to study the building construction in detail and evaluate
proposed cleaning methods with this information in mind.
Previous treatments of the building and its surroundings also should
he evaluated, if known. Earlier waterproofing applications may make cleaning
difficult. Repairs may have been stained to match the building, and cleaning
may make these differences apparent. Salts or other snow removal chemicals
used near the building may have dissolved and been absorbed into the masonry,
causing potentially serious problems of spalling or efflorescence. Techniques
for overcoming each of these problems should be considered prior to the
selection of a cleaning method.
Types Of Cleaning Cleaning
methods generally are divided into three major groups: water, chemical,
and mechanical (abrasive). Water methods soften the dirt and rinse the
deposits from the surface. Chemical cleaners react with the dirt and/or
masonry to hasten the removal process; the deposits, reaction products
and excess chemicals then are rinsed away with water. Mechanical methods
include grit blasting (usually sand blasting), grinders, and sanding discs,
which remove the dirt by abrasion and usually are followed by a water
rinse. Problems related to each of these cleaning methods will be discussed
later in this leaflet.
Planning A Cleaning Project
Once the existing conditions have been evaluated, including
the type of dirt and the building materials, planning for the cleaning
project can begin.
Environmental concerns: The potential effect of each proposed method
of cleaning should be evaluated carefully. Chemical cleaners, even though
dilute, may damage trees, shrubs, grass, and plants. Animal life, ranging
from domestic pets to song birds to earth worms, also may be affected
by the runoff. In addition, mechanical methods can produce hazards through
the creation of airborne dust.
The proposed cleaning project also may cause property damage. Wind drift.
for example, may carry cleaning chemicals onto nearby automobiles, causing
etching of the glass or spotting of the paint finish. Similarly, airborne
dust can enter surrounding buildings, and excess water can collect in
nearby yards and basements.
Personal safety: The potential health dangers of each method proposed
for the cleaning project must be considered, and the dangers must be avoided.
Both acidic and alkaline chemical cleaners can cause serious injury to
cleaning operators and passersby; injuries can be caused by chemicals
in both liquid and vapor forms. Mechanical methods cause dust which can
pose a serious health hazard, particularly if the abrasive or the masonry
contain silica. Steam cleaning has serious hazards because of high temperatures.
Testing cleaning methods: Several potentially useful cleaning methods
should be tested prior to selecting the one for use on the building. The
simplest and least dangerous methods should be included as well as those
more complicated. All too often simple methods, such as a low pressure
water wash, are not even considered, yet they frequently are effective,
safe, and least expensive. Water of slightly higher pressure or with a
mild non-ionic detergent additive also may be effective. It is worth repeating
that these methods should be tested prior to considering harsher methods;
they are safer for the building, safer for the environment, and less expensive.
The level of cleanliness desired also should be determined prior to
selection of a cleaning method. Obviously, the intent of cleaning is to
remove most of the dirt. A "brand new" appearance, however, may be inappropriate
for an older building, and may require an overly harsh cleaning method.
It may be wise, therefore, to determine a lower level of acceptable cleaning.
The precise amount of residual dirt considered acceptable would depend
upon the type of masonry and local conditions.
Cleaning tests, whether using simple or complex methods, should be applied
to an area of sufficient size to give a true indication of effectiveness.
The test patch should include at least a square yard, and, with large
stones, should include several stones and mortar joints. It should be
remembered that a single building may have several types of masonry materials
and similar materials may have different surface finishes; each of these
differing areas should be tested separately. The results of the tests
may well indicate that several methods of cleaning should be used on a
single building.
The cleaning budget should include money to pay for these tests. Usually
contractors are more willing to conduct a variety of tests if they are
reimbursed for their time and materials, particularly if the tests include
methods with which the contractor is not familiar.
When feasible, test areas should be allowed to weather for an extended
period prior to evaluation. A waiting period of a full year is not unreasonable
in order to expose the masonry to a full range of seasons. For any building
which is considered historically important, the delay is insignificant
compared to the potential damage and disfigurement which may arise from
use of an incompletely tested method (Figs. 25).
Potential Problems Of Cleaning
Water Cleaning: Water cleaning methods include: (1) low pressure
wash over an extended period, (2) moderate to high pressure wash, and
(3) steam. Bristle brushes frequently are used to supplement the water
wash. All joints, including mortar and sealants, must be sound in order
to minimize water penetration to the interior.
Porous masonry may absorb excess amounts of water during the cleaning
process and cause damage within the wall or on interior surfaces. Normally,
however, water penetrates only part way through even moderately absorbent
masonry materials.
Excess water also can bring soluble salts from within the masonry to
the surface, forming efflorescences (Fig. 2); in dry climates, the water
may evaporate inside the masonry, leaving the salts slightly in back of
the surface. The damage which can be caused by soluble salts is explained
in more detail later in this leaflet. Efflorescence usually can be traced
to a source other than a single water wash.
Another source of surface disfigurement is chemicals such as iron and
copper in the water supply; even "soft" water may contain deleterious
amounts of these chemicals. Water methods cannot be used during periods
of cold weather because water within the masonry can freeze, causing spalling
and cracking. Since a wall may take over a week to dry after cleaning,
no water cleaning should be permitted for several days prior to the first
average frost date, or even earlier if local forecasts predict cold weather.
In spite of these potential problems, water methods generally are the
simplest to carry out, the safest for the building and the environment,
and the least expensive.
Chemical cleaning: Since most chemical cleaners are water based, they
have many of the potential problems of plain water. Additional problems
of chemical cleaning agents have been mentioned in the discussion of environmental
concerns.
Chemical cleaners have other problems as well. Some types of masonry
are subject to direct attack by cleaning chemicals. Marble and limestone,
for example, are dissolved easily by acidic cleaners, even in dilute forms.
Another problem may be a change in the color of the masonry caused by
the chemicals, not by removal of dirt; the cleaner also may leave a hazy
residue in spite of heavy rinsing (Fig. 3). In addition, chemicals can
react with components of mortar, stone, or brick to create soluble salts
which can form efflorescences, as mentioned earlier. Historic brick buildings
are particularly susceptible to damage from hydrochloric (muriatic) acid,
although it is, unfortunately, widely used on these structures.
Mechanical cleaning: Grit blasters, grinders, and sanding discs all
operate by abrading the dirt off the surface of the masonry, rather than
reacting with the dirt and masonry as in water and chemical methods. Since
the abrasive do not differentiate between the dirt and the masonry, some
erosion of the masonry surface is inevitable with mechanical methods,
especially blasting. Although a skilled operator can minimize this erosion,
some erosion will still take place. In the case of brick, soft stone,
detailed carvings, or polished surfaces, even minimal erosion is unacceptable
(Figs. 4 and 5). Brick, a fired product, is hardest on the outside where
the temperatures were highest; the loss of this "skin" of the brick exposes
the softer inner portion to more rapid deterioration. Abrasion of intricate
details causes a rounding of sharp corners and other loss of delicate
features, while abrasion of polished surfaces removes the polished quality
of stone. Mechanical methods, therefore, should never be used on these
surfaces and should be used with extreme caution on others.
Grit blasting, unfortunately, still is widely used in spite of these
serious effects. In most cases, blasting will leave minute pits on the
surface of the masonry. This additional roughness actually increases the
surface area on which new dirt can settle and on which pollutants can
react.
Mortar joints, especially those with lime mortar, also can be eroded
by mechanical cleaning. In some cases, the damage may be visual, such
as loss of joint detail or increased joint shadows. Joints constitute
a significant portion of the masonry surface (up to 20% in a brick wall)
so this change should not be considered insignificant. In other cases,
however, the erosion of the mortar joint may permit increased water penetration,
leading to the necessity for complete repointing.
Other problems of mechanical methods have been mentioned in the discussion
of project planning. In addition, wet blasting or water rinses can create
the potential hazards of water methods.
Problems
Of Water Repellent And Waterproof Coatings Is waterproofing
necessary? Coatings frequently are applied to historic buildings without
concern for the requirement or the consequences of the coating. Most historic
buildings have survived for years without coatings, so why are they needed
now? Water penetration to the interior usually is not caused by porous
masonry but by deteriorated gutters and down spouts, deteriorated mortar,
capillary moisture from the ground (rising damp), or condensation. Coatings
will not solve these problems. In the case of rising damp, in fact, the
coatings will allow the water to go even higher because of the retarded
rate of evaporation. The claim also is made that coatings keep dirt and
pollutants from collecting on the surface of the building thus reducing
the requirement for future cleaning. While this at times may be true,
at other times the coatings actually retain the dirt more than uncoated
masonry. More important, however, is the fact that these coatings can
cause greater deterioration of the masonry than that caused by pollution,
so the treatment may be worse than the problem one is attempting to solve.
Types of coatings: Masonry coatings are of two types: waterproof coatings
and water repellent coatings. Waterproof coatings seal the surface from
liquid water and from water vapor; they usually are opaque, such as bituminous
coatings and some paints. Water repellents keep liquid water from penetrating
the surface but allow water vapor to enter and leave through the "pores"
of the masonry. They usually are transparent, such as the silicone coatings,
although they may change the reflective property of the masonry, thus
changing the appearance.
Waterproof coatings: These coatings usually do not cause problems as
long as they exclude all water from the masonry. If water does enter the
wall, however, the coating can intensify the damage because the water
will not be able to escape. During cold weather this water in the wall
can freeze, causing serious mechanical disruption, such as spalling. In
addition, the water eventually will get out by the path of least resistance.
If this path is toward the interior, damage to interior finishes can result;
if it is toward exterior cracks in the coating, it can lead to damage
from the buildup of salts as described below.
Water repellent coatings: These coatings also can cause serious damage,
but by a somewhat different mechanism. As water repellent coatings do
not seal the surface to water vapor, it can enter the wall as well as
leave the wall. Once inside the wall, the vapor can condense at cold spots,
producing liquid water. Water within the wall, whether from condensation,
leaking gutters, or other sources, can do damage, as explained earlier.
Further damage can be done by soluble salts. Salts frequently are present
in the masonry, either from the mortar or from the masonry units themselves.
Liquid water can dissolve these salts and carry them toward the surface.
If the water is permitted to come to the surface, efflorescences appear
upon evaporation. These are unsightly but usually are easily removed;
they often are washed away by the simple action of the rain.
The presence of a water repellent coating, however, prevents the water
and dissolved salts from coming completely to the surface. The salts then
are deposited slightly behind the surface of the masonry as the water
evaporates through the pores. Over time, the salt crystals will grow and
will develop substantial pressures which will spall the masonry, detaching
it at the depth of crystal growth. This buildup may take several years
to cause problems.
Test patches for coatings generally do not allow an adequate evaluation
of the treatment, because water may enter and leave through the surrounding
untreated areas, thus flushing away the salt buildup. In addition, salt
deposits may not cause visible damage for several years, well after the
patch has been evaluated.
This is not to suggest that there is never a use for water repellents
or waterproofings. Sandblasted brick, for example, may have become so
porous that paint or some type of coating is essential. In other cases,
the damage being caused by local pollution may he greater than the potential
damage from the coatings. Generally, coatings are not necessary, however,
unless there is a specific problem which they will help to solve. If the
problem occurs on only a portion of the masonry, it probably is best to
treat only the problem area rather than the entire building. Extreme exposures
such as parapets, for example, or portions of the building subject to
driving rains can be treated more effectively and less expensively than
the entire building. Washington, D.C. November, 1975.
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