Historic Preservation - Technical Procedures

Preservation Briefs: 1 The Cleaning And Waterproof Coating Of Masonry Buildings
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Preservation Briefs 1, National Park Service, Pad
Masonry Cleaning
Last Modified:
Preservation Briefs: 1 The Cleaning And Waterproof Coating Of Masonry Buildings
Last Modified:




The link immediately below connects to the latest version of National Park Service Preservation Brief 1:



Robert C. Mack, A.I.A.

This standard includes the bulk of information contained in the
original Preservation Brief developed by the National Park Service.
To obtain a complete copy of this brief, including figures and
illustrations, please contact:  

              Superintendent of Documents
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Please call the Publication Order Information Desk at 202/783-3238
or FAX 202/512-2250 to verify price and availability.  


The inappropriate cleaning and waterproofing of 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 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

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.


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, and the State
Historic Preservation Officer.

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 be 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.


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

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.  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 be 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.


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.


Once the existing conditions have been evaluated, including the
type of dirt and the building materials, planning for the cleaning
project can begin.


The potential effect of each proposed method of cleaning should be
evaluated carefully.  Chemical cleaners, even though diluted, 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 run-off.  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.


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 passers-by; 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.


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 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

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.



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 efflorescence.  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.  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

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.


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.  In addition, chemicals can react with components of
mortar, stone, or brick to create soluble salts which can form
efflorescence, as mentioned earlier.  Historic brick buildings are
particularly susceptible to damage from hydrochloric (muriatic)
acid, although it is, unfortunately, widely used on these


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
abrasives 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.  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.



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 downspouts, 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.


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

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 build-up 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, efflorescence 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 build-up 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 build-up.  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 waterproofing.  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 be 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.

                         END OF SECTION

Last Reviewed 2012-09-10