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Repointing Mortar Joints in
Historic Brick Buildings
Robert C. Mack, AIA, de Teel Patterson Tiller,
James S. Askins
Repointing is the process of removing deteriorating mortar from the
joints of a masonry wall and replacing it with new mortar. Properly
done, repointing restores the visual and physical integrity of the masonry.
Improperly done, repointing not only detracts from the appearance of
the building, but may, in fact, cause physical damage to the masonry
units themselves.
The purpose of this brief is to provide general guidance on appropriate
materials and methods for repointing historic brick buildings and is
intended to benefit building owners, architects, and contractors alike.
Because of its general nature, this publication should not be considered
a specification for repointing. Rather, it should serve as a guide to
prepare such specifications; to develop sensitivity to the particular
needs of historic masonry; and to assist historic building owners in
working cooperatively with contractors and architects.
Identifying the Problem
The decision to repoint is most often related to some obvious sign
of deterioration (figure 1) such as disintegrating mortar, cracks in
mortar joints, loose bricks, damp walls, or damaged plasterwork. It
is, however, erroneous to assume that repointing alone will solve all
these problems. Therefore the true cause of the deterioration should
be determined before beginning any repointing work. Leaking roofs or
gutters, differential settlement of the building, capillary action causing
rising damp, or extreme weather exposure should all be recognized as
sources of deterioration and should be dealt with immediately. Without
such action, mortar deterioration will continue and any repointing work
will have been a waste of time and money.
Budgeting and Scheduling
It is important to recognize that repointing will probably be both
expensive and time-consuming due to the extent of handwork and special
materials required; however, it should also be emphasized that it is
not only possible, but preferable to repoint only those areas that require
work rather than an entire wall, as is often specified. Recognizing
this at the outset may prevent many restoration/rehabilitation jobs
from becoming prohibitively expensive.
In scheduling, seasonal aspects need to be considered first. Generally
speaking, repointing should only be undertaken when the wall temperatures
are between 40 and 95 degrees (F) to prevent freezing or excessive evaporation
of the water in the mortar. During hot weather, repointing should ideally
be done on the shady side of the building in order to slow the drying
process of the mortar.
The relationship of repointing to other proposed work on the building
must also be recognized. For example, if paint removal or cleaning are
anticipated and if the mortar joints are basically in sound condition
and only need selective repointing, it is generally better to postpone
repointing until after completion of these activities. However, if the
mortar has badly eroded, thus allowing moisture to penetrate deeply
into the wall, repointing should be accomplished before cleaning. Related
work such as structural or roof repairs should be scheduled so that
they do not interfere with repointing and so that all work can take
maximum advantage of scaffolding.
Visual Examination
All repointing work on historic masonry buildings should be preceded
by an analysis of the mortar and by an examination of the bricks and
the techniques used in the original construction of the wall. For most
projects, a simple visual analysis of the historic mortar is sufficient
to allow an appropriate match for the new mortar. The exact physical
and chemical properties of the historic mortar are not of major significance
as long as the new mortar:
* matches the historic mortar in color, texture, and detailing;
* is softer (measured in compressive strength) than the brick;
* is as soft, or softer (measured in compressive strength) than the
historic mortar.
A simple method of analyzing the historic mortar to aid in developing
an appropriate repointing mortar for many restoration jobs and most
rehabilitation work follows (1):
1) Remove three or four unweathered samples of the mortar to be matched
from several locations on the building (set the largest sample aside--this
will be used later for comparison with the repointing mortar). It is
important to recognize that many historic buildings have been repointed
a number of times and that on any given wall surface there may be a
variety of mortars. Therefore, it is important to remove as many samples
as are representative to obtain a "mean" mortar sample.
2) Break the remaining samples apart with a wooden mallet until they
are separated into their constituent parts. There should be a good handful
of the material.
3) Carefully blow away the powdery material (the lime or cement matrix
which bound the mortar together).
4) With a low power (10 power) magnifying glass, examine the remaining
sand and other materials such as oyster shells.
5) Note and record the wide range of color as well as the varying
sizes of the individual grains of sand or shell.
Historic sand colors may range from white to grey to yellow within
a given mortar sample. Furthermore, the varying sizes of the grains
of sand or other materials such as shell play an important role in the
texture of the repointing mortar. Historic sand was not screened or
graded by size as it is today. Therefore, when specifying sand for repointing
mortar, consideration may need to be given to obtaining sand from several
sources and then combining them in order to approximate the range of
sand colors and grain sizes in the historic mortar sample. The role
of the sand in the overall appearance of the replacement mortar should
not be underestimated!
Pointing styles and the methods of producing them should be examined
(figure 2). It is important to look at both the horizontal and the vertical
joints to determine the order in which they were tooled and whether they
were the same style. Some late19th and 20th century buildings, for example,
have horizontal joints that were tooled while the vertical joints were
finished flush and stained to match the bricks, thus creating the illusion
of horizontal bands. It is significant to note that pointing styles often
differed from one facade to another. Front walls often received greater
attention to mortar detailing than side and rear walls.
Bricks should also be examined so that any replacement will match
the historic brick. Within a wall there may be a surprising range of
colors, textures, and sizes, particularly with handmade brick. Replacement
should match the full range of the historic brick rather than a single
brick. Although many bricks can be matched from existing stock, they
must often be customordered, a lengthy process that can seriously affect
the project budget and schedule. Here, there should be a note of caution!
The use of recycled brick from demolished buildings for replacement
brick often results in an excellent color and texture match: however,
it is important to remember that historic brick was manufactured in
varying grades, ranging from high-fired exterior brick to low-fired
interior "bat" or "clinker" brick. This lowfired brick was never intended
to be exposed to the weather, and, when used for replacement brick on
an exterior wall, will deteriorate at a rapid rate, often needing replacement
within a year or two. Great care, therefore, should be taken in choosing
the proper type of recycled brick.
Properties of Mortar
In general, mortars for repointing should be softer (measured in compressive
strength) than the masonry units and no harder than the historic mortar.
This is necessary to prevent damage to the masonry units. It is a common
error to assume that hardness or high strength is a measure of durability.
Stresses within a wall caused by expansion, contraction, moisture migration,
or settlement must be accommodated in some manner; in a masonry wall,
these stresses should be relieved by the mortar rather than by the bricks.
A mortar that is stronger or harder than the bricks will not "give,"
thus causing the stresses to be relieved through the bricks--resulting
in cracking and spalling (figure 3). Stresses can also break the bond
between the mortar and the brick, permitting water to penetrate the
resulting hairline cracks.
Constituents of Mortar
Sand: Sand is the largest constituent of mortar and the material that
gives mortar its characteristic color and texture. When viewed under
a magnifying glass or low-power binocular microscope, particles of sand
generally have either rounded edges, such as found in beach or river
sand, or sharp, angular edges, found in crushed or manufactured sand.
For repointing mortar, rounded or natural sand is preferred for two
reasons. First, it is usually similar to the sand in the historic mortar,
thus providing a better visual match. Second, it has better "working"
qualities or plasticity and can thus be forced into the joint more easily,
forming a good contact with the historic mortar and the surface of the
bricks. Although manufactured sand is frequently the only type readily
available, it is worth the search to locate a sufficient quantity of
rounded or natural saltfree sand for repointing .
Lime or Portland Cement: The two commonly used binders for mortar
are lime and portland cement. Of the two, lime produces a mortar that
meets nearly all the requirements for a good mortar for historic buildings,
while portland cement produces a mortar that does not perform as well.
High lime mortar is soft, porous, and changes little in volume during
temperature fluctuations. In addition, lime mortar is slightly water
soluble and thus is able to reseal any hairline cracks that may develop
during the life of the mortar. Portland cement, on the other hand, can
be extremely hard, is resistant to movement of water, shrinks upon setting,
and undergoes relatively large thermal movements. The use of a high
lime mortar, therefore, is recommended for nearly all repointing projects.
However, white portland cement can be substituted for up to 20 percent
of the lime (ex. 1 part cement to 4 parts lime). This will usually improve
workability or plasticity without adversely affecting the desirable
qualities of the lime mortar. Plasticity is important to ensure a good
bond between the new mortar, the historic mortar, and the brick.
Water: Water should be clean and relatively free of salts or acids.
Historic Additives: In addition to the color of the sand, the texture
of the mortar is of critical importance in duplicating historic mortar.
While modern mortars are finely ground and present a uniform texture
and color, historic mortars were not as well ground. They may contain
lumps of oyster shell, partially burned lime, animal hair, or particles
of clay. The visual characteristics of these additives should be duplicated
through the use of similar materials in the repointing mortar.
Matching Color and Texture
of Mortar
In matching the repointing mortar, the new mortar should match the
unweathered interior portions of the historic mortar. The simplest way
to check the match is to make a small sample of the proposed mix and
allow it to cure; this sample is then broken open and the broken surface
is compared with the broken surface of the largest "saved" sample of
historic mortar.
If it is not possible to obtain a proper color match through the use
of natural materials because locally available sands are not a close
match to the original sand, it may be necessary to use a modern mortar
pigment, and, in fact, some historic mortars did use such additives.
In the late 19th century, some mortars were colored with pigments to
match or contrast with the brick. Red, brown, and black pigments were
commonly used. Pigments are available as separate ingredients or already
mixed with mortar; however, the premixed mortars normally are not suited
for use on repointing projects because of their high portland cement
content. Only chemically pure mineral oxides, which are alkaliproof
and sunfast, should be used in order to prevent bleaching and fading.
Material Specifications
Modern materials specified for use in repointing mortar should conform
to specifications of the American Society for Testing Materials (ASTM)
or comparable federal specifications.
Sand should conform to ASTM C144 to assure proper gradation and freedom
from impurities. Sand color, size, and texture should match the original
as closely as possible to provide the proper color match without other
additives. Samples of sand proposed for use should be submitted for
approval prior to beginning work.
Lime should conform to ASTM C207, Type S, Hydrated Lime for Masonry
Purposes. This lime is designed to assure high plasticity and water
retention with a safe degree of strength. The use of quicklime which
must be slaked and soaked, does not necessarily provide better results.
Cement should conform to ASTM C150, Type II (white nonstaining) portland
cement. It should not have more than 0.60 percent alkali to help avoid
efflorescence.
Water should be potable--clean and free from acids, alkalies, or large
amounts of organic materials.
Historic Additives will require writing new specifications for each
project. If possible, suggested sources for special materials should
be included. For example, crushed oyster shells frequently can be obtained
in a variety of sizes from poultry supply dealers.
Mortar Mix: Specifying the proportions for the repointing mortar for
a specific job is often a perplexing task for the architect, engineer,
contractor, or preservation consultant alike. The following guidelines
can assist in writing specifications:
* Material proportions should be given in volumes, that is, 4 parts
of lime to 12 parts of sand, rather than 2 bags of lime to 6 cubic feet
of sand. This will avoid any confusion on the job site when substitution
of sources occur with differing packaging sizes.
* Repointing mortar for most historic buildings should ideally be
composed only of lime and sand. A proportion of 1 part of lime to 2
parts of sand is a useful starting point.
* ASTM C150, Type II (white nonstaining) portland cement may be added
to the repointing mortar to increase workability and to achieve whiteness
in color; however, no more than 20 percent of the total volume of the
lime and portland cement--combined--should be portland cement. Any greater
amount of portland cement increases the hardness of the repointing mortar
to a potentially damaging degree.
* For surfaces of extreme weather such as parapet walls or water tables,
a harder mortar (6 parts of sand, to 3 parts of lime, to 2 parts of
white portland cement) may be more desirable.
Execution of the Work
The Test Panel: In choosing a contractor or mason, perhaps the best
way to award the contract and for the contractor or mason to demonstrate
his or her work in a repointing job is the test panel: a small demonstration
section of joint preparation and repointing actually done on the historic
masonry. The test panel should be carefully selected to include all
types of masonry, joint styles, and types of problems to be encountered
on the job. Usually a 3foot by 6foot area located in an inconspicuous
yet readily accessible place is sufficient.
Joint Preparation: Old mortar should generally be removed to a minimum
depth of 2-1/2 times the width of the joint to ensure an adequate bond
and to prevent mortar "popouts." For most brick joints, this will require
removal of the mortar to a depth of approximately 1/2 to 1 inch. Any
loose or disintegrated mortar beyond this minimum depth should be removed
(fig. 4). The use of power tools such as saws with carbide blades or
impact hammers for the removal of mortar almost always results in damage
to the bricks by breaking the edges and by overcutting on the head,
or vertical, joints (fig. 5). Damage to the bricks not only affects
their visual character but can also lead to accelerated weather damage.
Where joints are uniform and fairly wide, it may be possible to use
a grinder to assist the removal of mortar; however, final preparation
of the joint should be done by hand.
Test panels are quite helpful, but they cannot adequately assess the
potential effect of using a grinder since such panels are not prepared
under actual working conditions. If there is any chance of damaging
the masonry, handmethods should be used exclusively. Although slower,
these methods are easier to control and less likely to cause irreversible
damage to the bricks. Mortar should be removed cleanly from the bricks,
leaving square corners at the back of the cut. Before filling, the joints
should be rinsed with a jet of water to remove all loose particles and
dust. At the time of filling, the joints should be damp but with no
standing water present.
Mortar Preparation: Mortar should be mixed carefully to obtain uniformity
of visual and physical characteristics. Dry ingredients should be measured
by volume and thoroughly mixed before the addition of any water. Half
the water should be added, followed by mixing for approximately 5 minutes.
The remaining water should then be added in small portions until a mortar
of the desired consistency is reached. The total volume of water necessary
may vary from batch to batch, depending on weather conditions. Mortar
should be used within 30 minutes of final mixing, and "retempering,"
or adding more water after the initial mix is prepared, should not be
permitted.
Modern Additives: In general, modern chemical additives are unnecessary
and may, in fact, have detrimental effects. The use of antifreeze compounds
is not recommended. They are not very effective with high lime mortars
and may introduce salts, which will later cause efflorescence. A better
practice is to warm the sand and water, and to protect the completed
work from freezing. The use of airentraining additives to resist frost
action and enhance plasticity, are also discouraged, since the air has
a detrimental effect on both bond and strength of the mortar. In areas
of extreme exposure requiring highstrength mortars (see formula for
"extreme weather exposure" under Mortar Mix section), air-entrainment
of 1016 percent may, however, be desirable. Bonding agents that increase
the bond of the new mortar to the old should also be avoided. If the
joint is properly prepared, there will be a good bond between the new
mortar and the adjacent surfaces; chemical agents do not significantly
improve this bond and are not a substitute for proper joint preparation.
In addition, some of the agent will inevitably become smeared on the
surface of the masonry and removal is very difficult.
Filling the Joint: Where existing mortar has been removed to a depth
of greater than 1 inch, these deeper areas should be filled first, compacting
the new mortar in several layers. The back of the entire joint should
be filled successively by applying approximately 1/4 inch of mortar,
packing it well into the back corners. This application may extend for
several feet. As soon as the mortar has reached thumbprint hardness,
another 1/4 inch layer of mortar--approximately the same thickness--may
be applied. Several layers will be needed to fill the joint flush with
the outer surface of the brick. It is important to allow each layer
time to harden before the next layer is applied; most of the mortar
shrinkage occurs during the hardening process and layering thus minimizes
overall shrinkage.
The rate of hardening can be controlled by dampening the brick and
the old mortar before beginning to fill the joint, but free water or
excessive dampness in the joint should be avoided. Too much water will
delay the tooling or cause excess shrinkage; too little water will be
absorbed from the mortar before it is properly set, thus reducing bond
strength.
When the final layer of mortar is thumbprint hard, the joint should
be tooled to match the historic joint. Proper timing of the tooling
is important for uniform color and appearance. If tooled when too soft,
the color will be lighter than expected, and hairline cracks may occur;
if tooled when too hard, there may be dark streaks called "tool burning,"
and good closure of the mortar against the brick will not be achieved.
If the old bricks have worn, rounded edges, it is usually best to
recess the final mortar slightly from the face of the bricks. This treatment
will help avoid a joint visually wider than the actual joint width;
it will also avoid creation of a large, thin featheredge which is easily
damaged, thus admitting water (figs. 6 and 7). After tooling, it is
frequently necessary to remove excess mortar from the edge of the joint
by brushing with a bristle brush.
"Aging" the Mortar: Even with the best efforts at matching the existing
mortar color, texture, and materials, there will usually be a visible
difference between the old work and the new, partly because the new
mortar has been matched to the unweathered portions of the historic
mortar. If the mortars have been properly matched, the best treatment
for surface color differences is to let the mortars age naturally. Another
reason for a slight mismatch may be that the sand is more exposed in
old mortar due to the slight erosion of the lime or cement. Several
methods of treatment have been attempted in an effort to overcome these
differences. As with all work, however, any proposed treatment should
bc carefully tested prior to implementation.
Efforts to stain the new mortar to produce a color match should, in
most cases, be avoided. Although such a process may provide an initial
match, the old and new mortars may weather at different rates, leading
to visual differences after a few seasons. In addition, the mixtures
used to stain the mortar may be harmful to the masonry, for example,
introducing salts into the masonry which can lead to efflorescence.
Cleaning: If repointing work is carefully executed, there will be
little need for cleaning other than the small amount of mortar brushed
from the edge of the joint following tooling. This type of cleaning
is best accomplished with a stiff bristle brush after the mortar has
dried, but before it is fully hardened (12 hours). Mortar that has hardened
can usually be removed with a wooden paddle or, if necessary, a chisel.
Further cleaning is best accomplished with plain water and bristle
brushes. If chemicals must be used, their selection should be made with
extreme caution. Improper cleaning can lead to deterioration of the
masonry units, deterioration of the mortar, mortar smear, and efflorescence.
New mortar joints are especially susceptible to damage because they
do not become fully cured for several months. Chemical cleaners, particularly
acids, should be used only once and should be flushed freely with plain
water to remove all traces of the chemicals.
Several precautions should be taken if freshly repointed mortar joints
are being cleaned. First, the mortar should be fully hardened before
cleaning--30 days is usually sufficient, depending on weather and exposure
(as mentioned above, the mortar will continue to cure even after the
mortar has hardened). Test panels should be prepared to evaluate the
effects of different cleaning methods. Only stiff natural bristle brushes
should be used, except on glazed or polished surfaces. Here, only soft
cloths should be used. Further information concerning masonry cleaning
is presented in Preservation Briefs 1 "The Cleaning and Waterproof Coating
of Masonry Buildings."
New construction "bloom" or efflorescence occasionally appears within
the first few months of repointing and usually disappears through the
normal process of weathering. If the efflorescence is not removed by
natural processes, the safest way to remove it is by dry brushing with
stiff natural or nylon bristle brushes and water. Hydrochloric (muriatic)
acid, is generally ineffective and should be avoided in the removal
of efflorescence. In fact this chemical may deposit additional salts,
which, in turn, can lead to additional efflorescence.
Scrub Coats: A variety of new and purportedly useful repointing techniques
for historic buildings are being offered by contractors which, appear
to have limited usefulness in historic preservation. These techniques
are identified under a variety of names that include: slurr coats, slurry
coating, and, most commonly, scrub coating. All involve the brushing
of a thinned, low-aggregate coat of mortar over the entire masonry surface
which, when dry, is scrubbed off the brick with a brush, presumably
leaving a residue in the mortar joint. These techniques have become
increasingly appealing as they are quick, inexpensive in comparison
to traditional repointing costs, and do not require particularly skilled
labor or craftsmanship. Their greatest attraction lies in repointing
large masonry surfaces such as highrise structures, but their benefit
to historic masonry is essentially cosmetic. A certain amount of crack
sealing in the mortar joint does occur, and for these limited applications
it is a useful technique; however, these techniques should not be confused
with, or substituted for, repointing. It is not the same process. Slurr
coats and slurry and scrub coatings, tend to mask joint detailing or
tooling, have a life expectancy of only a few years, and are extremely
difficult to clean from the surface of the brick without leaving a residue,
called "veiling." While of some limited use in specific instances, these
new techniques are not appropriate for historic buildings and should
therefore not be considered when a lasting and durable repointing job
is desired.
Summary
For the Owner/Administrator: The owner or administrator of an historic
building must constantly remember that repointing is likely to be a
lengthy and expensive process. The owner will need to allow adequate
time for evaluation by a qualified preservationist, for preparation
of plans and specifications for the work, and for a lengthy work period
with scaffolding in place. Schedules for both repointing work and other
activities will thus require careful coordination to avoid unanticipated
conflicts. The owner must avoid the tendency to rush the work or economize
if the building is to retain its visual integrity and the job is to
be durable.
For the Architect/Consultant: The architect/consultant must assist
the owner in planning for logistical problems relating to research and
construction. The consultant must also realize that older buildings
have special problems usually not encountered with modern building materials
or techniques of construction. Therefore, extra research will be required,
and nonstandard materials and procedures will need to be used in evaluating
the work of potential contractors to ensure that they are qualified
to work on projects of the type anticipated. The consultant must also
be prepared to spend more time than is customary in modern construction
to inspect the work.
For the Contractor/Craftsman: The contractor or craftsman must keep
in mind that a repointing project for a historic building will be slower
and more expensive per unit cost than work on a modern building. The
contractor must understand the reasons for these special requirements,
and must convey them to the workers at all levels. Understanding the
nature of the project and the potential problems will not only allow
the contractor to submit a more accurate bid, but will also provide
for the use of nonstandard methods in performing work.
Conclusion
First and foremost, a good repointing job is meant to last, often
in the range of 50100 years. Shortcuts and poor craftsmanship not only
result in a job that looks bad but also is one that will require future
repointing more frequently than if the job had been done correctly in
the first place. The mortar joint in a historic brick building has often
been called the wall's "first line of defense." Good repointing practices
guarantee the long life of the mortar joint, the wall, and the historic
structure. However, while careful preservation, restoration, and maintenance
will guarantee the long life of the freshly repointed mortar joints,
it is important to remember that these mortar joints will probably require
repointing some time in the future. It is the nature of mortar joints
to deteriorate. Nevertheless, if the historic mortar joints proved durable
for many years, then careful repointing should have an equally long
life, ultimately contributing to the preservation of the historic brick
building.
NOTE
(1) Two detailed methods for performing a mortar analysis are presented
by E. Blaine Cliver in "Tests for the Analysis of Mortar Samples" in
the Bulletin of the Association for Preservation Technology, vol. 6
(no. 1, 1974); and Morgan W. Phillips in "Brief Notes on the Subjects
of Analyzing Paints and Mortar and the Recording of Moulding Profiles:
The Trouble with Paint and Mortar Analysis" in the Bulletin of the Association
for Preservation Technology, vol. 10 (no.2, 1978).
Selected Reading List
Brick Institute of America. "Cleaning Brick Masonry" Technical Notes
on Brick Construction (no. 20) rev., Sept/Oct., 1977.
Cliver, E. Blaine. "Tests for the Analysis of Mortar Samples," Bulletin
of the Association for Preservation Technology, vol. 6 (no. 1, 1974),
pp. 6873.
Mack, Robert C. "The Cleaning and Waterproof Coatings of Masonry Buildings,"
Preservation Briefs: 1, Heritage Conservation and Recreation Service,
Washington, D.C., November 1975.
Mack, Robert C., and Askins, James S. "The Repointing of Historic
Masonry Buildings," Downers Grove, Illinois, Service Master Industries,
1979.
McKee, Harley J. "Brick and Stone: Handicraft to Machine," Building
Early America. The Carpenters' Company of the City and County of Philadelphia,
1976.
McKee, Harley J. Introduction to Early American Masonry. Washington,
D.C.: National Trust for Historic Preservation/Columbia University,
1973.
Phillips, Morgan. "SPNEAAPT Conference on Mortar," Bulletin of the
Association for Preservation Technology, vol. 6 (no. 1, 1974).
This brief was first written by Robert C. Mack, AIA, in 1976. It has
been revised and updated in 1980 by Robert C. Mack, de Teel Patterson
Tiller, Architectural Historian, Heritage Conservation and Recreation
Service (HCRS), Department of the Interior; and James S. Askins, Preservation
Craftsman, National Park Service, Department of the Interior. Other
HCRS staff members who contributed materially to the development of
this revised brief are Kay D. Weeks, Technical WriterEditor, and James
A. Caufield, Historical Architect. Washinton, D.C.
This publication has been prepared pursuant to the
National Historic Preservation Act of 1966, as amended, which directs
the Secretary of the Interior to develop and make available information
concerning historic properties. Technical Preservation Services (TPS),
Heritage Preservation Services Division, National Park Service prepares
standards, guidelines, and other educational materials on responsible
historic preservation treatments for a broad public.
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