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Historic Preservation - Technical Procedures

Spectitle:

Preservation Briefs: 2 Repointing Mortar Joints In Historic Brick Buildings

Procedure code:

0421107S

Source:

Preservation Briefs 2, National Park Service, Pad

Division:

Masonry

Section:

Brick Unit Masonry

Last Modified:

09/10/2012

Details:

Preservation Briefs: 2 Repointing Mortar Joints In Historic Brick Buildings



PRESERVATION BRIEFS: 2

REPOINTING MORTAR JOINTS IN HISTORIC BRICK BUILDINGS

 

The link immediately below connects to the latest version of National Park Service Preservation Brief 2:

 

http://www.nps.gov/history/hps/tps/briefs/brief02.htm

 




Robert C. Mack, AIA
de Teel Patterson Tiller
James S. Askins

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
              P.O. Box 371954
              Pittsburgh, PA  15250-7954

              GPO #024-005-01026-2
              Available ONLY in package sets
              Briefs 1-14 - $13.00

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


***INTRODUCTION***

Repointing is the process of removing deteriorated 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 such as disintegrating mortar, cracks in mortar
joints, loose bricks, damp walls, or damaged plasterwork.  It is,
however, erroneous to assume that repointing stone 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 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 degrees F (5 degrees C) and 95
degrees F (35 degrees C) 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  the 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 (measure 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.   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.  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 late-19th- 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 hand-made 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 custom-ordered, 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 low-fired 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.  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 the manufactured sand is frequently the only
type readily available, it is worth the search to locate a
sufficient quantity of rounded or natural salt-free 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 re-seal 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 and 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 later 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 alkali-proof and
sun-fast, 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 and
Materials (ASTM) or comparable federal specifications.

SAND should conform to ASTM C-144 to assure proper graduation 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 C-207, 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 C-150, Type II (white non-staining)
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 C-150, Type II (white non-staining) portland cement may
    be added to the repointing mortar to increase workability or
    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 3-foot by
6-foot 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 1/2-1 inch.  Any
loose or disintegrated mortar beyond this minimum depth should be
removed.  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.  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, hand-methods 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 "re-tempering," 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
air-entraining 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 high-strength mortars (see formula for
"extreme weather exposure" under Mortar Mix section),
air-entrainment of 10-16 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
thumb-print 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 thumb-print 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.

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 be 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 (1-2 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 50-100 years.  Shortcuts and poor craftsmanship not
only result in a job that looks bad, but also in 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.

                         END OF SECTION
 


Preservation Brief 2, mortar, mortar joints, repointing mortar joints, repointing