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Historic Preservation - Technical Procedures

Spectitle:

Repointing Masonry Using Lime Mortar

Procedure code:

0452002R

Source:

The Custom House/Portland, Or - Gsa/Facilities Support Ctr

Division:

Masonry

Section:

Masonry Restoration

Last Modified:

02/24/2012

Details:

Repointing Masonry Using Lime Mortar



REPOINTING MASONRY USING LIME MORTAR


PART 1---GENERAL

1.01 SUMMARY

    A.   This procedure includes guidance on repointing stone
         masonry using lime mortar.

    B.   Repointing is the process of removing deteriorated mortar
         from a masonry joint and replacing old mortar with new,
         sound mortar.

    C.   This process is sometimes referred to as "tuck pointing",
         though "tuck pointing", is actually a decorative
         treatment rather than a method of repair.  True tuck
         pointing is the process of adding a finish layer of
         mortar, occasionally tinted, to the outer portion of a
         newly laid joint.

    D.   Major reasons for mortar joint failures include:

         1.   Weathering action,

         2.   Settling,

         3.   Temperature cycles,

         4.   Poor original design and materials, and

         5.   Lack of exterior maintenance.

    E.   See 01100-07-S for general project guidelines to be
         reviewed along with this procedure.  These guidelines
         cover the following sections:

         1.   Safety Precautions

         2.   Historic Structures Precautions

         3.   Submittals

         4.   Quality Assurance

         5.   Delivery, Storage and Handling

         6.   Project/Site Conditions

         7.   Sequencing and Scheduling

         8.   General Protection (Surface and Surrounding)

         These guidelines should be reviewed prior to performing
         this procedure and should be followed, when applicable,
         along with recommendations from the Regional Historic
         Preservation Officer (RHPO).

    F.   For guidance on preparing lime mortar, see 04100-03-S.

1.02 SUBMITTALS

    A.   Manufacturers' literature describing packaged items.

    B.   Source and screen analysis of bulk aggregate.

    C.   Mortar sample:   Submit, for verification and approval,
         a sample of each type of mortar used, in form of 6" long
         by 1/2" wide sample strips of mortar set in aluminum or
         plastic channels.  

         1.   Provide record of mortar mix, composition and field
              procedures to be followed.

1.03 QUALITY ASSURANCE

    A.   Mock-ups:  Raking and Repointing Sample Work:

         1.   Test/Sample Area and RHPO Approval:

              a.   Initially perform sample joint raking and
                   repointing on each of a 100 sq. ft. test of
                   stone, brick, and terra cotta areas as
                   approved by RHPO.

              b.   Demonstrate proficiency with joint raking
                   tools and ability to not damage masonry units
                   with either hand or power tools.

              c.   Mix and cure test batch of repointing mortar
                   and place in joints; repeat test mix until
                   mortar color is approved.  Test mortar should
                   be matched, dried and approved before placing
                   in joints.

              d.   Demonstrate workmanship of repointing
                   procedures and joint finishing.

              e.   Gain written approval from RHPO for test area
                   before proceeding with remaining work.

         2.   Joint Raking Method:  Rake joints by hand ONLY
              using special joint cleaning chisels and hammer.

         3.   Repointing Method:  Repoint joints by hand ONLY
              using approved pointing trowels.  NO "BAGGING" OR
              CAULKING GUN POINTING METHODS APPROVED.

1.04 PROJECT/SITE CONDITIONS

    A.   Environmental Conditions:   Perform repointing only when
         the temperature is between 40 degrees Fahrenheit and 80
         degrees Fahrenheit.  If the temperature is below 40
         degrees, the mortar sets too slowly, and there is a good
         chance of freezing before it fully sets.  If the
         temperature is above 80 degrees, the mortar will set too
         quickly, and there is a strong chance of excessive loss
         of water prior to adequate setting.


PART 2---PRODUCTS

2.01 MANUFACTURERS

    A.   Repointing Tools:  Available from good hardware stores,
         building material suppliers or mail-order catalogues.

         1.   The Stanley Gold-blatt Tool Co.
              511 Osage Ave.
              Kansas City, KS  66105-2198
              913/621-3010

         2.   Marshalltown Trowel Co.
              P.O. Box 738
              Marshalltown, IA  50158
              515/753-5999

         3.   Masonry Specialty Co.
              4430 Gibsonia Rd.
              Gibsonia, PA  15044
              412/443-7080

2.02 MATERIALS

    A.   Lime mortar (See 04100-03-S for materials and procedures
         in preparing lime mortar)

    B.   Clean, potable water

2.03 EQUIPMENT

    A.   Trowels:  range in length from 10-12 inches

    B.   Chisels:

         1.   Joint chisels or a standard mason's chisel with a
              1-1/2 in. blade and a long narrow handle

         2.   Floor chisels

    C.   Hammers:

         1.   5# stone dressing hammer

         2.   2# striking hammer

         3.   "No-Bounce" hammer

         4.   Full size and one half size brick hammers

    D.   Joint Tools:  (see 2.01 MANUFACTURERS above)

         1.   3/8"-1/4" raised beaded tool

         2.   3/8"-1/4" beaded striking tool

         3.   1/2" raised beaded tool with offset handle

         4.   1/2" flat joint iron

         5.   Pointing tool should be about 1/16" narrower than
              the joint being filled to achieve good compaction

    E.   Hawks:  Plywood or steel hawk (mortar board)

    F.   Brushes:

         1.   Natural bristle brushes

         2.   Stiff bristle brushes (no wire)

    G.   Spray bottle

2.03 MIXES

    A.   See 04100-03-S for lime mortar mixes


PART 3---EXECUTION

3.01 EXAMINATION

    A.   Examine all existing exterior mortar joints.  If the
         answer to any of the following questions is yes, then the
         building's joints are deteriorated and need repointing:
         
         1.   Are mortar joints eroded back more than 1/4" from
              the masonry face?

         2.   Are there cracks running vertically or horizontally
              through the mortar?

         3.   Are mortar bonds broken or pulled away from the
              masonry?

         4.   Has mortar fallen out of joints?

         5.   Is mortar excessively soft, powdery or crumbling?

         6.   Is pointing badly-stained?

    B.   Typical exterior damage due to mortar deterioration
         includes open joints, efflorescence, spalling and
         loosened masonry units.

    C.   Typical interior damage due to mortar deterioration
         includes failing plaster and stained wall paper.

    D.   A professional pointer experienced in old masonry is
         required for any of the following areas or conditions:  

         1.   Chimneys need repointing

         2.   Window lintels must be rebuilt

         3.   Masonry is loose or missing

         4.   Work must be done from scaffolds or extension
                             ladders

         5.   The original mortar joints were "beaded"-tooled
              with a raised, round-profiled joint that projects
              out from the wall

3.02 PREPARATION

    A.   Preparing the Joints:

         1.   Clean area of loose dirt and debris using a stiff
              bristle brush and remove all extraneous fastenings
              and devices.

         2.   Install necessary protection of adjacent building
              materials, property and persons from joint cleaning
              work and dirt.

         3.   Control dust and dirt from raking work; dampen area
              being worked; and use curtains to limit spread of
              dust from joint raking and cutting operations.

    B.   Joint Cutting and Raking:

         1.   Cut and rake old mortar from existing joints by
              hand using a hammer and chisel.  NOTE:  POWER
              CHISELS AND POWER SAWS SHOULD NOT BE USED.

         2.   Place the chisel in the center of the joint and
              pound it with a striking hammer or "No-Bounce"
              hammer until the mortar disintegrates.

         3.   Rake out the loose material to a depth of about 1
              inch and never to a depth less than their width.
              Leave a clean, square face at the back of the joint
              to provide optimum contact with the new mortar.

              CAUTION:  AVOID OVERCUTTING ENDS OF VERTICAL
              JOINTS, WIDENING JOINTS OR CUTTING INTO BEDDING
              FACES OF MASONRY UNITS.

         4.   While raking out joints, remove all metal fittings
              such as nails, brackets and clips on both
              horizontal and vertical surfaces.

         5.   Carefully clean out the prepared face with a soft
              or stiff bristle brush, or blow the joints clean
              with low-pressure compressed air (40-60 psi).

         6.   Thoroughly flush out joint with clean, clear water.

3.03 ERECTION, INSTALLATION, APPLICATION

    A.   Filling Joints:

         1.   Dampen masonry surfaces and joints to control
              suction and evaporation before placing repointing
              mortars.

              NOTE:  THERE SHOULD BE NO FREE WATER PRESENT WHICH
              MAY CAUSE VOIDS IN THE MORTAR.

         2.   Using a pointing tool, push the mortar into the
              joint from a board and iron with the maximum
              possible pressure; The mortar should be applied in
              layers, each to a maximum thickness of 3/8".

              NOTE:  THE POINTING TOOL SHOULD BE ABOUT 1/16"
              NARROWER THAN THE JOINT BEING FILLED TO ACHIEVE
              GOOD COMPACTION.  IN SOME CASES, THE JOINTS WILL BE
              SO THIN THAT A STANDARD POINTING TOOL WILL NEED TO
              BE GROUND DOWN TO FIT THE JOINT.

         3.   Thoroughly compact each layer of mortar and allow
              to set until thumb-print hard before applying the
              next layer of mortar.

         4.   Fill the joints so that they are slightly recessed
              from the masonry face.  Avoid leaving a joint which
              is visually wider than the actual historical
              appearance.

         5.   Continuously keep all excess and spilled mortar
              brushed off the faces of masonry units, ledges and
              other surfaces before it sets or stains the work.

    B.   Joint Finishing:

         1.   Begin when mortar attains "thumb print" hardness.

         2.   Tool the joint to match the old mortar.

              NOTE:  IT IS IMPORTANT TO TOOL THE JOINT AT THE
              RIGHT STAGE; IF THE JOINT IS TOO SOFT, THE COLOR
              WILL BE LIGHTER THAN EXPECTED AND HAIRLINE
              SHRINKAGE CRACKS ARE LIKELY TO OCCUR; IF THE JOINT
              IS TOO HARD WHEN TOOLED, DARK STREAKS MAY APPEAR
              (TOOL BURNING) AND GOOD CLOSURE OF THE MORTAR
              AGAINST THE MASONRY WILL NOT BE ACHIEVED.
              EXCESSIVE TOOLING MAY BRING LIME AND FINE
              AGGREGATES TO THE SURFACE, CREATING A VISUAL CHANGE
              IN THE TEXTURE AND A SURFACE SUBJECT TO EARLY
              DETERIORATION.

         3.   To produce a roughened texture, lightly spray the
              mortar with water after the initial set, stipple
              the mortar with a stiff bristle brush or dab the
              mortar with coarse sacking.

         4.   Protect finished work from direct sun and rain
              until the face has dried and hardened.

3.04 ADJUSTING/CLEANING

    A.   Cleaning Up:

         1.   Use masking and drop cloths to prevent mortar
              stains on adjacent work and ledges.

         2.   Keep work areas clean and free from mortar drips,
              spills and residue of waste mortars or wash-off.

         3.   Clean off excess mortar as work proceeds using
              masonry brushes before mortar sets.

         4.   Wash completed repointing work when finished mortar
              joints are set with clean water and masonry
              brushes, scrubbing only as required to clean mortar
              stains off masonry without scouring the units and
              joint faces.

         5.   Do not use acid or detergent cleaning agent to aid
              mortar removal and clean-up without written
              approval from RHPO.

    B.   Curing:

         1.   Schedule work only when moderate weather is
              forecast.

         2.   Protect completed work from adverse weather, heavy
              rainfall, freezing, and drying by direct sunlight
              and winds until cured.

         3.   Sprinkle or mist repointed work as required to
              achieve cure in mortar joints for a minimum of 72
              hours after completion.

         4.   Lime Mortar:  Cures by drying and crystallization,
              not by hydration; and can be washed out of joints
              if not protected before it cures.

    C.   Final Cleaning:

         1.   After mortar has fully hardened, thoroughly clean
              exposed masonry surfaces of excess mortar and
              foreign matter using stiff nylon or bristle brushes
              and clean water spray applied at low pressure.

              NOTE:  USE OF METAL SCRAPERS OR BRUSHES IS NOT
              PERMITTED.  USE OF ACID OR ALKALI CLEANING AGENTS
              IS NOT PERMITTED.

    D.   Some efflorescence, called new construction "bloom,"
         occasionally appears on the surface within the first few
         months following a repointing project.  These deposits
         normally are harmless and are removed by the natural
         washing of the rain.  If not removed by natural
         weathering, they can be removed with dry brushing with a
         bristle brush.  The use of chemical cleaners to remove
         this type of efflorescence normally is not necessary;
         AVOID USING ACIDS, PARTICULARLY MURIATIC ACID.

                         END OF SECTION