Historic Preservation - Technical Procedures
- Removing And Replacing Deteriorated Stone Masonry
- Procedure code:
- Developed For HSPG (NPS - Southeast Regional Office)
- Last Modified:
- Removing And Replacing Deteriorated Stone Masonry
- Last Modified:
REMOVING AND REPLACING DETERIORATED STONE MASONRY
THIS PROCEDURE SHOULD BE PERFORMED UNDER THE DIRECTION OF AN
HISTORICAL ARCHITECT OR ENGINEER TO DETERMINE THE MOST EFFICIENT
AND LEAST DESTRUCTIVE MANNER FOR EXECUTING THE WORK. APPROVAL
THE REGIONAL HISTORIC PRESERVATION OFFICER IS REQUIRED.
A. This procedure includes guidance on removing and
replacing deteriorated stone masonry. It should be used
in conjunction with procedure 04520-02-R "Repointing
Masonry Using Lime Mortar" and 04100-03-S "Preparing Lime
Mortars for Repointing Masonry".
B. Damage to stone in the form of cracking, splitting and
spalling often occurs in areas sheltered from direct
C. 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
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).
A. American Society for Testing and Materials
1.03 PROJECT/SITE CONDITIONS
A. Environmental Requirements:
1. Weather: Do not proceed with patching or
replacement under adverse weather conditions, or
when temperatures are below or above manufacturer's
recommended limitations for installation; Proceed
with the work only when forecasted weather
conditions are favorable for proper cure.
2. Wet Weather: Do not apply or mix mortar on outside
surfaces with standing water or outside during
3. Cold Weather, winter construction is not allowed
without consent of Regional Architect; Winter
construction (midwest region) is defined as any
time between December 1 and March 1 and/or when
surface temperature of masonry is below 40 degrees (F). or
air temperature is predicted to be below 40 degrees (F).
within 48 hours; Heat mortar materials to above
40 degrees (F). if necessary.
4. Hot Weather: The surface temperature of the work,
not the ambient temperature, should not be higher
than 100 degrees(F).; Mortar mixing should be done only in
the shade; Cover mortar in hot weather to reduce
evaporation; Pointing work should be done in the
shade; Work around the building during the day so
that the fresh work will be shielded from direct
sunlight to reduce evaporation rate.
5. All work must be suspended during frosty weather
unless a heated enclosure is provided. Work should
not be done in full sun at temperatures above 27 degrees (C).
unless shading of the walls is provided and the
masonry wall temperature is kept below this point;
Burlap sacking and water misting may be necessary
to control evaporation; High temperatures can cause
flash setting of cements and rapid evaporation of
water in the mix, leading to lack of development of
final strength by the cement.
6. Special Precautions and Notes: Do not allow
masonry to freeze until mortar is thoroughly dry
and hardening almost complete (approx. three days
time); The setting of lime mortar is very much
slower than that of cement mortar because the
curing requires the absorption of carbon dioxide to
form hard lime carbonates; It is a very lengthy
process, so do not expect it to become hard
immediately, especially at the core of large masses
A. Repointing Tools: Available from good hardware stores,
building material suppliers or mail-order catalogues.
1. Goldblatt Tool Co.
2. Marshalltown Trowel Co.
A. Salvaged Stone: Approved, sound, crack free, clean stone
without face chips larger than 1/2 inch, salvaged from
removed face stone work.
B. Replacement Stone: Upon approval of Regional Historic
Preservation Officer, provide natural building stone of
type (classification) and with color, and surface texture
to match existing stone on building. Match existing
stones in size and shape.
D. Naturally colored binder such as (PFA) low sulphate
pozzolanic fly ash
E. HTI Powder (white refractory brick powder - "High
Temporary Insulation") - may be used as a pozzolanic
additive to make a weak hydraulic mortar.
F. Mortar and Grout Materials:
1. Portland Cement: ASTM C150, Type I.
a. For stonework and other masonry indicated,
provide non-staining white cement complying
with staining requirement of ASTM C91 for not
more than 0.03% water soluble alkali.
2. Hydrated Lime: ASTM C207, Type S.
3. Aggregate for Mortar: ASTM C144, unless otherwise
a. Colored Mortar Aggregate: Natural or
manufactured sand selected to produce mortar
4. Colored Mortar Pigment: Natural and synthetic iron
oxides and chromium oxides, compounded for use in
mortar mixes. Use only pigments with record of
satisfactory performance in masonry mortars.
5. Water: Clean, free of oils, acids, alkalis and
6. Aggregate for Grout: ASTM C404.
G. Manufactured and raw materials accessory to the primary
1. Supports, fasteners, installation accessories, and
anchors: Stainless steel only for use with
granite. Brass, bronze, or stainless steel for use
with limestone and other masonry.
2. Flashings or barriers: Sheet copper or as
A. Trowels: range in length from 10-12 inches
B. Chisels: Joint chisels or a standard mason's chisel with
a 1-1/2 in. blade and a long narrow handle
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)
1. Natural bristle brushes
2. Stiff bristle brushes (no wire)
G. Spray bottle
A. Measurement and Mixing: Measure cementitious and
aggregate material in a dry condition by volume or
equivalent weight. Do not measure by shovel, use known
measure. Mix materials in a clean mechanical batch
B. Consistently and accurately measure materials for each
C. Mix for at least five minutes in a mechanical batch mixer
or mortar box.
D. Mix trowel workable consistency for unit masonry setting
1. Mixer Pointing Mortar: Thoroughly mix cementitious
and aggregate materials together before adding any
water. Then mix again adding only enough water to
produce a damp, unworkable mix which will retain
its form when pressed into a ball. Maintain mortar
in this dampened condition for 1-to-2 hours. Add
remaining water in small portions until mortar of
desired consistency is reached. Use mortar within
30 minutes of final mixing; do not retemper or use
partially hardened material.
E. Mix firm dry consistency for repointing.
F. Discard mortar not used within two hours after mixing; do
not retemper at mixer.
G. Dispose of waste mortar and cleaning water in approved
manner; do not contaminate the site, adjoining property,
or waste to sewers.
H. Colored Mortar: Produce mortar of color required by use
of selected ingredients. Do not adjust proportions
without RHPO's approval.
1. Color Pigmented Mortar: Where colored mortar
pigments are indicated, do not exceed pigment-to-cement ratio of 1-to-10, by weight.
2. Match original color and texture for each kind of
mortar and masonry work.
3. Match to inner mortar color which has been
protected from weathering and soiling, not face
4. Conform to the dry mix color matching, "A Fast
Method of Matching Mortar Colors Using Heat," APT
Bulletin, Vol. X, No., 2, 1978, pp. 79-85, copy
provided by RHPO; or other method approved.
I. Do not use admixtures of any kind in mortar, unless
J. Mortar Proportions:
NOTE: VERIFY PROPORTIONS WITH MORTAR ANALYSIS TO MATCH
OR RESEMBLE HISTORIC MORTAR.
1. Pointing Mortar for Stone: One part white portland
cement, 1 part lime, 6 parts colored mortar
2. Rebuilding Mortar: Comply with ASTM C270,
proportion Specification, Type N, unless otherwise
indicated with cementitious material content
limited to portland cement-lime.
3. Grout: ASTM C476.
A. Surface Preparation: Where fresh masonry joins existing
work, clean the exposed surface of the set masonry by
removing loose stone and mortar and wet lightly to obtain
the best possible bond with the new work.
A. Carefully remove by hand, as indicated, any stone which
has deteriorated, shifted, or is damaged beyond repair.
1. Carefully cut out perimeter joints of stone by hand
using a hammer and chisel.
2. Break down the rest of the damaged stone with
vertical saw cuts and break it up with a hammer and
B. Carefully clean out the cavity to receive the stone.
Remove mortar, loose particles and other debris from in
preparation for resetting.
C. Thoroughly wet the cavity with water; Dampen the new or
salvaged stone with water.
D. Spread a 1/2 inch thick mortar bed into the open cavity;
the new mortar should consist of coarse sand and grit to
match the original mortar. Butter vertical joints for
full width before setting and set unit(s) in full bed of
mortar, unless otherwise indicated.
E. Repair and/or replace removed stone with salvaged stone,
where possible, or with new stone to match existing,
including size. Set the stone in the cavity.
F. Fill the joints with a grout of lime and low sulphate
pozzolanic fly ash (PFA) or lime and HTI powder.
G. Tool joints after setting to match joints of surrounding
H. Repoint new mortar joint to comply with requirements for
repointing existing masonry, except rake out joints
before mortar sets.
A. Wipe all excess mortar as the work progresses. Dry brush
at the end of each day's work. After mortar is
thoroughly set and cured, clean new masonry surfaces,
walls, sills, overhangs, etc., of all loose mortar, and
dirt and point up all nail holes, cracks, etc., after
which wash down all masonry walls, leaving them clean and
END OF SECTION