Historic Preservation - Technical Procedures
Removing Loose Stucco And Patching
Hspg Prepared For Nps - Sero
Lath & Plaster
Removing Loose Stucco And Patching
REMOVING LOOSE STUCCO AND PATCHING
A. This procedure includes guidance on patching loose stucco
by removing deteriorated areas and applying new stucco.
B. Historic Structures Precautions:
1. When choosing the type of stucco to be used, the
Regional Historical Preservation Officer (RHPO)
should be consulted, to provide chemical analysis
of the existing stucco and information as to how to
match for color, structure and texture.
2. Contact RHPO for stucco analysis, as well as
historic practices and technology characteristic of
3. RHPO will make provisions for analyzing stucco and
will furnish proportions and detailed material
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 (ASTM), 100
Barr Drive, West Conshohocken, PA 19428, (610) 832-9585
or FAX (610) 832-9555.
1. ASTM C207, Type S
2. ASTM C150, Type I or II
3. ASTM C144
1.03 PROJECT/SITE CONDITIONS
A. Environmental Requirements:
1. Weather: Do not proceed with patching 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. Do not apply or mix
mortar on outside surfaces with standing water or
outside during rain.
2. Cold Weather, winter construction is not allowed
without consent of RHPO. Winter construction
(midwest region) is defined as any time between
December 1 and March 1 and/or when surface
temperature of masonry is below 40o F. or air
temperature is predicted to be below 40o F. within
48 hours. All work must be suspended during frosty
weather unless a heated enclosure is provided. Do
not expose curing stucco to freezing temperatures.
3. Hot Weather: The surface temperature of the work,
not the ambient temperature, should not be higher
than 100o F.; Mortar mixing should be done only in
the shade; Cover mortar in hot weather to reduce
evaporation; Work around the building during the
day so that the fresh work will be shielded from
direct sunlight to reduce evaporation rate. Work
shall not be done in full sun at temperatures above
80o F unless shading is provided. Burlap sacking
and water misting may be necessary to control
evaporation. Keep curing stucco out of the hot sun
and away from harsh winds.
4. All materials must be kept above 40o F.
A. Brooklyn Animal Hair Manufacturing Company
175-185 Beard Street
Brooklyn, NY 11231
A. Hydrated Lime
B. Portland Cement
D. Cow hair (Brooklyn Animal Hair Manufacturing Company)
E. Clean, potable water
A. Mortar box
B. Mixing hoe (with two holes in the blade)
C. Masonry bit
D. Wire cutters
E. Garden hose or spray bottle
I. Hawks: Plywood or steel hawk (mortar board)
1. Natural bristle brushes
2. Stiff bristle brushes
3. Wire brush
NOTE: CONSULT WITH RHPO TO DETERMINE WHICH OF THE FOLLOWING
MIXES IS APPROPRIATE. ACTUAL MIX SHOULD BE DETERMINED BY
ANALYSIS. IF ORIGINAL MIX IS KNOWN, OR IF PAST PATCHING MIX
HAS BEEN USED SUCCESSFULLY, USE IT; OR ALTER STANDARD MIX TO
A. Lime/Sand Stucco Mix:
1. Scratch and Brown Coats: Two coats doubled up to a
thickness of about 5/8 inch.
a. 5 parts hydrated lime
b. 15 parts aggregate (match to original)
c. 6 lb./cu. yd. hair (1/2- 2 inch length, free
of dirt, grease and impurities)
d. 2-3 parts (max.) Type II portland cement for
2. Finish Coat:
a. 1 part hydrated lime
b. 3 parts aggregate (match to original)
B. High Lime Mortar Mix:
1. 1 bag of hydrated lime
2. 1 shovelful of white portland cement
3. 3 cubic feet of sand (matched to original)
4. Coarse aggregate matched to original (not to exceed
15% of total volume of hydrated lime)
5. Hair or fiber (for scratch coat) matched to
original if possible, about 1 pound of hair per 100
lb. bag of hydrated lime
C. Lime/Portland Cement Mortar (More lime makes the mixture
more "plastic" but more likely to crack from shrinkage;
more sand or aggregate makes the mixture harder to trowel
smooth and weakens the mortar).
1. 1 to 1-1/2 bags hydrated lime
2. 1 bag portland cement
3. 5 to 6-1/2 cubic feet of sand
4. Coarse aggregate, hair, and fiber as above
A. Most stucco damage is caused by water infiltration.
B. Sometimes cracks occur due to different expansion rates
of two surfaces. These cracks can then allow water
C. When identifying the source of infiltration, examine:
1. Flashing: Check for holes, splits, or general
corrosion. Replace if required. Copper, lead-
coated copper, terne metal and terne-coated
stainless steel are the best; aluminum is
questionable. To prevent corrosion due to galvanic
action, flashing metal should be compatible with
other metals, such as gutters and downspouts or
other flashing, used on the building.
2. Drip edges: A discontinuity formed into the
underside of a window sill or wall component to
force drops of water to fall free of the face of
the building rather than move farther toward the
interior. Check to see that they are free from
paint or dirt build-up.
3. Gutters: Check to see that gutters are clear of
debris (rust, tar patches) and have no open joints.
4. Walls outside of kitchens, bathrooms and chimney
flues: Look for damaged stucco caused by water
vapor migration. Remedy by altering water vapor
a. Apply a vapor barrier paint on the interior
b. Caulk joints along interior window trim and
c. Properly vent bathrooms and kitchens.
d. On a chimney, line the flue with a non-porous
liner like stainless steel.
5. Termination of stucco at ground level: Stucco
should terminate at least 4 inches above the
6. Site Grading: Make sure the ground slopes away
from the stucco wall.
7. Joints between parapet walls and roofs: Look for
deteriorated and improperly installed flashing.
8. Plumbing: Repair any leaks.
D. Determine the extent of the damage:
1. Check for spongy areas by pushing against the
stucco with your hand: any areas that move back
and forth while making a squishy sound will need to
2. Tap the stucco with a hammer handle: a succession
of sounds indicates loose stucco; Areas that do not
move and make only one solid sound indicate good
3.02 ERECTION, INSTALLATION, APPLICATION
A. Removing Damaged Stucco:
1. Make cuts through the stucco around the damaged
area either with a cold chisel or by drilling a
series of holes with a masonry bit.
2. If possible, cut back the coats of old mortar in
square layers; Undercut the area to provide a firm
bonding for the patch. Cut to the lath; Pry off
the old stucco with a broad flat tool like a nail
3. Clean out all dust, dirt, and loose material with a
4. Nail back to the sheathing any loose wood or metal
lath under the old stucco.
5. Repair any small areas of lath that were damaged by
nailing wire lath in place.
6. Replace any rusted corner beads with new corner
B. Hand-Mixing the Mortar:
1. Place 1/2 the sand required for one bag of cement
in one end of the mortar box; Spread the cement
over the sand.
2. Lay the balance of the sand over the cement.
3. Place the amount of coarse aggregate or hair
required for a bag of cement over the top of the
4. Repeat as necessary until all of the required
material is in the box.
5. Using a hoe, start at one end of the box and pull
the hoe toward you in short choppy strokes until
all of the materials are thoroughly mixed.
6. Pour the water into the box, and pull the dry
material into the water using short choppy strokes;
Continue to add water as needed to bring the mix to
a soft, plastic mass.
7. Chop the hoe through the wet material until all the
dry material has been wetted and pulled to the end
of the box.
8. Change direction, and pull the mortar to the other
end of the box.
9. Well combined materials will produce a uniform
C. Applying Stucco Patch Using a Three-coat System:
1. Dampen wood lath by spraying lightly with a garden
hose set for fine spray or use a spray bottle; A
better method is to wet the lath with water
containing photographer's wetting agent, i.e. Kodak
2. Apply first coat of stucco (scratch coat) 3/8 to
1/2 inch thick, matching the thickness of the
original scratch coat.
3. Cross-hatch the first coat of mortar with a trowel
or piece of wire lath to provide good keys for the
4. Cure for 18 to 24 hours.
5. Moisten the surface with water before applying the
6. Apply the second coat (brown coat) 3/8 to 1/2 inch
thick, matching the thickness of the original
7. Finish the second coat with a wood float that has a
small nail driven through it (only the nail tip
protrudes) to provide good keys for the finish
8. Cure coat for several days; Sprinkle it with water
occasionally so that direct sun or dry weather does
not cause it to dry too rapidly and crack.
9. Moisten the surface with water right before
applying the top coat so that the first two coats
do not draw water out of the fresh stucco.
10. Apply the top coat (finish coat) to a thickness of
at least 1/8 inch, to be flush with surface.
11. Wait 1-3 hours, then wire brush, float or trowel
top coat using mild pressure to duplicate existing
A. Wipe all excess mortar as the work progresses. Dry brush
at the end of each day's work. After stucco is
thoroughly set and cured, clean new masonry surfaces,
walls, sills, overhangs, etc., of all loose stucco, and
dirt. Patch all nail holes, cracks, etc., after which
wash down all masonry walls, leaving them clean and neat.
END OF SECTION