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Removing Loose Stucco And Patching

Technical Procedures Disclaimer

Prior to inclusion in GSA’s library of procedures, documents are reviewed by one or more qualified preservation specialists for general consistency with the Secretary of Interior Standards for rehabilitating historic buildings as understood at the time the procedure is added to the library. All specifications require project-specific editing and professional judgement regarding the applicability of a procedure to a particular building, project or location. References to products and suppliers are to serve as a general guideline and do not constitute a federal endorsement or determination that a product or method is the best or most current alternative, remains available, or is compliant with current environmental regulations and safety standards. The library of procedures is intended to serve as a resource, not a substitute, for specification development by a qualified preservation professional.


We’ve reviewed these procedures for general consistency with federal standards for rehabilitating historic buildings and provide them only as a reference. Specifications should only be applied under the guidance of a qualified preservation professional who can assess the applicability of a procedure to a particular building, project or location. References to products and suppliers serve as general guidelines and do not constitute a federal endorsement nor a determination that a product or method is the best alternative or compliant with current environmental regulations and safety standards.



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 the region.
  3. RHPO will make provisions for analyzing stucco and will furnish proportions and detailed material specifications.

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
  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).


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


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

C. Sand

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

F. Trowels

G. Chisels

H. Hammers

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

J. Brushes:

  1. Natural bristle brushes
  2. Stiff bristle brushes
  3. Wire brush

2.04 MIXES


A. Lime/Sand Stucco Mix:

  1. Scratch and Brown Coats: Two coats doubled up to a thickness of about 5/8 inch.
    • 5 parts hydrated lime
    • 15 parts aggregate (match to original)
    • 6 lb./cu. yd. hair (1/2- 2 inch length, free of dirt, grease and impurities)
    • 2-3 parts (max.) Type II portland cement for workability
  2. Finish Coat:
    • 1 part hydrated lime
    • 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 infiltration.

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 transmission:
    • Apply a vapor barrier paint on the interior walls.
    • Caulk joints along interior window trim and baseboards.
    • Properly vent bathrooms and kitchens.
    • 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 ground.
  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 be removed.
  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 stucco.


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 puller.
  3. Clean out all dust, dirt, and loose material with a wire brush.
  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 beads.

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 sand.
  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 mortar color.

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 Photo-Flo.
  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 second coat.
  4. Cure for 18 to 24 hours.
  5. Moisten the surface with water before applying the second coat.
  6. Apply the second coat (brown coat) 3/8 to 1/2 inch thick, matching the thickness of the original scratch coat.
  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 coat.
  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 finish appearance.


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.