Patching Small Holes, Minor Chips And Spalls In Terra Cotta

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.



  1. This procedure includes guidance on patching small holes (no greater than fist size), minor chips and spalls in terra cotta with mortar to match existing units.
  2. Make patch repairs only where the area and depth of damage is large enough to make a patch which is at least 1/2-inch thick; otherwise make surface finish repairs to protect from more weather damage and to maintain a uniform color and appearance. For guidance on patching masonry cracks, see 04200-03-R; for guidance on reinforcing existing patches, see 04214-03-R.
  3. 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).


  1. American Society for Testing and Materials (ASTM)


  1. Patching Mortar Sample: Cured and dried sample of mortar proposed for use in masonry patching; provide with mix design and complete list of all materials and proportions with details on how mix proportions will be followed in the field; a sample area on the building may be used for this test.


  1. Environmental Requirements: When outside air temperature is below 5 degrees C (40 degrees F), maintain mortar temperature between 20 degrees C (70 degrees F) and 50 degrees C (120 degrees F) by heating water and/or aggregates.



  1. Thoro Consumer Products (BASF Construction Chemicals) 


  1. Setting and Patching Mortar Materials: Mortar should meet standard compressive strength and permeability requirements for terra cotta. General ASTM Designation C 270-88a.
    1. Portland Cement: ASTM Designation C150-89, Type I or II. Natural light gray or buff color.
    2. Hydrated Lime: ASTM Designation C207-79, Type S or equivalent lime putty.
    3. Masons Sand: The sand used is the primary giver of the cured mortar color. Match samples provided by RHPO from original mortar materials.
    4. Mineral Colorants: Only as necessary and approved to match terra cotta matrix or surface color.
    5. Other admixtures: No accelerators (Calcium Chloride) or additives may be used unless approved by the RHPO.
  2. Patching Accessories:
    1. Bonding Agents: "ThoroBond" (Thoro Consumer Products), or approved equal.
    2. Glaze Replacement Coating for Terra Cotta: Portland cement base, polymer modified, water vapor permeable, masonry coating; Thoroseal (Thoro Consumer Products) or approved equal.
      1. Color to be selected by RHPO to match existing terra cotta finish after curing.
    3. Microcrystalline wax
    4. Water-tolerant paint
  3. Clean, potable water


  1. Masonry drill
  2. Hacksaw blade
  3. Chisels
  4. Wooden mallet
  5. Glass cutter
  6. Wire brushes
  7. Hand-held water bottle
  8. Steel trowel



  1. When inspecting for external damage, look for:
    1. Missing units
    2. Deteriorated or missing mortar
    3. Large cracks running through multiple units
    4. Material failures such as spalls, and parallel crazing
    5. Presence or absence of water-shedding devices like flashing, gutters and downspouts
    6. Bulges in the terra cotta, particularly at floor levels and at piers
    7. Rust stains from failing anchors
    8. Efflorescence from excessive moisture in the wall
  2. When inspecting for internal damage:
    1. Strike the surface of a unit with a wooden mallet.
    2. Undamaged units give off a clear ringing sound, while broken units and those with internal damage give off a flat thud.
    3. Consistent tonal quality among similar pieces is the key to locating damaged units.


  1. Surface Preparation:
    1. Clean broken, spalled, and cracked terra cotta units by scraping, chipping, and wire brushing the damaged area to expose sound, clean, terra cotta surface.
    2. Wash the cleaned surface with clean, clear water or blow compressed air to remove dusts.
    3. Tool work areas to be patched to roughen for promoting bonding to patch material.
    4. Undercut edges of area to be patched (roughly 30 degrees).


  1. Using a glass cutter, cut a line through the fireskin or glaze around the damaged area at right angles to the edges of the block.
  2. Using a masonry drill and a hacksaw blade, remove all damaged material.
  3. Undercut at least 2 of the edges to provide a good key for the repair.
  4. Mix patching mortar to a stiff consistency, to be workable, but not to run, sag, or be friable.
    1. Consistently and accurately measure materials for each batch according to approved samples.
    2. Mix for at least five minutes in a mechanical batch mixer or mortar box.
    3. Mix trowel workable consistency for unit masonry setting and resetting.
    4. Mix firm dry consistency for repointing.
    5. Discard mortar not used within two hours after mixing; do not retemper at mixer.
  5. Hand-spray the surface with water before placing the mortar.
  6. Apply bonding agent to the clean terra cotta surface.
  7. Completely fill patch area (no less than 1/2" thick, and no greater than fist size) by hand placing patching mortar, working and compressing it to remove air pockets and to effect a tight bond to surfaces.
    1. Place patching mortar in maximum of 3/8-inch thick lifts, allowing an initial set, before proceeding with additional lifts, successively, until the whole patch is filled. Hand-spray the mortar with water before each application.
    2. Slightly over-fill the patch to allow for shrinkage and finishing.
      NOTE: Depth of lifts should be adjusted to type of mortar in order to allow for adequate curing.
  8. Finish and dress patch surface to match adjacent surfaces, and begin curing.
    1. For unglazed units, finish with 1 or 2 applications of microcrystalline wax.
    2. For glazed surfaces, paint to match the glaze color and texture with water-tolerant paint.
    3. For damaged areas that are smaller than 1/2 inch deep or for old patches where the color does not match the terra cotta, repair by applying one coat of colored masonry acrylic-cement finish to the surface to match terra cotta color.



  1. Wipe away all excess mortar as the work progresses.
  2. Dry brush at the end of each day's work.
  3. 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.