Repairing Spalled 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 repairing small spalls in terra cotta using a cementitious patching mortar. For alternative guidance in repairing spalls in terra cotta using field-prepared mortar, see 04214-01-R.
  2. 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. Samples: Color-match patch samples fabricated on pieces of appropriate masonry from or on the building using the specified repair mortar as required.



  1. Jahn Restoration Mortars
  2. Edison Coatings, Inc.


  1. Terra Cotta Patch Mix: "Jahn M100 Terra Cotta Repair Mortar" (Jahn Restoration Mortars, Cathedral Stone Products, Inc.), "Custom System 45" (Edison Chemical Systems, Inc.), or approved equal. Prepare samples for approval. The RHPO will select the color most closely matching the existing original terra cotta.
    1. Mortar mix should be formulated to match color and texture of existing masonry.
    2. Mortar mix should not contain any synthetic additives.
    3. Mortar mix should be vapor permeable, frost and salt resistant, shrink resistant, and be physically compatible with the substrate, including, but not limited to, porosity, tensile, and compressive +strength.
  2. Water: Clean, free of oils, acids, alkalis and organic matter.


  1. Stiff, natural bristle brushes
  2. Hammer
  3. Chisel
  4. Trowel
  5. Putty knife

2.04 MIXES

  1. For Jahn Restoration Mortar: Follow manufacturer's printed instructions.
  2. For Custom System 45: Follow manufacturer's printed instructions.



  1. Protection:
    1. Protect persons, motor vehicles, surrounding surfaces of building whose masonry surfaces are being restored, building site, and surrounding buildings from injury resulting from masonry restoration work.
    2. Erect temporary protection covers over pedestrian walkways and at points of entrance and exit for persons and vehicles which must remain in operation during course of masonry restoration work.
    3. Contractor shall test those areaway drains, window well drains, etc., which will be used to assure that drains are functioning properly prior to performing masonry restoration operations in those areas. The Contractor shall report immediately to the Construction Engineer the location of drains which are found to be stopped up or blocked.
    4. Prevent grout or mortar used in repointing and repair work from staining face of surrounding masonry and other surfaces. Remove immediately grout and mortar in contact with exposed masonry and other surfaces.
    5. Protect sills, ledges, windows, and projections from repair material droppings.


NOTE: See manufacturer's product specifications for additional requirements in making terra cotta patches.

  1. Remove loose mortar and masonry at areas to be patched.
  2. Cut back 1/2" to sound terra cotta with chisel and hammer. Score surface to receive patch with chisel to provide a mechanical key with patching material.
  3. Thoroughly clean all loose terra cotta particles, mortar and other dust and debris from surfaces to be repaired by air and then with a soft brush and water.
  4. Pre-moisten terra cotta with clean water and a stiff natural bristle brush to prevent patching mortar from drying out prematurely. Avoid over wetting masonry, which inhibits adhesion. Prepare test patches as required for approval by the Contracting Officer's Representative to determine the correct degree of moistening.
  5. Mix the cementitious patching material according to manufacturer's instructions.
    1. The mixture should be compatible in texture and porosity to the adjacent terra cotta.
    2. Patching mortar should be shapable without using molds and as it is being applied should hold its shape right away.
    3. Do not mix more material than can be used within 30 minutes, or as recommended by manufacturer.
  6. Apply mortar mix with a trowel. Apply patching mortar in layers no greater than 3". If a second layer is required, allow the mortar to cure before applying the second layer. If a cement skin forms, remove 1/16" of the surface using a metal straight edge or scraper.
  7. Build up layers so that patch is slightly higher than adjacent surfaces. Do not feather edges.
  8. Allow to harden for length of time as recommended by manufacturer, then carefully cut back and tool to match adjacent surfaces. Maintain mortar joints; do not install patch across joints.
  9. Keep patches damp for 72 hours after installations.


  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.
  2. Use of metal scrapers or brushes will not be permitted.
  3. Use of acid or alkali cleaning agents will not be permitted.