Patching Spalled Limestone

Procedure code:
National Capitol Region Specifications
Last Modified:

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A. This procedure includes guidance on repairing spalls in limestone using a cementitious patching material.

B. 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. Sample Stone Repair: Reattach stone fragments and patch stone at locations selected by the Contracting Officer's Representative using methods specified. Provide one sample each for granite, limestone and marble.



A. Jahn Restoration Techniques and Research


A. Cementitious patching material such as "M70 Stone Restoration Mortar" (Jahn Restoration), or approved equal; mixed and applied following manufacturer's guidance.

B. Clean, soft cloths

C. Clean, potable water


A. Stiff natural bristle brush

B. Trowel

C. Chisel

D. Hammer

E. Putty knife



A. Cut back to sound stone with chisel and hammer. Score surface to receive patch with chisel to provide a mechanical key with patching material. Undercutting if possible is recommended to hold patch more securely.

B. Thoroughly clean all stone dust and debris from areas that are to be repaired by air and then with a soft brush and water.

C. Pre-moisten stone with clean water and a stiff natural bristle brush to prevent patching mortar from drying out prematurely. Avoid over wetting stone, which inhibits adhesion. Prepare test patches as required for approval by the Contracting Officer's Representative to determine the correct degree of moistening.

D. First mix the cementitious patching material well in a dry state, then add water as per manufacturer's instructions, depending on the porosity of the limestone; between 150-200 cc of water to one kg of dry material. Patching mortar should be shapable without using molds and as it is being applied should hold its shape right away. Do not mix more material than can be used within 30 minutes.

E. Apply with trowel so that patch is slightly higher than adjacent surfaces. Jahn Patching mortar can be put on from 3 mm to any required thickness at once. Allow to harden for 7 days, then carefully cut back and tool to match adjacent surfaces.

Last Reviewed: 2019-07-30