Mortar Patching Holes In Stone Masonry

Procedure code:
The Old Custom House, New York, Ny - Gsa/Pbs, 2/91
Last Modified:



  1. This procedure includes guidance on mortar patching holes in stone masonry where fastenings have been removed by other trades or damage has occurred for some other reason.
  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. American Society for Testing and Materials (ASTM)



  1. Patching Mortar:
    1. Portland cement: Type I, ASTM C150, white and/or gray as necessary to match mortar color to adjacent limestone color.
    2. Hydrated masons lime: ASTM C207.
    3. Aggregate: Clean sharp sand free of loam, silt, soluble salts and organic matter.
      1. For limestone, aggregate shall conform to the following size gradations:

        Sieve Size Percent Passing

        No. 4 100

        No. 8 100

        No. 16 97

        No. 30 76

        No. 50 17

        No. 100 6

        No. 200 0

      2. For granite, aggregate shall conform to the following size gradations:

        Sieve Size Percent Passing

        No. 4 100

        No. 8 97

        No. 16 85

        No. 30 65

        No. 50 28

        No. 100 13

        No. 200 2

        Color and size of aggregate to match existing, based on RHPO approval.

    4. Clean, potable water
  2. Anchor dowels, studs
  3. Wire armature reinforcing - 16 gauge
  4. Epoxy cement or grout


  1. Masonry drill
  2. Stiff bristle brushes (non-metallic)
  3. Steel trowel
  4. Pneumatic or hand tools

2.03 MIXES

  1. Proportions for Limestone Patching Mortar:
    1. Portland cement: 1 part by volume
    2. Lime: 2 parts by volume
    3. Aggregate: 9 parts by volume
    4. Water: to form a workable consistency
  2. Proportions for Granite Patching Mortar:
    1. Portland cement: 1 part by volume
    2. Lime: 1 part by volume
    3. Aggregate: 6 parts by volume
    4. Water: to form a workable consistency
  3. Mixing Procedures:
    1. Measure materials by volume or equivalent weight.
    2. Mix ingredients in clean mechanical batcher for 5-10 minutes.
    3. Let mortar sit for 1 hour prior to use to allow for initial shrinkage. Remix mortar to workable consistency. Use mortar within 1.5 hours of initial remixing. Discard left-over mortar. DO NOT REMIX.



  1. Protection:
    1. 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.
    2. Protect sills, ledges and projections from mortar droppings.
  2. Surface Preparation:
    1. Remove loose particles, soil, debris, oil and other contaminants from existing stone units at locations indicated by cleaning with stiff brush.
    2. Prepare the patch substrate by carving with pneumatic stone working tools, "Barre" short stroke or by hand tools to make a clean surface, roughened to bond with patch material, and with edges at least 3/8-inch thick (1 cm), no feather edges, and undercut to provide a mechanical key for the patch.


  1. Clean any exposed anchors of corrosion by scraping and brushing with stiff bristle brushes. Replace any unsound anchors as necessary with new stainless steel anchors of same approximate size and shape. Bed new anchors in epoxy grout.
  2. Clean all stone areas to be patched of all debris, dust, oil, etc. by brushing with a stiff fiber bristle brush.
  3. Provide mechanical keys or create wire reinforcing armature to hold and attach mortar patch.
  4. Place patching mortar in layers no thicker than 1/2". Roughen surface of each layer to provide key for next.
  5. Keep each layer damp for 72 hours or until mortar has set.
  6. Dry pack and firmly compact patching mortar in the patch area, fully embedding reinforcing, free from voids, to fill the patch area slightly over full for smoothing and finishing when partially set.
  7. Work and finish the patch when mortar has achieved its initial set and is firm, to match adjacent surfaces and finishes.
  8. Form patches on edge of masonry units to restore the original unit's line and to maintain clear mortar joint lines, without bridging any mortar joint, so each stone unit is separate and not joined to the next one; joints to be filled by repointing, not mortar patches.
  9. Unacceptable patches are defined as those with hairline cracks or showing separation from stone at edges. Remove patches and refill to provide patches free of those defects.

Last Reviewed: 2017-08-13