Repointing Sandstone

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 raking out and repointing the deteriorated or inappropriately repointed horizontal and vertical joints on exterior sandstone.
  2. Deteriorated mortar includes that which is excessively soft, crumbling, cracked, badly stained or missing.
  3. This procedure should be used in conjunction with 04520-02-R, which describes in more detail the purpose of repointing in general, the materials required and precautions that should be taken.
  4. 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. Product Data: Submit manufacturer's technical data for each product indicated including recommendations for their application and use. Include test reports and certifications substantiating that products comply with requirements.
  2. Restoration Program: Submit written program for each phase of restoration process including protection of surrounding materials on building and site during operations. Describe in detail materials, methods and equipment to be used for each phase of restoration work.
  3. Protection Program: Submit for approval methods for protecting exterior surfaces and elements not being repointed, such as: windows, light fixtures, decorative metal work, plantings, cars and landscape elements.
  4. Samples: Submit, for verification purposes, prior to mock-up erection, samples of the mortar for pointing and masonry rebuilding and repair, in a form of 6" long by 1/4 inch to 1/2 inch appropriately wide sample strips of mortar set in aluminum or plastic channels. Width of channel forms should be similar to the existing mortar joints.


  1. Qualifications:
    1. Work must be performed by a firm having not less than five years successful experience in comparable masonry restoration projects and employing personnel skilled in the restoration processes and operations indicated.
    2. All technicians planned for use on the job will be required to successfully complete five linear feet of cutting and raking of mortar joints in the presence of the Contracting Officer prior to working on the job. Unsuccessful performance in this test area will be grounds for rejection of this technician for this job.
  2. Field Samples: Prior to start of general masonry restoration, prepare the following sample panels on building where directed by Contracting Officer. Obtain Contracting Officer's acceptance of visual qualities before proceeding with the work. Retain acceptable panels in undisturbed condition, suitable designated during construction as a standard for judging completed work.
    1. Repointing: Prepare two separate sample areas of approximately 3' high by 6' wide for each type of repointing required, one for demonstrating methods and quality of materials and workmanship expected in removal of mortar joints, and the other for demonstrating quality of materials and workmanship expected in pointing mortar joints.
  3. Source of Materials: Obtain materials for masonry restoration from a single source for each type of material required to ensure match of quality, color,pattern and texture.


  1. Storage and Protection:
    1. Protect masonry restoration materials during storage and construction from wetting by rain, snow or ground water, and from staining or intermixture with earth or other type materials.
    2. Protect grout, mortar, and other materials from deterioration by moisture and temperature. Store in a dry location or on waterproof containers. Keep containers tightly closed and away from open flames. Protect liquid components from freezing. Comply with manufacturer's recommendations for minimum and maximum temperature requirements for storage.


  1. Existing Conditions: No stone work will be performed when the air temperature is 40 degrees F and falling or when stone surface temperature is 40 degrees F or below.


  1. Coordinating Work: Perform masonry restoration work in the following sequence:
    1. Provide approved protection of adjacent building materials and features.
    2. Repair existing masonry including replacing existing masonry with new masonry materials.
    3. Rake out existing mortar from deteriorated joints to be repointed.
    4. Point existing mortar joints of masonry to be restored.
    5. Clean up masonry surface as required after repointing.



  1. Mortar for Pointing:
    1. Portland Cement: Type 1, ASTM C150, non-staining white as approved by RHPO to achieve proper color.
    2. Hydrated Masons Lime: ASTM C207, Type S.
    3. Aggregate: Clean, sharp sand free of loam, silt, soluble salts and organic matter. Select aggregate and pigment to match the final mortar color to the color of the original mortar. Sand should contain a full range of sizes from coarse to fine.
    4. Water: Clean and free of deleterious amounts of oil, acid, alkalis and organic matter.


  1. Brushes: Natural fiber or nylon bristle only.
  2. Mixing Vessels: Polyethylene plastic or as recommended by adhesive manufacturer.
  3. Measuring Boxes
  4. Joint tools
  5. Chisel
  6. Hammer
  7. Hawk
  8. Trowel
  9. Putty knife
  10. Stiff fiber bristle brushes
  11. Spray bottle

2.03 MIXES

  1. General: Replacement mortar MUST be weaker than the sandstone. Mortars exposed to the weather should be formulated to resist erosion and freeze-thaw deterioration. Mortars not exposed to severe weathering should be soft, flexible and durable.
    1. Measurement and Mixing: Measure cementitious and aggregate material in a dry condition by volume or equivalent weight. Do not measure by shovel. Use known measure. Mix materials in a clean container, by hand. See also 04100-03-S for additional guidance.
    2. Mixing Pointing Mortar: Thoroughly mix cementitious and aggregate materials together before adding any water. Then mix again adding only enough water to produce a damp, unworkable mix which will retain its form when pressed into a ball. Maintain mortar in this dampened condition for 1-to-2 hours. Add remaining water in small portions until mortar of desired consistency is reached. Use mortar within 30 minutes of final mixing; do not retemper or use partially hardened material.
    3. Colored Mortar: Produce mortar of color required by use of selected ingredients. Do not adjust proportions without RHPO's approval.
    4. Do not use admixtures of any kind in mortar, unless otherwise indicated.
    5. To insure proper color match between mixed mortars, the same person should mix all mortar for an entire job.
  2. Pointing Mortar Mix Proportions (Type O): Typical for most wall surfaces, not directly exposed to the weather. Mix by volume the following:
    1. 1 part Portland cement
    2. 2 parts lime putty
    3. 12 parts sand
    4. Add pigments as required, but NEVER more than 10% of the total weight of portland cement. Much smaller amounts are usually enough. Always use the smallest amount necessary to produce the desired color and limit carbon black to 2%. Pigments should be of mineral oxide composition.
  3. Pointing Mortar Mix Proportions (Type N): More common for surfaces directly exposed to the weather. Mix by volume the following:
    1. 1 part Portland cement
    2. 1 part lime putty
    3. 10 parts sand
    4. Add pigments as required, but NEVER more than 10% of the total weight of portland cement. Much smaller amounts are usually enough. Always use the smallest amount necessary to produce the desired color and limit carbon black to 2%. Pigments should be of mineral oxide composition.



  1. Protection:
    1. Protect persons, motor vehicles, landscaping, 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 mortar droppings.


  1. Joint Raking:
    1. Cut out mortar from deteriorated joints by hand to depths equal to 2 times their whole width but not less than 5/8" nor less than that required to expose sound, unweathered mortar. NOTE: POWER OPERATED ROTARY HAND SAWS AND GRINDERS WILL NOT BE PERMITTED.
      1. Use chisels with a 1 inch maximum head for cutting out the mortar. Sharpen chisels hourly to minimize the chipping. One (1) chip per linear yard of cutting will be the standard of acceptable skill.
      2. Do not spall edges of masonry units or widen joints. Replace any masonry units which become damaged.
    2. Rinse masonry joint surfaces with water to remove any dust and mortar particles. Time application of rinsing so that, at time of pointing, excess water has evaporated or run off, and joint surfaces are damp but free of standing water.
  2. Joint Pointing:
    1. Apply first layer of jointing mortar to areas where existing mortar was removed to depth greater than surrounding areas. Apply in layers not greater than 3/8" until a uniform depth is formed. Compact each layer thoroughly and allow to become thumbprint-hard before applying next layer.
    2. After joints have been filled to a uniform depth, place remaining pointing mortar in 3 layers with each of first and second layers filling approximately 2/5 of joint depth and third layer the remaining 1/5. Fully compact each layer and allow to become thumbprint hard before applying next layer. Take care not to spread mortar over edges onto exposed masonry surfaces, or to feather edge mortar.
    3. When mortar is thumbprint hard, tool to match original appearance of joints, unless otherwise indicated. Remove excess mortar from edge of joint by brushing.
    4. Cure mortar by maintaining in a damp condition for not less than 72 hours.


  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 only tools and equipment which are clean and free of hardened or partially hardened material. Use of metal scrapers or brushes will not be permitted.
  3. Clean sandstone only with fiber bristle brushes and water. Use no acids, detergents, or other cleaning agents