Repointing historic masonry

Technical procedures are supplied by GSA only as a reference. All procedures have been reviewed for general consistency with federal standards for rehabilitating historic buildings. 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.

Part 1 – General

1.0 Description

1. This specification provides guidance for the repointing of historic masonry materials.
   1. This specification has been developed for use on historic properties (defined as any district, site, building, structure, or object that is listed in or eligible for listing in the National Register of Historic Places) and provides an overview of accepted practices. Site-specific specifications, when appropriate, will be provided by the Architect.
   2. All work described herein, and related work must conform to the Secretary of the Interior’s Standards for the Treatment of Historic Properties.
   3. The Contractor shall provide all labor, material, equipment, and operations required to complete the rehabilitation work indicated herein.
   4. All work described herein, and related work must have the approval of a Cultural Resources Manager, Conservator, Historic Architect, or other professional who meets the standards outlined in the Secretary of the Interior’s Standards – Professional Qualifications Standards pursuant to 36 CFR.

1.1 Section includes

1. Repointing of historic masonry materials

1.2 Related sections

Section 01100-07-S for general project guidelines to be reviewed along with this procedure. These guidelines cover the following sections and should be reviewed prior to performing this procedure and should be followed, when applicable, along with recommendations from GSA Historic Preservation.

1. Safety Precautions
   1. Historic Structures Precautions
   2. Submittals
   3. Quality Assurance
   4. Delivery, Storage and Handling
   5. Project/Site Conditions
   6. Sequencing and Scheduling
   7. General Protection (Surface and Surrounding)
   8. Historic Treatment Procedures
2. Masonry Cleaning Specification
3. Temporary Sealant Specification

1.3 Quality assurance

1. Minimum of five years experience in masonry cleaning and restoration including repointing and shall have successfully completed at least three projects of similar scope within the previous five years. He/she shall demonstrate a working knowledge of The Secretary of the Interior’s Standards for the Treatment of Historic Properties with Guidelines for Preserving, Rehabilitating, Restoring & Reconstructing Historic Buildings.
2. Field Supervisor Qualifications: Full-time supervisors experienced in historic treatment work similar in nature, material, design, and extent to that indicated for this project. Supervisors shall be on project site during times that historic treatment work is in progress. Supervisors shall not be changed during project except for causes beyond the control of the specialist firm.
3. Worker Qualifications: Persons who are experienced in historic treatment work of types they will be performing.
4. All repointing must be performed by a contractor that is familiar with historic  mortar formulations, curing conditions and performance characteristics. Contractor shall provide proof of such knowledge to the Architect by submitting a certificate from a U.S. Heritage Group Lime Mortar Workshop, similar workshop course, or sufficient proven project experience.
5. Only skilled journeymen masons who are familiar and experienced with the materials and methods specified and are familiar with the design requirements shall be used for masonry restoration. One skilled journeyman mason, trained and Certified by the specified manufacturer, shall be present at all times during masonry restoration and shall personally direct the work.
6. Source of Materials: Obtain materials for stone repair and mortar repointing from a single manufacturer source to ensure match quality, color, grain size, texture, and detailing.
7. Contractor shall provide manufacturer’s standard warranty for not less than one year, commencing on Date of Substantial Completion.
8. Mortar is to be replaced in kind as determined by a laboratory analysis and submitted to GSA Historic Preservation. Mortars used shall not have a strength in excess of adjacent stone nor original mortar mix as determined by laboratory analysis. Historic mortars are to be lime based.

1.4 Submittals

1. The Contractor shall submit to the Architect, GSA Approving Official or GSA Historic Preservation:
   1. A detailed schedule of the areas to be repointed, including an assessment of the problem surfaces, as well as proposed testing and  mockups and associated procedures, methods, and products.
   2. Once repointing mockups are completed and approved by GSA Historic Preservation, Contractor shall submit proposed masonry repointing procedures, and mortar mixes for full scale project.
   3. The manufacturer’s product literature for all proprietary mortar products. Product literature shall include specification data, Safety Data Sheets (SDS), and instructions for storage, handling, and use.
   4. A project safety plan to include at minimum: any personal protective equipment to be used by the contractor staff such as disposable clothing, gloves and safety glasses, procedures for emergencies and accidents, documentation of staff training in use of equipment.

1.5 Testing and mockups

1. Contractor, at inconspicuous locations designated by the Project Team and GSA Historic Preservation, shall perform small test patches (no larger than 6 inches by 6 inches) for all products to be used. Testing will determine the best method(s) to perform the work and ensure that the chosen products do not cause harm.
2. Following testing, mockup panels shall be created (mockup panels may incorporate successful test patches) using the chosen methods. Size and location of mockup panels shall be determined by the project Team and GSA Historic Preservation.
3. The contractor shall submit pucks of fully cured proposed mortar mixes prior to full-scale mockup.
4. Following mockups, the contractor, at inconspicuous locations designated by GSA Historic Preservation, shall prepare and repoint a sample area of not less than 10 feet high and 10 feet long, using the procedures, materials, proposed colors, grain size, texture, finish and workmanship as approved by the architect and GSA Historic Preservation. Deviations to this sample size need to be approved by GSA Historic Preservation.
5. Mockups should include:
   1. Damage free removal of existing mortar performed by raking joints by hand using joint cleaning chisels and hammers, to be approved by GSA Historic Preservation prior to repointing.
   2. Repointing of masonry to match existing mortar in strength, color, grain size, historic tooling, and texture when dry.
   3. Mockups should cure for a minimum of 14 days prior to GSA Historic Preservation approval.
   4. One mockup should be performed for each proposed color of mortar. Contractor may do separate mockups for mortar removal and repointing, or have GSA Historic Preservation inspect mortar removal prior to repointing.
   5. Power tools may be allowed for use when applicable, especially for horizontal joints, however the parameters for use should be outlined and approved by GSA Historic Preservation and a mockup must be completed and approved prior to use.
6. Approved mockup panel(s) shall become part of the work and shall serve as the quality standard for all similar work.
7. Any testing, demonstration, or mockup that is rejected by the project team shall be removed in its entirety without damage to the surrounding masonry and a new demonstration/mockup shall be performed for the project team’s review.

1.6 Delivery, storage, and handling (as applied to products and materials)

1. The Contractor shall:
   1. Deliver prepackaged, dry-blended mortar mix to project site in clearly labeled plastic-lined bags each bearing the name and address of the manufacturer, production codes or batch numbers, and color or formula numbers.
   2. Store mortar off the ground and in accordance with manufacturer’s instructions to prevent contamination by foreign materials. Maintain packaged materials in a clean, dry state protected against weather, traffic and foreign materials.
   3. Deliver all proprietary products to the project site in manufacturer’s or distributor’s unopened packaging, undamaged, complete with application instructions and Safety Data Sheets (SDS).
   4. Collect and dispose of any waste material, packaging, debris, and effluent associated with the masonry cleaning work in accordance with local, state, and Federal environmental regulations.
   5. Comply with the manufacturers written specifications and recommendations for mixing, application, and curing of repointing mortars.

1.7 Safety

1. The contractor shall only use chemical products whose safety data sheets (SDS) have been reviewed and considered acceptable by the Government. Any project surfaces containing lead-based paint, as disclosed by the Government, shall be handled by the contractor in accordance with OSHA (29 CFR 1926.62), EPA (40 CFR 261) and State regulations. Any lead-based paint or coating that is removed in a project resulting from this scope shall undergo Toxicity Characteristic Leaching Procedure (TCLP) testing by the contractor. The contractor shall dispose of any lead-paint waste as hazardous if it fails the TCLP.
2. Any paint or coating required to be removed which has not been disclosed by the Government to contain lead, shall be tested for lead by the contractor. All paints or coatings testing positive shall be handled and disposed of as lead-based.
3. The contractor shall isolate the work area with cones, tape or similar means as possible for the duration of the project, to prevent unauthorized individuals from entering the immediate work area.
4. For any work required on elevated surfaces (4 feet or more above the ground) the contractor shall install and use any ladders, scaffolding or platforms in accordance with OSHA regulations (29 CFR Subpart M).
5. Contractor and sub-contractor employees shall wear all required and appropriate PPE while on site.
6. All applicable OSHA and GSA safety regulations must be followed and safety plans must be approved.

1.8 Environmental requirements

1. Cold Weather Requirements: When ambient air temperature is below 40 degrees F, heat mixing water to maintain mortar temperature between 40 and 120 degrees F until placed. If necessary, store materials in a heated area to allow mortar temperatures to remain above 40 degrees F throughout the placement and finishing cycle and for at least 48 hours after completion of work.
   1. Do not use frozen materials or materials mixed or coated with ice or frost. Do not lower the freezing point of mortar by the use of admixtures or anti-freeze agents, and do not use chlorides in the mortar.
2. Hot Weather Requirements: In accordance with ACI 530.1 pointing shall not be performed when ambient temperature is greater than 100 degrees F or surface and ambient air temperature is greater than 90 degrees F with wind velocity greater than 8 mph. Phase repointing during hot weather by completing the process on the shady side of the building or schedule installation of materials during cooler evening hours to prevent premature evaporation of moisture from the mortar.
3. Do not perform pointing under conditions that fall outside the manufacturer’s requirements, which include:
   1. Surfaces that are frozen; allow complete thawing prior to installation.
   2. When surface or air temperature is not expected to remain above 40 degrees F for at least 8 hours after application.
   3. Wind conditions that may blow materials onto surfaces not intended to be treated.
   4. Less than 24 hours after rain or when rain is expected less than 6 hours after installation.
4. Any exceptions to this specification must be reviewed and approved by a historic masonry professional and the GSA Project Team.

1.9 Project/site conditions

1. Perform repointing after cleaning masonry materials.
2. Contractor shall protect persons, motor vehicles, building site, and surrounding building materials and buildings from injury or damage resulting from masonry restoration work. This includes surface areas on adjacent wall surfaces or roofs not included in this scope of work.
3. Prevent repointing mortar from staining the face of masonry or other surfaces to be left exposed. Immediately remove all repointing mortar that comes in contact with such surfaces.
4. Cover partially completed work when work is not in progress.
5. Protect sills, ledges and projections from drips and droppings.
6. Damage occurring to the building as a result of work of this section of Contractor’s failure to protect against such damage shall be the Contractor’s responsibility. The contractor shall restore damaged areas to the complete satisfaction of the Architect at no expense to the Owner
7. All Contractor personnel performing masonry cleaning operations shall be provided by the Contractor with gloves, respirators, protective clothing and any other personal protective equipment (PPE) as recommended by the manufacturer of the masonry cleaning products and required by local, state, and Federal regulations.
8. The Contractor shall complete installation of temporary sealants, as per sealant specification, at window and door perimeters prior to starting cleaning operations where required to prevent leakage to the interior.

Part 2 – Projects

1. Materials: Deviations may be approved by Project Team and GSA Historic Preservation in accordance with results of the mortar analysis.
   1. Portland cement: ASTM C150 Type I, gray or white as required to match original mortar. Fly ash, slag and pozzolans are not permitted as substitutes for Portland cement.
   2. Hydrated Lime: ASTM C207 Type S, incorporated as a finely divided powder in uniform particle size, free of lumps, flakes or other inconsistencies.
   3. Mortar Aggregate: ASTM C144 Natural sand blend, rounded to sub-angular in shape, washed, screened and dried, with zero or near zero -270 crystalline silica content. Aggregate to be selected to match the color, grain size, and texture of the original mortar aggregates as closely as possible while remaining in compliance with ASTM C144 grading and soundness requirements.
   4. Mortar Colors: Inorganic mineral oxides meeting the requirements of ASTM C797, at levels not to exceed 10% on cement weight, except for carbon black, which may not exceed 2% on cement weight.
   5. Admixtures: NO admixtures shall be used without the express written consent of the Architect and the mortar manufacturer. Calcium chloride is not permitted in any mortar. Admixtures containing more than 0.1% chloride ions are NOT permitted.
   6. Water: Potable, free of deleterious quantities of materials which may affect mortar performance or appearance.

Part 3 – Execution

3.1 General

1. Joints shall be raked out using hand tools only. If needed, a circular power blade can be used to relieve pressure by cutting down the middle of the joint. Work must be preapproved by GSA Historic Preservation and proficiency must be demonstrated to ensure that historic masonry is not damaged.
2. The repointing mortar shall match the original in color, grain size, and texture. The compressive strength of the repointing mortar shall be equal or less than the compressive strength of the original mortar and surrounding brick or stone. The replacement mortar shall contain approximately the same ingredient proportions of the original mortar. The type of mortar will be determined by the laboratory analysis of the historic mortar and approved by the architect and GSA Historic Preservation. Testing will be the responsibility of the contractor unless it has already been performed by the AE.
3. All replacement mortar ingredients and mortar formulations will be established from test data gathered from the original materials sampled from site as determined by a mortar testing laboratory.
4. The testing laboratory shall supply a ready mixed mortar sample sufficient in size for a mock up sample at the site.
5. Mixing of individual mortar ingredients at the construction site will not be permitted.
6. Repointing mortars shall be pre-blended in single containers in a factory-controlled environment. All ingredients will be converted from volume measurements to weight measurements to ensure quality production of the mortar.
7. All containers shall be marked including manufacturing date and batch number. Manufacture is required to maintain production-sampling procedures for each batch for quality control purposes. Manufacturer to provide samples of proposed materials for mock up panels at the site. All pre blended products are to meet applicable ASTM standards and project specification requirements.
8. Thoroughly mix mortar in quantities needed for immediate use, using mechanical mortar mixer or paddle mixer. Add approximately half the required water and mix mortar for a minimum of 5 minutes, and then slowly add water as needed to reach the desired working consistency. Do not exceed a mix time of 10 minutes. Ready mix mortars are prohibited.
9. Add only clean, potable water at the project site. Do not add cement, lime, bonding agents, coloring admixtures, set accelerators, plasticizers, air entraining admixtures or other materials.
10. Use mortar within 1-1/2 hours of mixing, after which unused mortar is to be discarded. This period must be reduced to 30 minutes in hot weather. Do not retemper mortar.

3.2 Joint cutting and raking

1. All mortar is to be removed by hand using hand tools such as appropriately-sized chisel and resilient mallet.
2. All joints (unless otherwise noted) shall be raked back to sound, solid, back up material.
3. All raking out should leave a clean, square face at the back of the joint to provide for maximum contact of pointing mortar with the masonry back up mortar. Shallow or feather edging shall not be permitted.
4. Existing mortar joints shall be raked out a minimum depth of 2.5 times the height of the existing mortar joints, however, so as not to compromise the structural stability of the wall, the joint should not be raked out more than half the width of the masonry unit. EXAMPLES:
   1. 1/16” Mortar joint needs to be cut out to a depth of 3/16” minimum
   2. 1/8” Mortar joint needs to be cut out to a depth of 5/16” minimum
   3. 1/4” Mortar joint needs to be cut out to a depth of 5/8” minimum
   4. 1/2” Mortar joint needs to be cut out to a depth of 1-1/4” minimum
   5. 3/4” Mortar joint needs to be cut out to a depth of 1-7/8” minimum
   6. 1” Mortar joint needs to be cut out to a depth of 2-1/2” minimum
5. Utilize hand tools and power tools only after test cuts determine no damage to masonry units results. GSA Historic Preservation or approved alternative to approve.
6. Vertical joints (head joints) SHALL NOT be raked out using rotary power saws. All vertical head joints must be removed by hand in masonry unless a demonstration can be made that rotary use can be implemented without over cutting the joint, i.e. “over running.” Vertical joints exceeding 6” in height may be approved for cutting with rotary power saws pending a successful demonstration to GSA Historic Preservation.
7. Do not damage masonry units.
8. Existing horizontal mortar joints (bed joints) that are filled with a hard Portland mortar may be raked out using a diamond blade that is narrower than the joint width. The middle one-third of the mortar joint may be cut using a rotary power saw. The remaining mortar shall be removed from the masonry joints by hand using masonry chisels or pneumatic carving tools powered by air. Contractor must receive permission for the use of power tools from GSA Historic Preservation to be received after contractor has successfully demonstrated that power tools can be used without causing damage.

3.3 Repointing

1. Rinse joints with water to remove dust and mortar particles. Rinsing should be performed prior to repointing so that the joints are still damp but there is no standing water when joints are pointed. If joints dry, contractor shall dampen them prior to repointing.
2. Contractor shall thoroughly wet the wall below the joints prior to repointing to avoid soiling and prevent staining. During hot or windy weather, walls and joints should be wet several times prior to pointing.
3. Pointing shall be performed by hand using hand tools. Grout bags shall not be used to fill joints.
4. Joints shall be pointed in layers or “lifts” where deeper than 3/4”.
   1. Joints greater than 3/4 inches deep shall be pointed with an initial lift to bring the joint depth to a uniform 3/4 inches deep.
   2. Compact each layer at the time it is placed in the joint by applying firm pressure with the pointing tool.
5. Allow each lift to become thumbprint hard before applying the next lift.
6. Tool joints uniformly to match historically finished joints as shown in the approved mock up.
7. Remove excess mortar from edge of joint by brushing with a stiff natural bristle brush. Metal brushes are not to be used.
8. Cure mortar by maintaining in thoroughly damp condition for a minimum of 72 consecutive hours including weekends and holidays.
   1. To maintain wetness during curing, mortar may be covered with wet burlap and plastic sheeting and periodically handmisted.
9. Inspect mortar for hairline cracking during and after during. Hairline cracking within the mortar or mortar separation at edge of a joint is unacceptable. Completely remove such mortar and repoint.

3.4 Final cleaning

1. Maintain clean surfaces on the face, sills, ledges, and projections of masonry on a daily basis.
2. Exposed masonry surfaces shall be thoroughly cleaned of excess mortar and foreign matter, after the mortar has fully hardened. Use wood scrapers, natural fiber brushes, and clean water, spray applied by hose, hand pump sprayer, or sponge.
   1. Do not use metal scrapers or brushes.
   2. Do not use acidic or alkaline cleaners.
3. Remove minor mortar marks from masonry by misting with water and brushing with a small, stiff-natural bristle brush. Metal brushes are not to be used.
4. Wash adjacent woodwork and other non-masonry surfaces. Use potable water and soft brushes, sponges or cloths.
5. Sweep and rake adjacent grounds to remove mortar and debris. Do not disturb sub-surface material during site cleaning.

3.5 Final report

Contractor shall:

1. Provide a final report of completed work, including all approved submittals, mockups and photographs of the areas repointed that were taken before, during, and after the work. Report shall also include:
   1. A written summary of the project
   2. Results upon final inspection and approval
   3. Discussion of steps taken or new findings not specified in the initial documentation
   4. Any applicable ongoing care and warranty requirements