Repairing Corroded Copper Sheetmetal Roofing Materials

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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.



A. This procedure includes guidance on repairing corroded copper sheetmetal roofing materials that are either stained or damaged as a result of acid rainwater run-off from other materials.

B. Copper roofing materials usually retain a long life. Copper’s longevity can be attributed to its surface patina that develops naturally over time and protects the surface against corrosion from atmospheric conditions or acids derived from organic growths on other roofing materials.

  1. Deterioration of the patina may be caused by acidified rainwater runoff, bituminous roofing cements, and alkali, ammonia or sulfate compounds.
  2. Corrosion attacks may also occur on copper roofing (especially in the valleys) where rainwater passes over roofing materials covered with mosses or lichens. If the moss or lichen buildup is great enough, it may reduce the pH level of the rainwater, causing the copper oxide film to dissolve.
  3. Rainwater passing over tile or slate onto a copper roof can also negatively affect copper sheetmetal roofing materials. Deposits from the tile or slate can abrade the sheetmetal surface and inhibit the formation of a protective oxide coating.
  4. Renewal and dissolution of the surface oxide progressively thins the metal at the drip points, leading to perforation unless steps are taken to prevent this.

C. Safety Precautions

  1. Wear rubber-soled shoes that have non-slip or grid type tread (preferably sneakers with a high top for good ankle support). Avoid wearing loose clothing.
  2. Wear a safety belt or harness and secure it to a substantial chimney or to a window on the opposite side of the house. Leave only enough slack so you can work comfortably in one area, and adjust the slack as you work on other sections of the roof.
  3. Be sure the roof is clear of debris and water.
  4. Do not work on wet or snow covered roofs. Work on cleated walkboards.
  5. Steep roofs: On roof slopes greater than 4 inches rise per foot, special consideration must be given to both footing and materials handling.
    1. Secure chicken ladders or cleats at the top for adequate footing.
    2. Hang and secure approved safety lines with ropes of sufficient strength.
    3. Carry a limited number of materials so that balance and footing are not impaired.

D. 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 before performing this procedure and should be followed, when applicable, along with recommendations from the Regional Historic Preservation Officer (RHPO).


A. Copper is highly resistant to corrosion caused by the atmosphere or salt water. It combines with hydrogen sulfide and oxygen or sulfur dioxide to form a protective copper carbonate or copper sulfate coating, which resists further corrosion and generally does not change further in appearance.

B. There should be no evidence of pitting or breaking down of the patina. There should be no sign of wearing, holes, or rust around drip points.


A. Qualifications: Copper should be applied by qualified sheetmetal mechanics using methods devised or approved by the manufacturer of the metal. Details may vary depending on the properties of the metal, local custom, and architectural effect required.


A. Storage and Protection:

  1. Material storage: Keep uninstalled roof materials under cover, dry, free from scratches, condensation, and distortion during delivery, storage, and handling. Protect sheetmetal edges from damage during delivery and storage.
  2. Salvage storage: Historic material to be used as example of original construction shall be stored as directed by the RHPO.
  3. Manufacturers’ delivery or job markings on metal, and adhesives for manufacturers’ labels shall be either a neutral or slightly acidic material. Never shall such material be alkaline; any staining of the metal by alkaline materials will be cause for the rejection of the piece.


A. Wash copper metalwork at regular intervals to remove corrosive elements, especially areas which are not effectively washed by rainfall to remove dust, grime, and soot. Carry out such cleaning with materials noncorrosive to copper or the copper patina.

Note: Avoid cleaning copper with alkaline soaps that do not contain sodium hydroxide. Avoid detergents containing pyrophosphates, such as Tide or ammonia solutions, as they will attack the copper.

B. Clean the roof of dirt build-up annually by rinsing with clean, clear water.

C. Keep the roof clear of debris, and trim all overhanging branches that might cause mechanical damage.

D. Inspect for and eliminate ant hills and/or bird droppings that can corrode sheet metals while stored. Caution: Do not use bleach to remove bird excrement. Bird droppings contain ammonia and, if mixed with bleach, can form toxic gases.

  1. Bird droppings can cause localized corrosion on copper because of the acids found in the droppings.
  2. Remove droppings using a wooden spatula; wash copper surface with a neutral detergent.
  3. Rinse with distilled water and wipe dry with a clean soft cloth, to prevent water spots and streaks.

E. Inspect the secureness of cleats and fasteners and the condition of the sheet metal after particularly heavy storms.

F. Never use any "black goop" (asphaltic roofing compound) or caulk to seal joints on a metal roof. Asphalt attacks metal roofing, and no caulk lasts long enough for this application.



Note: Chemical products are sometimes sold under a common name. This usually means that the substance is not as pure as the same chemical sold under its chemical name. The grade of purity of common name substances, however, is usually adequate for stain removal work, and these products should be purchased when available, as they may be less expensive. Common names are shown below by an asterisk (*).

A. 16–24 oz. sheet copper (weight of sheetmetal will vary depending on the situation)

B. Nails: Nails used for fastening copper shall be copper or hardware bronze of stronghold type or equal, with large flat head. They shall not be smaller than No. 12 stubs gauge (0.109 inches) and of sufficient length to penetrate roofing boarding.

C. Copper sulfate crystals (CuSO45H20) - review MSDS for safety precautions:

  1. A sulfate of copper especially the normal sulfate that is white in the anhydrous form but blue in the crystalline hydrous form and that is often used as an algicide and fungicide.
  2. Other chemical or common names include Cupric Sulfate; Blue stone*; Blue vitriol*; Roman vitriol*.
  3. Potential Hazards: TOXIC
  4. Available from chemical supply house, drugstore or pharmaceutical supply distributor, garden and lawn supply center, hardware store, swimming pool supply distributor, or water and sanitation supply distributor.

D. Clean, potable water

E. Clean, soft cloths


A. Chicken ladder, safety belt or harness

B. Protective gloves and gear

C. Straight snips for cutting straight or curved lines in sheet metal 24 gauge or lighter

D. Spray bottles

E. Garden hose



A. Whenever possible, make inspection from ground or from above if possible.

B. Look for characteristic rusty looking stain marks on the metal at the drip points that can be easily removed by rubbing them firmly with one’s finger. The bright metal surface will then be visible.

C. Look for areas of thinned metal. Press a finger firmly against the thinned area to test if the area needs to be patched.

  1. If the area resists indentation, it does not need to be patched.
  2. If the area is easily indented with minimal finger pressure, a patch should probably be soldered over the area.

D. Inspect for buildup of moss or lichen. This is more apt to occur on north slopes.


A. Protection

  1. Use scaffolding, ladders, and working platforms as required to execute the work. Ladders shall not be supported on hanging gutters. They may be distorted which can affect the slope to drain.
  2. Establish regulations for roof foot traffic. Many roofing materials should not be walked on.


A. If the metal has not thinned and is only slightly stained, kill any moss or lichen that has accumulated on the roof (see Section 3.03 E.1. below). No other remedial action should be necessary.

B. If the metal has thinned or has developed small perforations, solder a patch over the damaged area (see Section 3.01 C to determine if metal has thinned sufficiently to require a soldered patch). See 05010-7-R for guidance on soldering a metal patch.

C. If the entire length of roof slope upstand is badly corroded (especially at valleys or parapet gutters), replace the damaged metal and install a sacrificial copper apron. Acids in the rainwater running over the copper apron will react with the copper to reduce the corrosive effects of the acids running directly onto other copper roofing materials. Note: This apron will also eventually be corroded, but it is easier and less expensive to replace.

D. To prevent future copper corrosion problems caused by acid rainwater run-off over biological accumulations on other materials, kill growth from these surfaces.

  1. Apply a copper sulfate solution over the shingles or tiles supporting biological growth.
    1. Mix 1 part copper sulfate crystals in 10 parts water. Add the crystals slowly to the water and stir until dissolved.
    2. Spray the solution over the roof material covered with growths so it is thoroughly wet. Allow to drain from the roof.
    3. Thoroughly rinse gutters and pipes with clean, clear water.
    4. Repeat this treatment every three years.


  1. Install strips of copper cut to fit under roof shingles or tiles to treat rainwater run-off and deter growth of mosses and lichen on those surfaces.
    1. Cut long, 3" wide copper strips and slip under a course of shingles so that only 1/2" is exposed.
    2. Place strips every five to six courses, or even every 10 to 15 feet vertically along the slope of the roof.
    3. As rainwater washes over these strips, copper particles will be dissolved, acting as a biocide as it continues to wash over mosses and lichen below.


A. At the end of each work day, provide building protection for any exterior roofing element removed for repair.

B. Keep trees trimmed to prevent branches from scuffing roofing surfaces.