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Historic Preservation - Technical Procedures

Spectitle:

Repairing Corroded Copper Sheetmetal Roofing Materials

Procedure code:

0761006R

Source:

Hspg Prepared For Nps - Sero

Division:

Thermal And Moisture Protection

Section:

Sheet Metal Roofing

Last Modified:

12/02/2014

Details:

Repairing Corroded Copper Sheetmetal Roofing Materials



REPAIRING CORRODED COPPER SHEETMETAL ROOFING MATERIALS


PART 1---GENERAL

1.01 SUMMARY

    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.

              a.   Secure chicken ladders or cleats at the top
                   for adequate footing.

              b.   Hang and secure approved safety lines with
                   ropes of sufficient strength.

              c.   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).

1.02 SYSTEM DESCRIPTION

    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.

1.03 QUALITY ASSURANCE

    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.

1.04 DELIVERY, STORAGE, AND HANDLING

    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.

1.05 MAINTENANCE

    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
         THESE 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.

         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.

         CAUTION:  DO NOT USE BLEACH TO REMOVE BIRD EXCREMENT.
         BIRD DROPPINGS CONTAIN AMMONIA AND IF MIXED WITH BLEACH
         CAN FORM TOXIC GASES.

    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.


PART 2---PRODUCTS

2.01 MATERIALS  

    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 oz. - 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

2.02 EQUIPMENT

    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


PART 3---EXECUTION

3.01 EXAMINATION

    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.

3.02 PREPARATION

    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.

3.03 EXECUTION, INSTALLATION, APPLICATION

    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 05010-07-
         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.

              a.   Mix 1 part copper sulfate crystals in 10 parts
                   water.  Add the crystals slowly to the water
                   and stir until dissolved.

              b.   Spray the solution over the roof material
                   covered with growths so it is thoroughly wet.
                   Allow to drain from the roof.

              c.   Thoroughly rinse gutters and pipes with clean,
                   clear water.

              d.   Repeat this treatment every three years.

         -OR-

         2.   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.

              a.   Cut long, 3" wide copper strips and slip under
                   a course of shingles so that only 1/2" is
                   exposed.

              b.   Place strips every five to six courses, or
                   even every 10 to 15 feet vertically along the
                   slope of the roof.

              c.   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.

3.04 PROTECTION

    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.

                             END OF SECTION
 


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