Historic Preservation - Technical Procedures
- Repairing Corroded Copper Sheetmetal Roofing Materials
- Procedure code:
- Hspg Prepared For Nps - Sero
- Thermal And Moisture Protection
- Sheet Metal Roofing
- Last Modified:
- Repairing Corroded Copper Sheetmetal Roofing Materials
- Last Modified:
REPAIRING CORRODED COPPER SHEETMETAL ROOFING MATERIALS
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
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
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
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
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
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
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.
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
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
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
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
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
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
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
D. Inspect for buildup of moss or lichen. This is more apt
to occur on north slopes.
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,
d. Repeat this treatment every three years.
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
a. Cut long, 3" wide copper strips and slip under
a course of shingles so that only 1/2" is
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.
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
END OF SECTION