in the public domain and posted from the site 'copper.org'
Fundamentals: Architectural Considerations
Weathering, Corrosion, Staining, Substrate, Solder, Sealants
of the most important issues concerning the use of copper
is the chemical reaction between copper and other materials.
Chemical reactions are responsible for corrosion, staining,
and even the green patina that develops on copper surfaces
The oxidation process that gives copper its characteristic
green patina is a result of exposure to an acidic atmosphere.
The process is, therefore, faster in some metropolitan, marine,
and industrial areas, where higher concentrations of pollutants
exist. When acidic moisture comes in contact with exposed
copper surfaces, it reacts with the copper to form copper
sulfate. The acid is neutralized during the reaction with
the copper. This patina eventually covers the surface and
adheres tightly to it, thus providing a protective layer against
All metals have a property called nobility. It is a measure
of a metal's resistance to corrosion when in contact with
another metal. A greater relative difference in nobility between
the two metals in contact indicates a greater corrosion potential.
Table 1.1.4 ranks the most common metals used in construction
in increasing nobility, called the galvanic number.
1.1.4 - The Nobility of Common Metals
5. Stainless Steel - Active
9. Stainless Steel - Passive
||The galvanic scale to the left contains a partial list of commonly used construction metals.
dissimilar metals are in contact with one another in the presence
of an electrolyte, galvanic action occurs, resulting in the
deterioration of the metal with the lower galvanic number.
The electrolyte may be rain water running from one surface
to another, or moisture from the air containing enough acid
to cause it to act as an electrolyte.
copper has one of the highest galvanic numbers or nobility
of the active metals, *it will not be harmed by contact with
any of them.(*New information from other sources: Please take note that some facts are now becoming evident that may contradict this.) It will, however, cause corrosion of the other
metals if in direct contact. The solution is to prevent such
direct contact with the use of separating materials, such
as specific paints or gaskets.
is not necessary to isolate copper from lead, tin or stainless
steel under most circumstances. The principal metals of concern
in terms of direct contact are aluminum and zinc.
paints or coatings are used for isolation, they must be compatible
with both metals. Bituminous or zinc chromate primers can
be used between copper and aluminum. Either of these or a
red lead primer can be effective in separating copper from
iron and other ferrous metals.
or gasketing with nonabsorptive materials or sealants are
effective methods of separating copper from all other metals.
In areas with severe exposure, lead or similar gasketing materials
should be used, except between copper and aluminum.
of the method used to separate the metals, wash from copper
surfaces should be prevented from draining onto exposed aluminum.
Traces of copper salts in the wash may accelerate corrosion
of the aluminum.
type of corrosion, which affects copper, is caused by the
flow of acidic water concentrated on a small area of copper.
This type, often called "erosion corrosion", occurs
when rain falls on a non-copper roof such as tile, slate,
wood, or asphalt. The acidic water is not neutralized as it
flows over the inert material. When water, collected over
a large surface, is diverted or collected by a relatively
small copper flashing or gutter, the copper may deteriorate
before it develops a protective patina. Another type of corrosion
occurs at the drip edge of inert roofing material conducting
water into a copper gutter or valley. If shingles rest directly
on the copper, the corrosive effect is amplified because moisture
is held along the edge by capillary action resulting in "line-corrosion".
The solution is to raise the lower edge of the shingles with
a cant strip, or to provide a replaceable reinforcing strip
between the shingles and the copper.
The wash of water over copper surfaces can have additional
impact. Moisture in contact with copper surfaces tends to
pick up small quantities of copper salts. When this moisture
contacts porous material, such as marble or limestone, it
is absorbed. As the moisture evaporates, it leaves behind
the copper salts as a stain on these materials. The green
stain is particularly visible on light colored surfaces.
condition does not occur with heavy rains or similar rapid
run-off, since the dwell time of the moisture on the copper
is short and little copper salt is picked up. Staining results
from the slow bleeding action of copper laden moisture.
are a number of ways to reduce staining or its visual impact.
Two common methods are: collecting run-off in gutters and
directing it away from the building via downspouts; and designing
drip edges to a minimum of one inch, helping reduce the amount
of copper laden moisture that comes into contact with material
below. Coating the adjacent surface of the porous material
with a clear silicone sealant can reduce staining by minimizing
the amount of moisture absorbed into the surface. The use
of lead-coated copper results in a black or gray stain, which
may blend better with some building materials.
The preparation of the substrate onto which copper will be
applied depends in part on the substrate selected and the
copper application. A number of considerations, however, must
always be taken into account.
selecting the substrate, a key consideration is the method
of attachment of the copper. All applications that rely on
nails or screws to attach the copper or cleats to the underlying
structure require a nailable deck, nailing strips within the
deck, or wood blocking at specific locations. Such applications
include standing seam roofs, batten seam roofs, flat seam
roofs, continuous edge strips and cleats, and flashings around
of the attachment method used, the structural integrity of
the substrate should not be compromised. It must be able hold
the roof under sustained design wind conditions, as well as
to conform to all other required codes and standards.
most common substrate for copper is wood, usually 1/2"
to 3/4" plywood. Lumber should be kiln-dried and laid
with all joints true and even to provide a smooth surface.
It is recommended that wood be allowed to weather for a few
days after installation. During this period it should be protected
from rain, allowing it to conform to atmospheric temperature
and moisture level, while settling into place.
have been many recent developments in fire retardant treated
(FRT) plywood and lumber. Most of these products use wood
or plywood that is pressure-impregnated with chemical salts
in water solution to inhibit combustion. Many of these salts
are corrosive to copper, as well as other metals and materials.
If leaching of theses salts brings them into contact with
the copper, corrosion will occur. This is particularly likely
in areas with high humidity, if condensation occurs, or if
water is introduced during construction or at a later time.
Any areas where salt laden moisture can collect then evaporate,
thereby increasing the concentrations of salts, will accelerate
the corrosion process. For a complete and updated report on
Fire Rated Plywood and Corrosion contact CDA.
materials used as substrates for copper include: concrete,
brick, masonry units, terra cotta, and stucco. The guidelines
discussed above apply to these materials as well. Smooth,
dry surfaces, compatibility with copper, and provision for
fasteners are all required for an acceptable substrate.
Sheet and strip copper applications in construction are inevitably
required to provide some level of resistance to water penetration.
Anything that can cause punctures or openings in the copper
membrane should be avoided. Copper roofs, valley flashings,
and gutter linings should always be applied on a smooth, dry,
stable surface with no projecting nail heads or other imperfections.
Movement in the substrate should be accommodated by properly
designed expansion joints.
such applications, an approved underlayment, usually saturated
felt, must be applied to the substrate. The felt acts as a
cushion for the copper sheets. A sheet of rosin-sized building
paper should be inserted between the copper and the underlayment.
This will prevent bonding between the two surfaces that would
otherwise restrict the thermal movement of the copper. The
only exceptions to this requirement are applications where
the copper is not intended to move, not even under thermal
stress. For example, continuous cleats and edge strips are
nailed down, usually in a staggered pattern of nails 3 inches
on center, to limit movement.
Solder and Sealants:
Copper construction methods have traditionally relied on solder
to ensure water-tightness and to strengthen joints and seams.
The solder used is common 50-50 tin-lead bar solder for uncoated
copper, and 60-40 tin-lead for lead-coated copper. It is typically
applied to mechanically fastened or formed, rigid joints.
Soldered seams and joints are permanent; they should last
the life of the copper. Continuous, long runs of soldered
seams should be avoided to limit stress fractures.
the weathering process, the lead contained in solder turns
gray. Exposed solder in the finished joints can be minimized
with the use of blind soldering. In this technique, solder
is applied to the back or concealed edge of copper surfaces.
An alternative to solder, where its
additional strength is not required, is the use of sealants.
Sealant filled joints have been used successfully for standing
seam and batten seam roofing applications where roof slopes
are less than three inches per foot. Sealants can also be
used in joints that are primarily designed to accommodate
thermal movement of the copper.
sealants used should be tested by the manufacturer and designated
as compatible for use with copper. Many elastomeric polyurethane,
silicone, butyl, polysulfide or other inorganic or rubber
based sealants have shown acceptable performance. Acrylic,
neoprene, and nitrile based sealants have been observed to
actively corrode copper. The use of such sealants is, therefore,
|One more Source of good
U.S. General Services
Historic Preservation Technical Procedures
A. This procedure
includes guidance on repairing chimney
flashing that is missing, deteriorated or damaged.
B. Flashing is an
integral part of a roofing system and
protects against water infiltration at roof hips and
valleys; penetrations such as chimneys; where roofing
meets a vertical surface such as a parapet or at a porch
roof; or at drip edges.
C. Copper, with copper
nails, galvanized sheet metal with
hot-dipped galvanized nails, or terne-coated stainless
steel with stainless steel nails are the primary
materials used for flashing.
CAUTION: THE DIFFERENT
METALS SHOULD NOT BE MIXED.
RAINWATER RUNOFF FROM COPPER FLASHING WILL CORRODE BOTH
GALVANIZED SHEET METAL AND STAINLESS STEEL THROUGH
GALVANIC ACTION. TAKE CARE TO USE A FLASHING MATERIAL
COMPATIBLE WITH THE REST OF THE ROOF SYSTEM