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-Atmospheric corrosion (the most common form, caused by
the sulfur compounds found in industrial exhausts and
the salt air and seawater in coastal areas).
Not all corrosion is bad.  Oxidation sometimes forms a
protective coating for the metal, limiting further damage.
The greenish patina that forms on copper roofs is one
example of this "healthy" corrosion.
o Mechanical breakdown.  Metals may break down because of
abrasion from moving dirt, dust, sand, grit, sleet and
hail, or rubbing.  Abrasion is especially critical with
metal flashings and valleys used on slate roofs.  Human
use is another cause of abrasion.  Roof areas that must be
walked on for maintenance should be protected with wood
decking.  Metal fatigue is caused by repeated low-level
stress.  Metal fatigue often causes the failure of copper
roofs, which expand and contract in response to
temperature changes.  It also may lead to structural
failures of metal railroad bridges.  Creep, overloading,
fire damage, weathering, and connection failure are other
common metal problems.  Cast iron may not only rust, but
can also be split by freeze-thaw cycles if water gets
behind the rust as in a column or baluster.  When iron
pieces are attached to masonry, such as stair railings set
in stone or concrete, rusting iron may expand and split
the masonry.  Most architectural cast iron is made of many
small pieces bolted together.  Many problems, such as
wobbly stair rails, can be solved by taking the element
apart, and cleaning and tightening all the bolts.  You may
have to use a larger bolt or screw to make up for metal
lost through rusting.
4.2.3 Wood.  Fire may be the fastest way to destroy wood, but
water is by far the most common.  Besides the damage it can do
directly, it encourages fungus, mold, and insects; and it hastens
structural failure by weakening wood members.  Dry rot, wet rot,
brown rot, fungus, and mold are all signs of excessive moisture.
Insect infestations require professional treatment.  Be alert for
evidence of bugs or damage.  Powderpost beetles leave little
piles of sawdust at their exit holes.  Old-house borers make
large oval holes in coniferous wood.  Termite damage can be found
by sticking a pen knife into the wood.  Professional inspection
and treatment for insect infestation is needed to prevent further
damage.  (See Figure 4-9.)
Decayed wood should be replaced or repaired whenever it is
found.  Epoxies can be used to stabilize damaged pieces.  In-kind
replacement (i.e., with wood) is usually fairly simple.  Molded
and decorative shapes are widely available from commercial shops.
When necessary, large decorative features high on a building may
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