"Jay Spenchian" wrote in message
om...
Hi-
For the past week or so, and as I speak, we have been reroofing our
25-year old Colonial. Much of the house is brick, however, some parts
do have cedar siding. In these areas there is a lot of
roof-meeting-the-siding going on, and they had been originally flashed
with galvanized steel. However, it is impossible to remove the
flashing without severely damaging the cedar siding. We had choosen
to use copper flashing on all parts of the house, and the roofer
thought he probably could just put new copper flashing, for the new
fiberglass roof, over the existing galvanized steel flashing, which
was for the existing shake roof and is incompatable with the new roof.

So my question is, is it safe to flash a different metal on top of an
existing one? We are worried about creating current or some unforseen
problem with what should be a fairly mundane thing.

Is it safe from the stand point of the new flashing being prematurely corroded?

Nothing will happen until the galvanized coating is gone, which probably will be
pretty quick once it is in contact with the copper. After that the underlying
steel will be next. Since the copper acts as a cathode it will not be consumed
during this galvanic process (copper only supplies electrons the zine and iron
supply the chemical energy during the oxidation process which causes the loss of
material, also known as rusting).

Painting both the new and old flashing surfaces that will be in contact will
help slow the corrosion process. Any nails which pierce both materials should
be copper nails--otherwise the nails will quickly be destroyed by the
concentrated galvanic currents present at the point of contact between the two
materials created by the nails.

Regards,

John