Jay Spenchian wrote:

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.

I'd appreciate a really quick reply from anyone who has an experience
at all in this, as the roofing is being done as I write this.

Thank you.

Don't know the answer but your question strikes as being very apt
and to add my two cents;
With the prevalence of acid rain and/or a salty maritime climate
the possibility of corrosion between the zinc galvanizing and
copper; I believe the two metals are somewhat apart on the 'table
of elements' the possibility of electrolytic corrosion may be
high?
However it may come down to practical experience and whether you
can find unbiased advice that exactly fits your situation. Maybe
we are talking 50 years before any corrosion would, in practical
terms (i.e. your grandson will have to fix it?) have any effect.
Whereas, if 5 years from now you have discoloured water leaking
down the cedar siding due to holes in the 'new' flashing you will
be most disappointed?
Terry.