"bob haller" wrote in message
...

.

up till a number of years ago no roof around pittsburgh had that wierd
discoloration, now every roof does that faces north. something big
must of changed.

It did. The makeup of the shingles changed. I think it was copper, but
could have been something else that was added to the shingles to keep the
moss and mold down. In order to make them cheaper , that was left out of
the shingles, or greatly reduced.

I have seen it mentioned that if you take a copper wire and weave it in the
shingles about every 4 or 5 rows, it will help.