"bob haller" wrote in message
...

roofers can add some metal stips toprevent it from re growing.

its a effect of global warming

While the thread is very old, the main reason for the growth is the shingles
used to have copper and maybe zinc in them. Over the years they cut way
back or eliminated the elements that stopped the growth. I just had a roof
put on and they have started putting more growth preventive chemicals back
into the shingles and have a long warrenty about that growth.