Perhaps more shade and moisture. Or maybe not enough strong sunlight to dry the deck off in between moist weather bouts. I bet the painting and deck people would be able to zero in on the reason.
Thank you for the suggestions but this doesn't answer my question because b/c it is not the weathr conditions weather conditions we are having. It hasn't rained here much, snowed vwey little, and I am not the only one with tis isssue - except for mine seems to be worse. It is warmer than usual here my lhouse gets lots of natural sun...However, I was looking at my neighbor's roof last night= old aspestas (spelling?) an noticed it on her roof from the back, - pehaps that is the start, and it is just blowing around in the wind, and my hous is next in line to catch it. I don't know either though for sure.
Have you had a major blowy storm before you noticed it, like even a month before?
Very weird to have moss where it is dry and bright.
Terri read my article on roof moss...you might be able to apply some tricks to your deck and siding.
if its the north side of your house, thats exactly why!! bleach the wood and your siding..thats what i do on my north side ..ever notice moss only grows on the north side of trees...
@Margie Moss grows where ever the conditions are right for it. I have moss growing in FULL brutal Eastern thru most of the day full sun. In Upstate New York--and it stays green thru the winter. We have "mossy" stuff that grows on our deck in the same conditions and in planters etc; and some sort of odd stuff that grows on our soffiting that we finally managed to power wash off. the stuff also came off the deck with power washing with just water; now will have to re-stain as the stain is getting old but the moss--well the moss is very happy.