I have cedar siding on my house. The panels have overlapping vertical
seams, and look like they are made up of several thinner pieces. Sorry,
don't know what they are properly called.
Where the deck is attached to the house, I have had rain come in through
the siding and run down the wall of my walk-in basement. I presume the wind
blows the rain againt the house; it cannot run off and must go somewhere, so
it goes through the siding.
This has only happened twice in the 13 years I have owned the house, but it
still annoying. I am having my house painted by someone who is also a
carpenter, so I had him look at it.
He says they should have put flashing in, but there is no way to do that
now. He thinks caulking it will probably just make it worse; so his advice
is to clean it out as thoroughly as possible so the water can run through
rather then being forced in, and then just live with it.