Thank you all for the good ideas and facts.

It seems to me that if I can put a flashing on the upper entries to a joint and extend these down below the lower entry to a joint while leaving the lowest entry open, any water that enters will have a ready avenue of escape. I think that good ventilation will be a key factor.

Now what I can't figure out is how to attach the upper edge of the flashing material to the wood above the joint without water running down between the wood and flashing.

Is there a technique for letting in the flashing material into the wood surface with bedding compound to seal the upper edge?

Thanks,