I live near Seattle and so probably have similar weather. My ducks used to live in a raised shed, and now live in a room in a barn. We put linoleum down on the floor so that we can actually wash it. We also put a thick layer of straw down inside the barn. Our vet recommended straw to prevent bumblefoot.
I can't imagine using straw outside. It seems like it would just be wet, mucky, and moldy. If my ducks ever track any straw outside I pick it up and compost it.
I think optimally you would build or buy something for them that is off the ground, fully covered/rain/weather proof. I wouldn't want any rain to be able to reach my ducks' bedding.
Have you heard about sweet pdz? Even though my 3 ducks have an enormous duck palace I still use it on their straw to reduce odors. I still clean it regularly, but it helps keep things fresh.