Hello,
You can find out how far and how badly the wood is damaged by poking and prying around. When you move it inside it will stabilize pretty quickly just as it develops fungus pretty quickly. I don't believe the fungus will continue to live on the residual moisture in the wood once it is in a dry place. When you poke in and find sound wood, that is more or less the wood you will get from the time it's no longer exposed to the rain and weather onwards. In any case birch should not be used where it would be exposed to or come in contact with non atmospheric moisture.
Greetings,
Don Wagstaff
