Your picture shows a fruiting body with a wrinkled or ‘merulioid’ fertile surface.
It looks to me like Serpula lacrymans or dry rot. Try googling some images of this species to see the likeness.
The fungus itself grows in damp wood and can then spread to dry wood, causing lots of damage.
I would advise you to get a specialist in to sort this out.
Good luck.
Ken