Well, wood by itself is combustible - at least as far as WE know, we've never actually run into a kind of wood that wasn't. However, even in our world, we have wood that is specially treated to be fire resistant - such wood can be found used in the construction processes of homes built in mountains or even in areas that have a lot of forest fires.
Although I will stress this that even if such wood doesn't exist, even in our world, I would still consider it a realm of possibility given we reside in a fictional setting =)