I just pulled up my carpet to install 3/4 solid oak hardwood and found my floors have been built badly, very un-level. 2/3 of the floor is okay in one direction but then it drops off. It is so bad I will not be able to lay the hardwood until I fix it. From current research it look like I could build it up with 1/4 ply and glue sand it but I have no idea!!
What is the correct way to fix this issue please? The home was built this way rather than any foundation problems!
I have a pic to help show the issue-