Thank you all for the great information and help on this forum! I am going to install 5" 3/4 solid hickory upstairs in my 1971 split level house in the Pacific Northwest. I removed tile and hardibacker to expose 2'x8' car decking that goes north to south on the majority of the floor that is 16'x25' rectangle. Adjacent to this is a 5'x8'rectangle where the 5' is additive to the 16' for a total of 21' long in this area. This small area has the car decking installed east to west because the timbers downstairs are going the opposite direction. Car decking is on 4'x8' timbers installed on 48" centers. The small rectangle transitions into two carpeted bedrooms, a closet and a tiled bathroom.

First Question: I plan on sanding the car decking clean to get rid of all the mortar that remains so it doesn't crumble underneath the new floor and compromise the integrity. Is this necessary/overkill?

Second Question: The new floor elevation is close to matching up with the bathroom tile floor elevation, which would be optimal. The problem is, I plan on installing the hardwood perpendicular to the car decking in the larger portion of the room and continue it into the smaller portion of the room where the subfloor does a 90degree shift to being parallel with the hardwood. So, if elevation allows I could lay the hickory directly (with moisture barrier) down onto the car decking however it would be parallel with a small portion of the subfloor. Or, I could sand the smaller portion of the subfloor so I can fit some 5mm underlayment on top of that section. Or, should I cut out the small section of the floor, sister some joists to the timbers and install a new subfloor adequate for the hardwood so i can continue my courses of wood across the floor? Or, any other suggestions?

Will continue trying to figure out how to post a picture if my description isn't clear.

Thanks in advance!

Hopefully this Picture will help:

http://www.hardwoodflooringtalk.com/gal ... cking5.jpg