i'm trying to level/plane a subfloor for a hardwood installation. the subfloor is 1x8 shiplap installed diagonal to the floor joists. the house is old and has settled a bit. it's out up to 3/4" over a 12'x18' room. roof drainage that likely contributed to some settling and sag has been addressed. for time and budget i hope to avoid tearing into the flooring structure if it is possible. plus i want to stay at the current elevation for the installation to match existing floors in other rooms. it seems the advice out there includes patching and leveling compounds, shims of wood, cardboard (drywall type), asphalt, or just layering tar paper.
can anyone help with these questions:
i'm leaning towards sanding high spots and using the drywall shims in the dips but would they possibly compress over time? if not should i install them perpendicular to hardwood flooring or parallel? if perpendicular, at what spacing? with 3/4 maple hardwood can i safely bridge gaps of say 6" or so?
what type of asphalt shingles are recomended? are people talking about 3-tab tiles??? that seems a bit thick to me to work with. am i missing something here?
what about patch/leveling compounds? wouldn't portland cement based products not allow nails to go through? can anyone recommend a specific product that would work with a traditional nail down installation?
thanks for any help!!!