questions about best way to level sub-floor for hardwood installation

cbendock

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!!!

KCB

Is there anyway to fix the floor by raising the low area's from underneath?.
Maybe correct that problem so a year or 2 down the road your not dealing with the same issue?.
Build for the future is allways a good investment and not a waiste of your time and money.

nel262

I would try to level the floor by raising the low areas using the right thickness plywood glued and screwed.

cbendock

thanks for the input guys.

i'm afraid that jacking up the floor structure would cause cracking in my finished walls and mess up a window. but i fully agree about doing things right from the ground up. so i have fixed roof drainage issues and added extra support under the floor to prevent any further sinking.

i'm reluctant to add extra plywood because i would not be able to match the new flooring to the elevation of the existing flooring in other rooms, and still the issue of sags and bumps would remain.

anyone out there with experience using drywall shims or flooring patch compounds under nail down hardwood floors?

KCB

Well sense you dont want to add plywood or raise the floor underneath you have very few options.You could rip some 2x4's to compensate for the drop in the floor. Place them over each floor joist , rough sand the Ship lap for better bonding , pre-drill glue then screw them down. This would require a custom cut on every joist probably and a very skilled individual with a skill saw for a clean level cut.Use a lazer level to assure proper measure for each cut.Then use a 4 1/2 to 6 inch hardwood plank for better weight deflection where your 2x4' are very thin , adjust your planks so they dont break/join at these weak points.Thats the only other thing i can think of.
Now if you had some liquid wood or somthing you can nail to and level it out you'd be set .