There is a hallway with an uneven floor tilt to one side due to foundation settling more on that side. The settling had since been permanently stabilised.
The goal is to nearly level the hallway by shimming one side about 3/4 inch. A 36" wide sheet of plywood with thickness of 3/4" on one side of the 36" width and 0 inches on the other side would seem to fit the bill. It would be glued to the existing plywood subfloor.
How can this plywood be "shaved" to achieve that 3/4" to 0" gradient ?
I know from hard experience that it's very challenging to get a flat outcome doing something like this. Your existing floor isn't flat or level, and you need to remedy both of those things. A simple sheet shim probably won't do it and will be difficult to create in the first place.
Don't try to create wedge shims for each joist, either. That's a huge pain and never comes out perfect unless you measure thickness at frequent intervals from a straight line between reference lines, then cut very accurately freehand.
Instead, remove the existing subfloor by cutting as close to the walls as you can and start fresh. Pull carpenter's lines from end to end on each side, leveling across between them. Now evaluate the larger situation to decide whether flat and level is appropriate, or whether you should compromise with adjacent floor levels.
Once you have that plane established, sister 2x4s to the joists with 3" screws (and construction adhesive, if you like), aligning them with your reference lines. Now you can simply lay in your new 3/4" subfloor, fill the edges where you have the original subfloor remaining, and move on.
Build it up, don't shave it down. Depending on how smooth and level you want to get it, do some or all of the following steps:
That's the approach I would take (and have taken) when confronting low spots in an otherwise solid subfloor. Basically, build it up a layer at a time. Cutting pieces to weird shapes is far easier than trying to shave down a piece, and for most flooring types the subfloor only needs to be 'pretty close' to provide a solid base for the floor. For sure you can't tell where I had to build up layers under the hardwood floor I did this on, and it's still holding up just fine seven or so years later.
Glue a 3/4" inch block to one side of your ply wood, place a 1/2" block at 1/3 of of the way, and a 1/4" block at 2/3 of the way and then run the 36" sheet of wood thru a planer, cutting a 1/32" off each time... Might be able to do it faster, but planers eat plywood for dinner so, slow and steady wins the race.
Purchase self leveling concrete, mix it up as directed then pour it on your floor, make sure you get it to 3/4" on the side that is not level and spread it across the hallway... Let it cure then put flooring over it.
Not the prettiest solution, but it works.
instead of using self leveling concrete, use a 2 part, thin plastic resin, it will not grind to dust like the concrete, will bend and flex (depending on the type of resin) with the subfloor, and can be screwed through more easily. It will just cost an arm and a leg. Though this cost can be reduces by putting strips of wood, or some other filler into space to be leveled with resin to reduce the amount of resin required...
Advantages: Can get much thinner then self leveling concrete. Will not crumble. Disadvantages: Cost... Also, also future remodelers will look at what you did and wonder why in the ***** you did this. Will either take far to long to cure, or not long enough... Slight danger of fire... Will make the hallway unusable for a long period of time. Also might summon Bob the god of Builders to smite you for heresy.
Shaving a 3 foot wide sheet of plywood into a shim is not a realistic solution. You could probably get it done but there are easier and better ways of doing it. For one thing the plies are going to start delaminating and chipping as you're working the plywood and it's going to be a colossal pain to get the taper right over 3 feet. And that floor is probably not a straight gradient so you'd end up with lumps.
The better way is to use a patching compound, for example Mapei's Planiprep SC. There are a lot of manufacturers making their own analogous products so take your pick but you want something that feathers down to skim coat, can be laid down as thick as you need (3/4"+) and probably is modified for flexibility if your floor has some bounce. Patch any big holes in the floor, and either build up the patch or lay a straight edge along the low side and screed it. You can grind it down some if you overshoot (but do wear a respirator).
You could also use self-leveling compound but that seems trickier and has more gotchas. For a small-ish area like you're working with I would go (and have gone) with the manually applied patch compound.
You could make a square router plate that rides on two pieces of wood. As you across the room, lower it a little bit at a time. Once done, use a self leveling compound to even it out more.
