You will have to make sure your joists can handle the addition of considerably more weight from the plywood subfloor and any floor coverings.
Floor levelling shims.
On the top side of the floor another fix it idea to lay down new hardwood over the existing floor.
If the uneven floor is due to floor joists that have warped or twisted over time or were incorrectly installed the only way to level the floor is to remove the subflooring and shim the joists.
Step by step process for cutting a six foot ling shim from a 2x4.
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Contractor cedar shim eastern white cedar shims are virtually a eastern white cedar shims are virtually a maintenance free wood.
This may require either sanding high spots or shimming with shingles in the low spots to build them up.
To shim the floor take a straightedge or a level and look for imperfections in the flatness of the floor and determine if it needs to be raised or lowered.
Find a straight board.
A plywood subfloor will bridge any minor waves in the existing floor and leveling compound would help too.
Figure out how much taper you ll need.
You would set shims such that when you level across those shims the tops of all the shims are level.
What he is describing is to lay shims over your existing floor.
You do want them fairly close together so yes 12 to 16 inches apart.
Self leveling compound is a cement based mixture that is applied in a near liquid state.
Clean the floor area to be filled with a broom or vacuum and dampen it with a wet sponge to avoid rapid drying.
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On center means you measure center to center not strictly between the shims.
These thick shims are scored to make size adjustments easy break off the excess shim for a custom fit around fasteners or corners.
They are easy to work with and have always been the first choice for all your home renovation projects.