Floor Prep - Wood Sub Floors
If you haven't learned by now surfing through the site, one key to 
successful wood floors is floor preparation. Without do so, chances are pretty good you may begin to hear strange sounds as the new floor is not comfortable with the sub floor. Always follow manufacturer specifications for a trouble free and long lasting one.
See the one minute quickie video
Flat Sub Floors
"Okay that's great but..what can I do to flatten out my wood subfloor? There are high and low spots here and there, especially where the plywood sheets meet." During original framing some floor joists may not have been straight or had a slight bow running the length. This will cause some humps in the final sub floor itself and should be corrected to insure you don't have a see-saw effect in your home. Any voids under a hardwood floor are also likely to create squeaks you don't want to hear.
Power Tools To Fix
One great tool that works well to remove minor high spots is the use of a flooring edger. If you live near one of our large home centers most of them will rent edgers for a four hour period or daily rate. Thinking of using a belt sander? Sure it will work, but the power of this tool can't be matched. The picture above shows a few plywood seam areas that were knocked down with 30 grit paper (also available with rentals). Another option is a power planer, but can be dangerous unless all nails are set deep enough not to be hit by the blades of the planer.
But We Have Severe Dips
If the situation is too severe, removal of the subfloor may be needed and attention to the floor joists should be addressed. However in some cases with major dips in a few areas, different thicknesses of plywood can be fastened into the valleys to create a bridge effect. For example; you have an area 36" wide and runs the length of one room along an outside wall. Let's say it slopes one inch. One could opt to install strips of plywood in the area in varying thicknesses so it tapers back to height of the rest of the floor.
Minor Dips and High Spots
In the next example we've encountered a high floor joist and an area adjacent to it that sags. Without corrections, squeaks are sure to be heard. The flooring edger knocked down about an 1/8" of an inch at the plywood seams (1) where #30 roofing felt was used (2) to bridge the low area with standard #15 felt paper rolled out prior to the installation.
About the floor prep before I continue. If you have any squeaky areas before the hardwood is installed, take care of it before it's too late. Often plywood will loosen from the floor joists over time. Make sure they're snug. Decking screws work the best to keep both tight against one another.
Shim Areas Increase Vertical Height
The better installers will view transition areas with a keen eye. After all, they will be the most visible locations in the home. There's nothing worse than a great installation with sloppy or inadequate methods used to address these areas. The next example is an area that leads to ceramic tile. Our 3/4" solid hardwood will fall about 1/4" below the marble threshold exposing the cement backer board underneath. Caulk it? I don't think so. A simple solution is to shim with our #30 felt once again.
In this case, four layers of the #30 felt were tapered back to bring the flooring up so the durock underlayment (under threshold) isn't shown. When using any material to shim it's important to taper back from all angles. Not shown in this case is the tapering we did on the sides of the felt. In order for this to work you have to create a gradual incline so the hardwood will engage properly by tongue and groove.
View The One Minute Quickie Video
Why Not Self Leveling Compound?
I do not advocate the use of any self leveling compound over wood sub floors. The problems one may or will encounter over time is the slight expansion and contraction of the wood sub floor. Essentially any solid leveling compound is bound to fracture with this movement causing sounds underfoot you’d rather not want to deal with. Furthermore, any nails used for a nail down installation are also likely to break apart the compound enough to cause potential problems.
Other Materials That Can Be Used
Other materials can include non corrugated cardboard and newspaper, or anything that does not compress easily or crumble. The focus is the maintaining the integrity of what goes under the actual hardwood floor. For floating floors some manufacturers over the years have approved clean, dry masonry sand to be used as a self leveling material. For those contemplating going this route I urge you to get approval from the hardwood manufacturer. Some of these companies can be very fickle when it comes to warranties. Any misstep and they’re likely to void any warranty regardless of the reason for failure.
