Floor Leveling On Concrete - Preparation. Uneven Floors
Correct floor preparation with hardwood floor installations is the single most important part of the job. It insures the floor
will be a success in one year or twenty.
One Minute Quickie Videos
Grinding concrete
Mixing floor leveling compound
What Is Floor Preparation?
Floor preparation involves steps that must be followed before any actual installation. Checking for flatness or securing loose wood subfloors are all key aspects making sure the subfloor is in sound condition before it is covered with any new final floor. An age old saying in the flooring business sums it up well; "your final floor will only be as good as your subfloor." The quality of any new floor will be compromised just as much as a cheaper priced floor if these steps are not followed.
But The Other Guy Said I Need A Level Subfloor
I have yet to run into a hardwood installer that has a level in his tool bag. Levels are for block masons, trim carpenters, or marble tile setters. Sure some installation instructions may say level but they're really trying to say flat. Specifications call for subfloors being within a tolerance of 3/16 of one inch over a span of eight to ten feet. In other words, if there is a dip in the middle that measures 1/4" from each side in a ten foot range it should be taken 
care of.
Okay. How To Check For Flat?
A simple string line works well to check for flatness, or a straight 2 x 4. More on the subject at the page links below.
Other Forms Of Floor Prep
It may be as simple as removing all those clumps of mud the drywall dudes left behind. New floors do not function well when there is a void underneath. They need to lay as flat as possible to the sub floor. Without true flatness and once they are walked on, boards will flex or may rub together. This is where those squeaks may come into play. You may not notice it right away but a few months or a year, they will haunt you.
For glued type installations to concrete all paint should be removed. Plywood subfloors should be flat especially where the plywood sheets meet. Nails should be set if they are protruding from the subfloor. These may also exist between the subfloor and the floor joist system. If any areas feel squishy or sag before the hardwood is installed, fix them. Think VOIDS. We do not need any voids. For older homes that do not have actual subfloors, floor joist systems should be checked for plane.
Floating Floors
Don't let misleading instructions fool you. I never understood why some underlayment manufacturers mention their product helps correct minor subfloor irregularities. What many do it yourselfers do not realize is what minor means. It may help to fill the void in the short term but once that floor is installed and it begins to feel squishy, it's just a matter of time before the boards' interlocking mechanism is compromised and the squeaking starts.
Other Notes - Builders, Growl
Some tract home builders don't want to hear about floor preparation because they have to pay to have it corrected. I've even known some retailers to balk at paying an installer that has put in extra time to insure proper floor preparation. Why? They did not figure it into the costs of the installation and it cuts into their profit margin. They often never get paid from the builder either.
Folks, lack of floor preparation is rampant in the flooring business, specifically with hardwood and laminate installations. When interviewing potential installers ask what type of floor preparation may be needed. If they're covered by carpet it will be difficult to assess, but if you get a reply along the lines of just scraping or sweeping floors, you should look elsewhere.
Make it clear to anyone performing the work you want it done right and don't shriek at the costs. Floor preparation can get expensive as installers charge by time and material. Taking 
shortcuts by failing to look at floor prep seriously will only cause headaches at a later date.
How To Correct Concrete Sub Floors
Low areas should be filled with a Portland based cement compound that cures quickly. High areas should be grinded, otherwise you're likely to get a seesaw feeling after the installation. Grinding creates an enormous amount of dust, but hand held grinders with vacuum attachments can be used. It will not collect all the dust but a major portion of it. In any event be prepared and cover everything you value.
View The One Minute Quickie Video
Big Daddy Concrete Grinder
Close off other areas of the home and open windows for good ventilation when grinding. Dust from 
grinding will also get into kitchen cabinets. Don't expect these hand held concrete grinders to remove a big hump in a few minutes. Time and patience will endure. For major problems one can bring in the Levtec Expander grinder (shown) for flattening concrete subfloors in a flash.
Finding where to rent one of these beauties may be troublesome, but recently a few companies that specialize in floor removal have begun to offer this service.
Floor Leveling Compound?
Now that I've turned your attention from level to flat, we have leveling compound. Don't let it confuse you. Let's just call it floor patch. However it is called leveling compound in many circles. There are many choices out there. Home Depot seems to provide the most accessible source from my last visit around town. A local flooring supply store is another option and most sell to the general public. It's important to select a compound that will cure quickly. Ordinary cement mixed products will not cure quickly enough and will cause moisture problems after the installation.
Some names include Mapei and Ardex, with the latter providing a true floor leveling characteristic for those that may have severe problems that require complete work on the entire concrete slab. It's important to note, these fast setting compounds require fast working times.
For those that actually want to try doing this themselves, make sure you're ready, because this stuff will harden before you want it to. It may be wise to play with it first, getting a grasp on the properties. If there is an overabundance of work to be done I would highly suggest a professional. It took me a few jobs to get the hang of it in my earlier days. Luckily I had someone looking over my shoulder to point out what I was doing wrong.
Preparing The Patching Compound
Follow all manufacturers specifications listed on the product container. Most compounds are mixed with water. In our example I'm using Mapei. First add water to an empty five gallon bucket and a portion of the mixture. By using a paddle type mixer (attached to a drill) we're looking at getting milk shake consistency. By not adding water first, you will get a clumping mixture and won't be able to get to the compound that will be stuck on the bottom of the bucket.
Cautionary Note: When doing any mixing, it's preferable to do so outdoors or in the garage. This stuff will slop around on floors and walls if you're not familiar in working with it.
View The One Minute Quickie Video
Applying The Compound
While using the same straight edge, pour an amount in front of it, moving the material to the front edge with a float trowel. Pull the straight edge back over the low area. This is often termed screeding. It's best to have one person on each side of the straight 
edge/screed. Chances are you won't get a perfect screed but our area won't be visible after the installation, so looks aren't important.
Again, you won't have much time before the patch begins to harden. Once your first batch is used up, proceed directly to the hose for cleanup. There's nothing worse than dried compound in the bucket and on the tools. Allowing the material to harden for 20-30 minutes some areas can be trowled to a more desired appearance.
Correcting Different Floor Heights - Uneven Vertical Heights
Often in the midst of remodeling some are faced with different sub floor heights that were caused by previous house additions or just plain house settling. The question always remains how to fix and make corrective actions when considering installing hardwood floors through the areas without a transition.
Minor Differences
Typically if the difference is minor, shimming from the low side can take care of the problem. Shims can take the form of different thickness roofing paper or felt¹ for wood sub floors and leveling compound for concrete.
Shown on the right is a shim effect accomplished by 
tapering floor leveling compound back from the higher area. In this example, the concrete had to be raised 3/8 of one inch so our final finished floor height matched with the ceramic tile. The actual tapered areas started two feet back from the tile, creating a very gradual slope.
Keep in mind with concrete sub floors, an allowance must be made for any adhesive, moisture barriers, or underlayments used to achieve any success.
Sunroom Addition Wood Sub Floor
Let’s say we have a situation where a sunroom has been added in previous years and the difference is 5/16 of an inch that spans 20 feet. By installing hardwood over the area in this condition boards will tend to squeak after awhile because of the void underneath.
Adding felt paper parallel to the area can overcome the difference. With standard felt paper measuring 36 inches wide a solution can be adding one layer of heavier #30 felt against the lower area and cutting strips of #15 felt paper while layering and tapering back. For all intent and purposes in this scenario we would have three 20 foot long strips of #15 pound felt cut to widths of 27, 18, and twelve inches or thereabouts.
NOTE: using too many layers of #15 felt can create a spongy effect and should not be considered.
Larger Differences
Any sub floor differences greater should be addressed with different thicknesses of plywood. To fine tune any irregularities, the same method can be added under the plywood. Playing with smaller samples will provide the best idea what is needed.
The Floor Is Lower On Two Different Sides
Here’s a situation that drives many people bananas. By using the example above there are several areas that dip greater than 5/16 of an inch. Let’s say they are two and three feet wide. Instead of tinkering with more pieces of felt paper that usually ends up creating more problems, a simple solution is using sawdust, making sure it’s packed down before adding your felt paper.
Other Forms Of Minor Floor Prep
On wood sub floors, installers have also been known to use newspapers to offset any minor sub floor differences. Additionally, some floating floor manufacturers in previous years have also suggested the use of clean and dry masonry sand. If in doubt, I highly recommend contacting them directly before considering such. Any method, regardless of how effective it is, may be grounds for voiding a warranty.
¹ Can be used with floating floors on concrete.
Image Sources: levetec.com (3rd image from top)
Floor Preparation Wood subfloors
Floor Leveling Compound - Tools too
Moisture Barriers
Underlayment Types
