Carpet hides a multitude of sub-floor sins and so when buying new carpet you mostly don't incur extra costs for sub-floor preparation and leveling.  In the world of hardwood flooring things are very different.

In my experience for the most part, 7 out of 10 sub-floors will need a small to moderate amount of leveling, 3 out of 10 will need a significant amount and occasionally one will be so bad that it becomes cost prohibitive to fix the subfloor and so the choice is made to go back to carpet.

Ignoring the need for sub-floor levelling to save a little money is not a good idea. Remember, once the floor is installed the subfloor cannot be corrected.

The results of laying your new hardwood floor over an uneven sub-floor will be:
  • Excessive movement which can lead to premature failure of the flooring
  • Squeak and creaks
  • Gaps and separation
  • Veneer delamination for engineered flooring
  • Loss of wood flooring manufacturers warranty
The problems that come with an uneven sub-floor become evident once the floor is installed. If you install a floating floor over an uneven surface then you will notice soft spots where the floor dips when walked on.
If you install a glue down floor over high and low areas it can release under stress
Squeaks and creaks can develop when excessive movement is present in the floor. Note: that a small amount of movement is normal and to be expected with a floating floor because the underlay beneath the hardwood is supposed to compress. The problem of movement comes when the hardwood flooring is allowed to bridge two high spots in the sub-floor leaving the floor suspended over a low spot. 
The flooring is not designed to support wait in this way and so is easily pressed down into the low spot when someone walks over it. If the floor deflects more than 1/8" or 3mm this may lead to a problem in the future.

Leveling adds time and money to the whole project. A moderate amount of leveling will take a day or two to complete in a 500 square foot space.  Estimating how many hours of labour and how many bags of concrete leveling mixture will be needed is like asking 'how long is a piece of string'.  Therefore, I always price floor leveling by the man hour plus materials because it is the fair and equitable way for both parties. 
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Cosmetic cracks in concrete slabs (as shown in this image) typically  represent high spots in the concrete and are perfectly normal. This picture shows about a 1/8" width crack that is about 6' long. Such cracks don't pose a structural threat unless something is really obviously wrong such as a difference in height between one side of the crack and the other or if there is movement in the concrete or significant width to the crack such 1/4" or more.



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High spots in the concrete will typically need to be ground down using a grinder with a diamond tip blade.  Lots of dust is generated so a vacuum attachment should be used with the grinder.  It is still dusty work so hanging plastic and using fans placed in windows to blow dust outside are again a must.

Beware the 1970's +, three or four story, apartment buildings that are wood frame construction with light weight acoustical concrete, poured between floors. These are particularly bad for building settlement.  Over time the building foundations settle and sink slightly and by the time that slight foundation movement transfers to the upper floors it can result in significant floor unevenness.  This unevenness will pose big challenges for the hardwood flooring installation.

Subfloor levelling is equally important for nail down, glue down and floating floor installation methods.

Good luck with you installation.
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Chris
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9/19/2013 04:39:38 pm

I was not aware of the tips which you have shared regarding Subfloor levelling. Great details shared.

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10/14/2014 09:12:02 pm

Before installing your hardwood floor I strongly suggest you read this informative article. It helps you understand the need for levelling the subfloor beneath before installing hardwood.

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