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. 
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.

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.
I always tell my customers that ‘the first scratch is the worst scratch'.

Typically, when customers hear this they kind of sigh and visibly let go of the hope that their hardwood floor was special and would never scratch.....or..... that the scratch might just magically disappear if they rub it hard enough. 

The truth is - hardwood floors do scratch and continue to scratch if not taken care of.  With this said, customers need to see their hardwood floor as a fine wine aging and improving over the years.  A floor that shows signs of having been lived on and with can still be a beautiful thing. 

Here are some suggestions to minimize the issues that may come up regarding scratches.

1. Most importantly, the customers' expectations need to be properly set before they purchase the floor.  Don’t pull any punches on this topic, your client won’t thank you later.

2. Having established that all hardwood floor finishes scratch (pre-finished and site finished), you can begin to advise them on:

     a) how they can best care for their new flooring (click here for maintenance) and,
     b) advise them on a hardwood style or floor character that best suits their needs. Don’t leave them thinking 
         that a harder hardwood such as Brazilian Cherry will solve the scratch problem because it won’t.  Harder
         hardwoods do dent or compress less than their softer counterparts but the finish on top will still scratch.

3. Factory applied finishes are stronger than site applied finishes. Leave site applied, oil modified finishes, such as Polyurethane or Swedish finishes to cure for 48 hours before resuming normal foot traffic. Water-based finishes cure much faster  (24 hours) - but check the product instructions.

4. Aluminum oxide and ceramic finishes are very strong but still scratch.  It is ,however, unlikely that you will wear through these finishes to the wood. Be clear with your customer, set their expectations. Successful warranty claims based on premature wearing of floor finish are VERY rare in hardwood flooring. Read the 25 year warranty small print and help your customer understand why, because it will be you and your customer against the manufacturer should a claim be made later.

5. The darker the wood floor colour the more the scratches will show. Scratches are typically light in colour and therefore show up in strong contrast to a darker background.  Ever owned a black car?  It's no different in the world of hardwood flooring.

6. If your customer has 3 kids, a cat and a dog, then consider a floor with a busy grain that helps disguise inevitable wear and tear such as a plain sawn, natural Red or White Oak, Hickory, or Ash.  Busy grains distract the eye.

7. Maple (stained or natural), Brazilian Cherry, American Walnut or American Cherry are not good durability choices.  The grains in these woods are subtle and not busy like Oak, hence wear and tear and defects show up more. Always point out that American Walnut and American Cherry are softer hardwoods and will dent easily.

8. Recommend a low or satin sheen. This also helps to disguise wear and tear. Glossier finishes only highlight scratches and imperfections.

9. Dust or dry mop regularly. Spot clean sticky spills. Don’t wet mop the floor. Use a Swiffer in a ‘snow plow’ fashion to accumulate dust and grit.  Use a soft head vacuum cleaner.

10. Keep outside shoes off the floor.

11. Felt glides are a must on all furniture legs, even with laminate flooring.

12. New Kitchen appliances will scratch the hardwood flooring. If you have new flooring in the Kitchen and you are having new appliances delivered make sure the delivery people are take care not to drag or push the new fridge, stove or dishwasher across the hardwood floor. If they do they will leave scratch marks in the floor. It happens so often and is not easy to fix.

13. If you hardwood or laminate in your office and have a rolling office chair, use a plastic desk chair mat to protect the hardwood or laminate floor, they only cost around $40 from Staples.

Fixing scratches

If you have set your customers expectations properly then they know that scratches are par for the course with a hardwood floor   What do you do when the customer calls distressed about the first scratch?
Acknowledge the problem and be part of the solution.... this is good customer service.

Here are a few tips....
Surface scratches of this type caused by dirt and grit, poor maintenance, lack of felt glides etc. cannot be fixed locally but they can be fixed by re-coating the hardwood floor.  With a buffer and a 120 grit screen, the existing top coat of finish is carefully abraded and a fresh coat of finish applied over top.  My recommendation is to employ the services of a professional for this type of work and make sure the new finish to be used is compatible and will bond with the old finish before you commit. WARNING: If the re-coat fails then you are into a re-sand, a much bigger deal. Try a test spot first. 

Compression scratch marks from dogs or dragging furniture cannot be fixed by re-coating. You have to sand off the finish and into the wood to eliminate them.  Fixing this sort of wear and tear for the sale of a property or just to freshen up the floor is very common.  Locally sanding out the damage is not recommended as any patch work will show up like a sore thumb.  Either do the whole floor or live with the wear and tear until you can justify doing the whole floor.

Deep scratches or gouges for the most part can be filled with wood filler and then coloured with a Minwax stain stick (available at Home Depot).  This remedy is a good contender for a local repair because the deep scratch or gouge is often pretty ugly and easily improved with some simple attention.  Don't use black felt tips or sharpies on darker floors, the colour shows up purple!!  Always do a test patch in a hidden area first.

Remember, scratches are a fact of life with hardwood flooring. Help your customers understand this before they make a purchase.