Hardwood floor over sub floor with high spots

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Old 07-31-08, 07:43 AM
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Hardwood floor over sub floor with high spots

I'm installing hardwood floors on top of 3/4" CDX plywood sub floor that has several high spots. These bumps or high spots are right on top of the floor joists - for example if I lay a level down on the floor, centered on a floor joist, the level will rock on both sides (like a see saw). The sub floor has already been screwed and glued down. I'm not sure all these bumps are an issue because some of them are in a location where the original hardwood floor that I ripped up was. How do I know if these high sports will be a problem with the new hardwood floor I'm installing? If they will be problematic what can I do to address it?
 
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Old 08-01-08, 09:51 AM
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Couple of questions for you...

1) What type of flooring are you installing (ie solid prefinished, solid unfinished, or engineered?

2) How big of an area are installing?

The problem that you described is somewhat common in both NEW and OLDER homes. New homes settle and the main beam in the house may stay stationary. Older homes have similiar issues, but are usually more "complicated".

Deflection is measured using your 6' level and laying it (centered on the high spot) and holding one END of the level to the subfloor. Measure the height difference at the other end of the level (the end with the gap between the subfloor and the level). ALL MANUFACTURES have requirements regarding deflection and the maxium amount allowed over a certain span - so check your manufacture warranty or give them a call to see if you are going to void their warranty by installing over this "high spot".

Sometimes what I have done in the past is to use cedar shingles to "ease" the high spot. Install to within 6-8 rows of flooring to the high spot and then start installing thin pieces of cedar shingles to create a GRADUAL rise to the high spot and then do the same to create a GRADUAL decrease away from the high spot as you install past it. Not using this method, may increase the amount of flex in the new flooring, creating squeaks and eventual FAILURE of the wood floor.

Hope this helps,

Greg (Retired Floor Installer/Finisher)
Maine
 
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Old 08-01-08, 10:21 AM
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I'm installing solid prefinished flooring - 5" wide planks. The area is about 480 sq ft. The manufacturer suggests the sub floor be flat (within 3/16" in 10'). Can I use the above mentioned method with a 6' level to see if the floor is flat enough?
In addition to to using cedar shingles as shims, could I also use a belt sander on the plywood to knock down the high spots?
 
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Old 08-01-08, 10:37 AM
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Originally Posted by jaspers95 View Post
I'm installing solid prefinished flooring - 5" wide planks. The area is about 480 sq ft. The manufacturer suggests the sub floor be flat (within 3/16" in 10'). Can I use the above mentioned method with a 6' level to see if the floor is flat enough?
In addition to to using cedar shingles as shims, could I also use a belt sander on the plywood to knock down the high spots?

Well, first off... the wider the plank the harder it is to install over and area that has excessive deflection. I would use a 10' piece of string and check the deflection by nailing a nail into the high spot and checking the height over the span of the 10' length of string (attach one end of the string to the nail head). Do this in several areas over the length of the high spot.

Using a belt sander is a possibility. You would want to carefully remove all nails or screws prior to sanding though and then refasten with screws in new areas (dont go back into the old holes the screws came out of, they won't sufficiently hold the subfloor in place and you WILL get squeeking). Be carefull to take just enough off of the subfloor and feather the area out well with the belt sander. You may still have to use some cedar shingles (cut down to proper thickness - ie, if you cut the shingle to a length starting from the skinner taper end you end up with a less thick shingle - I think you get the idea).

Greg
 
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Old 08-01-08, 10:40 AM
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Originally Posted by jaspers95 View Post
I'm installing solid prefinished flooring - 5" wide planks. The area is about 480 sq ft. The manufacturer suggests the sub floor be flat (within 3/16" in 10'). Can I use the above mentioned method with a 6' level to see if the floor is flat enough?
In addition to to using cedar shingles as shims, could I also use a belt sander on the plywood to knock down the high spots?

Well, first off... the wider the plank the harder it is to install over and area that has excessive deflection. I would use a 10' piece of string and check the deflection by nailing a nail into the high spot and checking the height over the span of the 10' length of string (attach one end of the string to the nail head). Do this in several areas over the length of the high spot.

Using a belt sander is a possibility. You would want to carefully remove all nails or screws prior to sanding though and then refasten with screws in new areas (dont go back into the old holes the screws came out of, they won't sufficiently hold the subfloor in place and you WILL get squeeking). Be carefull to take just enough off of the subfloor and feather the area out well with the belt sander. You may still have to use some cedar shingles (cut down to proper thickness - ie, if you cut the shingle to a length starting from the skinnier taper end you end up with a less thick shingle - I think you get the idea).

Greg
 
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