# What material to fill the low spots on plywood subfloor?



## a1481155 (Dec 2, 2012)

Installing floating engineered wood on plywood subfloor in 2nd floor bedrooms and hallway. 

Regarding plywood subfloor prep, the manufacturer states: "It must be flat with no visible bumps or low spots; the subfloor should be flat to within 1/8” in 6 feet span or 3/16” in 10 feet." I have an 8' level, which shows level is good but there are a couple of areas that are not flat to within at least 1/4" in an 8 foot span, which I assume need to be filled in.

If you were installing floating engineered on a plywood subfloor on the 2nd floor, what material would you use to fill the low spots on plywood subfloor?


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## JazMan (Feb 17, 2007)

Hi,

There's many choices for your repair. You'l just have to go to your favorite hardware store and read a few labels. Since you forgot to enter your location we can't be very specific cuz we don't know what stores are near you.

Essentially you're going to use a cementitious patching compound. Be sure it says it's for what you specifically wanna do. It must say it's for over wood and you might have to mix it with a latex additive. 

Jaz


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## a1481155 (Dec 2, 2012)

Thanks!
I read about concerns with similar materials creating unwanted fumes, cracking, movement and/or noise, when used with floating installs.

Anyone have thoughts on using door skins, like this guy:
https://www.youtube.com/watch?v=Y4-5S6buKfg (Sorry, I wasn't sure how to embed the video.)


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## JazMan (Feb 17, 2007)

Stop, Stop, I can't take anymore of that nonsense. 

He just created a perimeter ridge and chances are the center is still low so he'll have to do this same thing again. The floor is gonna look like to topographical map.

Jaz


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## Fix'n it (Mar 12, 2012)

i have used tar paper.


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## a1481155 (Dec 2, 2012)

JazMan said:


> Stop, Stop, I can't take anymore of that nonsense.


With all due respect, that's not very helpful.


JazMan said:


> He just created a perimeter ridge and chances are the center is still low so he'll have to do this same thing again. The floor is gonna look like to topographical map.


The flooring manufacturer told me that with a floating installation, there's a little bit more room for error than with a glue-down. They said, if the "perimeter ridge" is less than 1/8” in a 6 foot span or 3/16” in 10 feet, there should be no "topographical map but there's always the option of sanding the "perimeter ridge", if needed.

FWIW, from Consumer Reports: Installing flooring: The inside story; June 07, 2011 "...*Make sure the subfloor is flat and quiet.* Screw a wood subfloor to the underlying joists to minimize creaking. Then pad low spots with plywood or shims. Fill dips in a concrete subfloor with leveling compound (also known as concrete patch)..." http://www.consumerreports.org/cro/news/2011/06/installing-flooring-the-inside-story/index.htm


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## Fix'n it (Mar 12, 2012)

Fix'n it said:


> i have used tar paper.


this is for very small dips. 

larger stuff, i have used thinset. so far it has worked real well. and its easy to use. 

btw. check all of the boards for warp, put these boards so they will be under furniture or something. warped boards in the walk area will make the floor feel like the subfloor was not flattened. ask me how i know this.


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## a1481155 (Dec 2, 2012)

Fix'n it said:


> i have used tar paper.


Thanks, that's very helpful!
Found this video, where the guy uses shingles, along with #15 & #30 tar paper to taper. https://www.youtube.com/watch?v=5MwqcYvhAAo


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