Adding 2nd layer to subfloor


Hi all!

We bought a market home last summer (been in 13 months now) and the builder just cannot seem to resolve problems with the subfloor on the main and second levels.

The joists (dimensional lumber) seem to run every 16" (basement is unfinished so I can access the joists for the main level) and the subfloor is 19/32" (osb I believe). Interestingly, where there is hardwood (kitchen) or tile (bathrooms) the floors are fine. I have dealt with problems in just about every carpeted area, however.

1). When I walk from the kitchen into the family room and step off of hardwood and onto carpeting (walking perpendicular to joist run), there is a lot of noise that can be heard from the basement. I fixed some of this by loosening the clamps holding the water line to the joists. I discovered that with the pipes clamped so tight, the water line was actually creaking as your walked across the floor. Now I have noise in the main trunk of the HVAC that runs...

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By Lee Wallender

Updated March 29, 2016.

Not the

finish floor

, not the subfloor--flooring underlayment occupies a middle position in the anatomy of the entire flooring "sandwich."

What Is Flooring Underlayment?

Floors are composed of many different layers. Flooring underlayment is a thin, hard layer of wood or cementboard upon which the top, or finish, flooring can rest.

Soft underlayments of foam

or cork will be discussed in brief, but do not fall into the same category.

Here is one typical composition of flooring layers:

Top Layer - Finish Floor: The finish floor, such as hardwood, laminate, or tile.

Second Layer Down - Underlayment: Plywood, fiberboard, or cementboard, and is usually either 1/4" or 1/2".

Third Layer Down - Subfloor: Subfloor will already be in place (unless this home is under construction). Subfloor is typically made of OSB and ranges from 19/32" to 1 1/8" thick.

Fourth Layer...

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Originally Posted by


....I'll just add a layer of 5/8 to the subfloor and let him move the ducts to get it up in there.

So you mean you'll be cutting out strips of 5/8" rock and gluing them directly to the subfloor? Just want to make sure you're not going to add 5/8" to the bottom of the joists which would then create a triple-leaf situation which would be a bad thing.

Assuming the plan is to do it correctly (not triple leaf), it is surely a decent amount of extra work to cut strips of 5/8" rock and get it in there between the joists. I have to question if it's worth it. If you have hardwood flooring above, you already have decent mass.

I think what might help the most is extra distance between the subfloor above and your new dual 5/8" rock ceiling below it. I think you're better off sacrificing a bit of ceiling height in the name of getting the ceiling / subfloor farther away from each other.

I recently built a basement...

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Ok, so that this thread doesn't just die, keeping many confussed and worried whether they should answer again, I will point out what I think is incorrect information or just lacking a bit. Please do not get offended, but I think it's important to give good info. I'm assuming those that gave wrong info are not tile setters and were just trying to help. I know most here are pros in some trade or another, and no one can know everything.

#2 Wrong because he mentions it may be ok to leave the vinyl in place if it's stiff enough. Vinyl and it's underlayment should be removed. What is fibro? Are you thinking of something like HardiBacker or another CBU?

#3 The advice to remove the particleboard is good, but not to install Hardi or cement board over the original 1/2" ply. If you install CBU over 1/2" ply you're gonna have some issues.

#4 I just think you got lazy on this one and just agreed. Love your site and sounds like you do great work.

#6 The method...

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I am installing 3/4" handscraped oak in my 2nd floor condo. Well, I was until I pulled up the carpet completely to find what I originally thought was plywood that had been oversprayed with primer, but some form of concrete substance. I say some "form of" because the room is approximately 320sq. ft. & there is not a single seam in the entire room leading me to rule out sheets of concrete backerboard. It looks like whatever it is, was poured. I assume there is plywood underneath this product, but don't know for sure. I'm a little hesitant to start hammering into the stuff because it already has hairline cracks in several places. Regardless, has anyone out there seen this product in condo/townhome developments, more importantly know exactly what it is, and how do you lay hardwood floors on top of this type of surface. The flooring contractor won't touch it because: a) he does not know the depth of the product when it comes to driving nails through it b) claims that the long term...

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If you aren't going to glue down the 2nd sheet because you might have to remove it later on, think real hard about that. If you need to remove that second sheet, you're probably going to remove the first sheet as well. You aren't making anything easier.

Say you have a hardwood floor over the subfloor of 2 sheet of plywood. After you've hacked way at the hardwood we're going to worry about the 2nd player of plywood? Not me. If there is a crawlspace, get under it, drill holes and cut that subfloor off. Believe me, you aren't going to want to keep it. Think about the reasons you are taking off the hardwood floor, bet you have some real issues if you also need to remove the 2nd layer of plywood. You are going to end up going down to the joists anyway, betcha.

On to movement. If you use T$G and screw it down, sorry, it isn't moving every so slightly that gluing the 2nd sheet down is going to matter. It doesn't work that way. If the 2nd sheet is moving through contraction/expansion...

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1) Bill: I'm about to move into an attached house (row house). I'm not sure if sound proofing will work in this situation. There is a cinder block and brick common wall between the houses. The current sheetrock is on 1" furring strips attached to the sheetrock. I can sometimes hear music or conversation at a very low level coming through. I am not sure if it is coming up from the basement where there are some gaps in the wall that need fixing or over the ceiling. It sounds like through the wall. If I retrofit, then will the sound go away? Or is at so low a level now that it would be impossible to reduce it further? Is green glue, acoustical caulk and regular sheetrock enough or should I use Quiet Rock. thanks

Trademark Soundproofing Reply: Hi Bill, The GG is a very good performer on low frequency sounds, as in the ones that you are hearing. Being that you do not have insulation in the wall, performance is not at it's maximum. Standard sheetrock and GG will suffice. If you...

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Which Way Should Hardwood Floors Run?

Do you notice the direction hardwood flooring runs when you walk into a house? It can make a big difference in how it looks and how it behaves over time. Rule number one in laying hardwood flooring is the wood boards should run perpendicular to the floor joists below. This allows the boards to “span” from one joist to the next and be much more solid. If the boards were run parallel to the floor joists, most of the boards would sit only upon the plywood subflooring and not on any of the joists. The plywood is flexible and will “give” fractionally when walked upon. This is a recipe for squeaks and large gaps.

But another rule in laying wood flooring has to do with the aesthetics or look of the flooring. As a general rule, wood flooring looks best when running in the same direction as the longest dimension of the room. For instance, if the room is ten feet by sixteen feet, the wood flooring will look best when running in the...

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