OSB/Plywood Subfloor?

I'm preparing a room that had carpet for LVP installation. Found that I have OSB for the most part but I also have a 2x14' section of plywood along the wall where a floor vent is. Where the OSB and plywood run together under a wall they appear to be the same thickness but away from the wall the OSB can raise as much as 200mils but its not consistent. Other than the thickness differences between OSB and plywood the OSB in general looks pretty rough and has a few dips that will need to be leveled.

What's the best way to deal with this?

Thanks in advance.
Don

200 mils is about .2 inches. So your plywood to OSB seam varies by a little less than 1/4”. I think I would feather it out using multiple layers of roofing felt. 6 layers of 30# roofing felt is .187”. Just shy of your .2”. Maybe you cheat it a little and go with 7 or 8 layers.

Or you could ease the lip with a sander then feather it out with patch.

Thanks for the reply.

My next step was to measure subfloor heights around the room. I used my rotary laser level and found the high point to the low point was just over an inch.

Floors don't care about being level, they just care about being flat. Then again, flat and level provides your best outcome.

havasu said:

Floors don't care about being level, they just care about being flat. Then again, flat and level provides your best outcome.

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I'm looking for best outcome. This first room is a test to determine if I want to DIY or pay someone. The following rooms are more of a challenge.

I'm thinking using a self leveler for the entire room might be an option.

SLC is a great choice, provided you have a thicker wallet.

Finding the right product that is compatible with osb and has a decent working time is going to be the challenge.

Michael Stager said:

Ardex Feather finish can help your low spots without the cost of self leveler, and if you have enough of a slope to the room, the self leveler might not be the best option as it may throw off the rest of the home. The OSB and plywood will expand and contract at different rates as well, so floating self leveler across them might cause issues at the joint. Another option would be to put down a 1/4" underlayment over everything.

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This is another option I would consider. If filling in the major low spots and using a 1/4" underlayment will give me the proper surface for a clean LVP install then I'm in. What 1/4" underlayment would you suggest?

Michael Stager said:

options abound. You could use 1/4" plywood stapled down. You could use quiet board, which would be taped together and float like a regular underlayment, but acts just like 1/4 Ply would. benefits would be easier install of underlayment, extra noise and thermal insulation plus a softer underfoot feel without having a "squishy" floor. Costlier, so only beneficial to someone staying in the home long term. Also can be used if wanting to install an electric heating floor ( plastic sheet style that looks like the defroster on your back window ) - more features/benefits at a higher cost. But the Manufacturer ( MP Global ) will back up your floor install over the top of it if the proper prep work has been done below it. Just need to figure out if you are getting the right ROI ( return on investment ) out of using it vs a plywood based underlayment.

One thing I would recommend is to speak with the manufacturer of the product you are going to use and find out what they recommend. If you follow their recommendations, then you will have an easier time with any kind of claim. Document how you install the floor with pictures and keep a file on it for future reference. Take measurements and photos.

As a really good friend of mine used to say..."CYA" ( Cover your ass )

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Thanks for the feedback!
We are in our forever home now but I have a lot of square footage to cover on the first floor so probably won't go with super expensive underlayment. I was recently shocked to see the cost of 1/4" plywood underlayment @ $33 per 4x8 sheet.

I do have squeaking floor syndrome I want to fix so I already screwed down the osb subfloor in the first room. Should I also screw down 1/4" plywood underlayment?

Michael Stager said:

squeaky subfloors are caused by nail movement, so any issues you have with the flooring should be addressed by running screws next to the loose nails into the joists to secure the subfloor/joist connection.
1/4" subfloor should be just stapled with 3/4-1" staples, but you could screw it down if you'd prefer, but it's thin, so you have to be careful not to overdrive the screws.

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Issues I've had with screws is frequently the premium grades of underlayment are much denser/stronger than some of those cheaper or degraded subfloors. So there's not enough grip on the screw threads to pull the screw heads down flush. They just "strip out" the wood rather than secure the ply where the screw head cant penetrate the surface of the underlayment leaving the head of the screw "proud.". So now you'd best pull that screw out or it could be a problem down the road. In addition those screw heads, IF you can sink them can create dimples that need to be smoothed before you can patch.

Staples at least can be set with the right PSI to flush out and for some flooring you wouldn't need to prep over each one. When I've encountered a soft ply subfloor, usually under very old and deegraded commercial flooring the only thing that really worked for me was ring shank nails. I don't have a compressor and gun rig so for me I hammered them in by hand. To be fair, most of those subfloors really needed to be replaced but these were typically what we call Portables--------prefab classroom or other school buildings. The Districts couldn't do change orders easily so we had to lump over a ton of substandard substrates. That's sort of the only way you survive in commercial work with a long term customers who needs things done on a tight budget and a tight schedule.

I dont recall ever encountering OSB in my commercial career or the less than 1% residential flooring I did so I dont really know how well the ring shanks would bite.

Michael Stager said:

Ardex Feather finish can help your low spots without the cost of self leveler, and if you have enough of a slope to the room, the self leveler might not be the best option as it may throw off the rest of the home. The OSB and plywood will expand and contract at different rates as well, so floating self leveler across them might cause issues at the joint. Another option would be to put down a 1/4" underlayment over everything.

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I'm looking for Ardex but it seems to be only available online. I saw a few posts that claims Ardex Feather finish and the Henry product is the same. Is this true? One of the other issues I find is that many of these products don't support OSB wood.

TNGuy said:

I'm looking for Ardex but it seems to be only available online. I saw a few posts that claims Ardex Feather finish and the Henry product is the same. Is this true? One of the other issues I find is that many of these products don't support OSB wood.

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Henry is a subsidiary of Ardex and the Feather Finish versions are very similar but I did notice a difference-----that's been about 4-5 years since I used the Henry FF so those things do change. Ardex would be available to any flooring professional where they buy SUPPLIES. Retail flooring stores and Big Box stores do not carry Ardex. It's a commercial product we would buy by the pallet or truckload.

I dont think for your purposed it could matter. Either way it's a learning curve of your mixture and working time. It can blow off faster then you can apply it under certain conditions. Out here in the Godforsaken California desert cities we have hard water which affect the chemistry. On top of that on new construction the water coming in off the hydrant can be WARM, if not hot. Good luck skimcoating 25K square feet with that. But in a small residence these things dont matter whatsover as soon as you learn the requirements (working time)

Incognito said:

Henry is a subsidiary of Ardex and the Feather Finish versions are very similar but I did notice a difference-----that's been about 4-5 years since I used the Henry FF so those things do change. Ardex would be available to any flooring professional where they buy SUPPLIES. Retail flooring stores and Big Box stores do not carry Ardex. It's a commercial product we would buy by the pallet or truckload.

I dont think for your purposed it could matter. Either way it's a learning curve of your mixture and working time. It can blow off faster then you can apply it under certain conditions. Out here in the Godforsaken California desert cities we have hard water which affect the chemistry. On top of that on new construction the water coming in off the hydrant can be WARM, if not hot. Good luck skimcoating 25K square feet with that. But in a small residence these things dont matter whatsover as soon as you learn the requirements (working time)

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I'm going to start one dip at a time until I get use to the process.

I'm familiar with the hard water in SoCal. Even the beach cities had it.

Michael Stager said:

To the best of my knowledge, Ardex isn't concerned as much with water quality as it is with the temperature of the water. Thinsets in general need water to activate the material so that the crystal matrix grows. The longer it stays wet, the deeper the crystals grow and the better the bond. Use a primer on the OSB if you want to get better adhesion. OSB has a waterproofing applied to the surface, so I'm sure that's part of it...it prevents a good bond.

Also, don't ingest the powder, it's terrible for your lungs ( remember, it grows as a crystal in the presence of water...ie your lungs )

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I was able to purchase Ardex on Walmart of all places, 3rd party seller at under 30 bucks per bag. What I'd like to do is first get my subfloor flat enough for the 1/4" plywood.