Soft subfloor prepping for LVT

Discussion in 'Floor Preparation' started by LizD, May 16, 2019.

  1. LizD

    LizD New Member

    We're about to start DIY replacing our kitchen & adding a wall/laundry room. My top choice for flooring at this point would be Coretec in a stone print since this floor will butt up against an existing hardwood foyer and other carpeted rooms. House was built in 2002. Current floor in designated areas is sheet vinyl over what I think would be 5/8" tongue in groove ply with 1/4" luan. (see photo from below) Subfloor is glued & nailed/screwed to joists, glued at seams, luan looks to be glued (from visible glue overflow at the floor registers). There is a load-bearing wall (blue in drawing) that runs halfway between the kitchen & sunroom, subfloor runs under this wall. New partition wall to be built (green) to run parallel to joists in the sunroom. The sunroom joists are all sistered (due to an 18" extension that was done by original builder before house was 1st sold), between 14.5" & 15.5"oc measuring the closest joists together. In the kitchen the joists are single and range between 14.5" oc & 17.5" oc. Some of the joists in both the sunroom & kitchen have plumbing/hvac running through. There is only 2x10 blocking near the 2 i-beams supporting the back of the house & the front of the sunroom, and some (but not all) 2x4 blocking at the end of the original joist before the foundation. Every gap between the joists in both rooms has sagged with a difference of 1/16" to 1/4" depending on the spot, even the 14.5"oc has 1/4" sag. There is deflection felt & squeaking heard when walking through the kitchen due to the 17.5" oc.

    Removing the current subfloor does not seem viable, when we removed water damaged subfloor in an upstairs bathroom the glue was stronger than the joists, they split and we had to sister 2 new joists & add blocking under the bathroom. Due to the open layout, unsure that adding another 5/8" ply layer is a good option either due to the carpet/hardwood in the other rooms.

    Obviously we need to add blocking where we can to prevent further deflection. What are my options to get this floor flat to allow the floating floor?

    Thank you for your help.

    Attached Files:

    Last edited: May 16, 2019
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  2. Jim McClain

    Jim McClain TFP Owner/Founder Administrator

    The first photo you posted is of the underside of the floor structure, which consists of 5/8" OSB (Oriented Strand Board, not plywood) glued and mechanically fastened to joists. I would imagine all the original joists were 16"oc, which is measured from center of joist to center of joist. There very well could be sag between the valleys of the joists, OSB has been known to do that. The best way to prevent sagging is to install blocking between joists prior to installing subfloor materials. The easiest way to fix sag after the subfloor is down is to bridge the valleys with a new layer of underlayment that is either thick enough on its own to support weight, or by filling the gap between the sunken valley and the new underlayment with a material that is capable of supporting the weight above. That filler could be tiered layers of 15# roofing felt or something similar.

    You say there's already an underlayment down and it might be glued, but you didn't share a photo of that. If it is glued down, as you said, that would have been a very wrong way to install underlayment. If it is though, you could add another layer of underlayment installed correctly, or you could fill the valleys and any other low spots/gouges, etc. with a floor patching compound, like Ardex Feather Finish (or its ugly brother, Henry Feather Finish). Spreading that stuff takes some skill, but it can be mastered.
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  3. LizD

    LizD New Member

    The glue that I'm seeing at the register might be from the vinyl and not under the underlayment, photo attached. In the sunroom most of the original joists are 16.5. Under the kitchen my biggest gap is actually 18" oc to a sistered joist. One of these must be hung at an angle as 2 feet away it was registering at 17.5" oc.

    Thank you for the clarification that the sub is OSB and not ply.

    If I am reading you right the 'easiest' way would be: lay 15# roofing felt in layers in the valleys, then underlayment again on top of that, then my Coretec. This is with the assumption of adding blocking underneath. If I am understanding correctly: Would we have to remove the existing vinyl prior to layering roofing felt or patching compound?

    If the current underlayment is not glued, would there be benefit (other than height) to pulling up the existing vinyl & underlayment prior addressing the valleys with the felt or compound?

    Again, thanks for your help.

    Attached Files:

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  4. Jim McClain

    Jim McClain TFP Owner/Founder Administrator

    That's all inconsequential at this point. Let's not worry about the joist spacing for now.

    Now that we think the existing underlayment is not glued, then I recommend it be removed (at the same time as the old vinyl). Then you can address the unevenness of the floor. How will it line up with the adjoining floor coverings (higher or lower or the same height)? A new underlayment might be advisable, but that, or the thickness if it is needed would be determined by the transition between your new flooring and the adjoining areas. We don't want to create tripping zones if we can avoid it.

    Pulling the old underlayment up should be fairly straight forward. Use a circular saw with a blade depth equal to the thickness of the old underlayment and sheet vinyl. Cut a grid pattern through both layers with each cut approximately 2' apart. Start prying up and pulling fasteners out as you go. You might need a chisel and hammer to separate grids you couldn't cut all the way to the wall or cabinet. Take your time, clean as you go and it should be an enjoyable workout.

    If you don't need new underlayment, you might be able to fill the low spots with a high quality floor patch. That would include the Ardex I mentioned, but maybe not the Henry product (I hope other pros chime in with their recommendations, as I haven't installed for a dozen years). But whatever cement based floor patch, it takes some skill to smooth it out right. You might want a pro to do that part.

    If you can add a half-inch layer of underlayment plywood without causing transition problems, that would be enough to bridge those subfloor undulations. The deepest ones might need a layer or 3 of roofing felt, but that's not just 2 or more layers of the same size of material. A low spot is curved, so you would make one layer narrower than the next and so on to make it more or less conform to the shape of the low spot.
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  5. Daris Mulkin

    Daris Mulkin The One and Only Charter Member I Support TFP Senior Member

    The easiest way to take up the old luan is use a roofing shovel. It will pop it up faster than fast. saves a lpot of cutting.Then it is a case of removing or pounding down the staple if nothing under them.


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  6. LizD

    LizD New Member

    Adjoining surfaces are padded carpet on top of the OSB subfloor and 3/4" hardwood on top of the OSB. I estimate that the new floor would be about 1/8" higher than the foyer (1/2" + 8mm). Where the hardwood meets the carpet is relatively the same level so I would think it shouldn't be too much difference for new floor vs. carpet. Would a reducer or flat transition work better for that type of gap?

    I understand the roofing felt issue, it's like looking at a topographical map.

    As far as hiring a pro for the cement patching, where we are it's extremely hard to find someone with experience who will take partial jobs and is reliable. Here it's really all or nothing and I can't afford to source out the whole job.

    Thank you so much.

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