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I'm planning to float LVP (6.5mm) on 2nd store of our house (30+ years old). I'm currently working on a room that is 13' by 10'. I removed the carpet & padding, scraped and vacuumed. This is what it looks like... I can sand it down a little to smooth it up but it has seen better days. enter image description here

I then used a 6' long level to measure the flatness. Generally the floor is somewhat flat, with a 5ft wide trench that runs thru pretty much the entire width with a depth of 1/8" (roughly indicated by the green shading in the 2nd plot). The flatness tolerance of the LVP we're interested in is 1/8" in 6' or 3/16" in 10'. So perhaps our 5ft wide trench isn't too bad. enter image description here

However, the floor does seem to have some sloping issue. One side has a gradual drop of 3/4" along the entire length (red shading in the plot). Along the other direction it also has a 1/4" drop over 5' on average (blue shading in the same plot). Both slopes are fairly flat (small dips no deeper than 1/8"). Forgive my very unprofessional plotting skills...

We recently had an engineer coming over for some other concerns. The engineer had this fancy laser thingy and found some ups and downs (no more than 1.5" over about 40') on the ground floor. But we were told it's not uncommon to see things like this over 30+ years and the engineer suspected that the house might have stabilized (house built on caissons). No cracks near window frames etc. So we were advised to monitor the situation. The engineer did not measure the 2nd floor (did examine the attic and whatnot).

Now back to LVP. I'm under the impression that generally sloping is not a major issue when it comes to LVP installation, but do correct me if I'm wrong. The 1/8" dip over ~5ft is close to the tolerance; the slopes shown in blue and red are pretty flat actually, i.e., the gap between the level and the floor is almost always 1/8" or less.

My questions are:

  1. I'm gonna keep an eye on the sloping issue of the house in general, but would like to ignore it for the sake of LVP installation. Am I an idiot?

  2. How should we further prep the floor? I have a palm sander but could rent a big one if needed. Could perhaps add another layer of plywood which would smooth things out I suppose.

  3. I'd like to avoid self leveler. Not only the messiness but also the slopes - the thickest place would be like 3/4". Unless self leveler is the only choice?

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  • This is a very well asked question, and your plotting skills, while maybe not professional are quite rather good! Then you went and used "gonna"... :( (lol)
    – FreeMan
    Jan 18, 2023 at 13:09
  • Sorry I'm new here, is "gonna" a taboo word here :D
    – dr.peppurr
    Jan 19, 2023 at 5:55
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    Nope, not at all. Just had to nitpick
    – FreeMan
    Jan 19, 2023 at 12:23

3 Answers 3

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I'd probably get some door skin at 1/8" and glue that in the valley. I would probably use a bag of self level and just bridge the 1/8 sheet back to the rest of the floor. It wouldn't take too much self level to do that. Depending on what SLC you use, you would need to prime the plywood first and then just feather the SLC between the door skin and the end of the valley.

It isn't a crazy amount of work and eliminating the valley will lead to a better install. If the floor is fairly flat for the slopes you could probably get away without addressing them. LVP is fairly forgiving and to fix those slopes would be a lot of SLC.

Houses do typically settle in the middle and stay the same on the outside where the foundation concrete has remained the same size but the interior beams/posts have dried and shrunk. I'd guess most houses tend to have a dip in the middle given framing takes place in all weather and more than likely gets wet during a build.

The 1/8" in 6' or 3/16 in 10' is fairly typically for all plank flooring. You rarely see manufacturers quote anything else. It is also what the NWFA - nation wood flooring association - wood flooring installation guide recommends. LVP is a plank flooring and I guess falls into wood flooring as opposed to TCNA - Tile Council of North America.

I would choose my plank longside layout from A to B so that the largest tongue and grove are spanning the change in slope.

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  • Thanks! If you pour self leveler in that valley, wouldn't it flow all the way to one side because of the big slope (3/4" over 10')?
    – dr.peppurr
    Jan 18, 2023 at 7:40
  • No. SLC doesn't really "self" level. It needs help. It will tend to spread out to some degree but it will also want to stick to the floor. It will spread out more than thinset but less than water. Pros use screeds, squeegees, trowels and spiked rollers to help it spread out. They also map out the floor and pour approximate amounts into the appropriate areas. SLC is also still quite tool-able after a day so if you have a ridge or build up in a certain area you can tool it down with a drywall knife or a chisel. Jan 18, 2023 at 7:49
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    You could also cut the 1/8 laminate larger to span the entire width of the valley. This would cause the laminate to stick up at the edges causing a ridge. Then, using a belt sander or power planer, take these ridges back down to flush. That would be an alternative to filling in the smaller gaps with SLC.
    – FreeMan
    Jan 18, 2023 at 13:07
  • @freshcodemonger thanks that's good to know! I'll think about it. What's your favorite self leveler? Looks like there are many options.
    – dr.peppurr
    Jan 19, 2023 at 5:52
  • Thanks @freeman this is a great idea! But might be tricky since the shape of the valley is actually irregular. I'll look into SLC.
    – dr.peppurr
    Jan 19, 2023 at 5:54
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I have done many a floor with the "issues" like yours. The 1/8 dip is asking for self leveling compound. The remainder of the floor is fine.

An important factor is the thickness of the LVP you are installing. 6.5mm is a good thickness, ( thicker is better ) It will not show slight imperfection through the floor. The new LVP and LVT is not like our parents vinyl flooring that needed a perfectly smooth surface underneath. It is far more forgiving and if there are a few dips underneath it will gradually "relax' and these will become less noticeable with time. The seams are tighter and more rigid than Laminate so a slight unevenness does not make the seams separate.

Do some slight leveling in the dip and lay the floor . You will be fine.

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  • Thank you! My pea-sizd brain thought self leveler would flow towards the lower end of the room and wouldn't be able to fill up the trench until all the low regions filled. Guess I'll have to sand the floor and remove the white paint like stuff...? Also would you recommend a few self levelers?
    – dr.peppurr
    Jan 19, 2023 at 5:50
  • I buy SLC in bags in the big box stores and some of the local flooring stores. It sets up pretty fast. ( 15-20 min and cures overnight) I use a carpet rake to "coax it" to where I want it to flow. You can rake it away from the 3/4 low side if it starts to build too high there. Mix it thinner than pancake batter but not like water. You don't have to sand the floor or remove the paint, just be sure it's swept clean.
    – RMDman
    Jan 19, 2023 at 13:09
  • Thank you! I thought SLC would require primer? Does the paint not need to be removed to apply primer? (Well I guess the part covered by paint perhaps was probably already sealed). Can I just apply the primer on the naked parts without the paint or is that not entirely necessary either? Sorry for the silly questions I have never used SLC before
    – dr.peppurr
    Jan 21, 2023 at 6:58
  • Not a silly question. The manufacturer always requires primer. In the real world of flooring installers the primer is sometimes used and sometimes not with the same results. I don't think you need to remove the paint if it is not pealing just be sure the floor is clean of debris. No sanding is needed, but in your situation I would probably use the primer right over the painted parts and bare wood. One coat should be good.
    – RMDman
    Jan 21, 2023 at 14:07
  • thank you very much! I've been working slowly on this project. I sanded the floor a little and can't seem to remove most of the paint so I'll leave it be as you suggested. P.s. are you able to tell if this is the actual subfloor or some underlayment? If former, what type? I'm asking this because mang self leveling compounds don't seem to work with particle boards or OSB. Just wanna make sure I'm not dealing with these... Sorry for the rookie questions. Thanks again!
    – dr.peppurr
    Jan 28, 2023 at 6:07
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LVP does not rally care about the slopes.

What it cares about are dips and holes and anything that sticks out.

For example a dime size or toothpick size will show in LVP surface.

To prepare the surface first sink the nails heads below the surface. (there is a tool for that).

Then sand the subfloor joins.

Apply SLC as needed, it does not spread all over, it needs help from you to do so, start with small portion and observe.

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  • Thanks! I'll look into SLC... Don't have any experience with it and it looks messy. But I'll look into it.
    – dr.peppurr
    Jan 19, 2023 at 5:58

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