floor prep for LVP installation: need advice

Question

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.

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.

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.