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Looking to put in Laminate flooring and seeing if my plan is ok or if it needs some correction.

Background

This is a half basement on grade, with about 4 feet underground on one side and walk out level on the other side. Slab with stub wall construction. We had interior ground tile installed when we first moved in per inspection showing that there was water intrusion (it may have been a ploy for business, but we also dealt with other things that needed to get away away from the foundation. In the PNW and out slab stays dry even after weeks of winter rain.

late 1970s housing (lucky no lead/asbestos) with not so great insulation and none on the stub walls. When they ripped up wall I placed XPS between the studs on one side and completely sealed the other wall with XPS and seal tape that I reframed (non load bearing and old wall came out of existing frame) XPS stub wall.

I removed the existing flooring and thinset and cut the old drywall that made contact with the cement, it severely reduced the slight musty smell of the room.

Slab Condition

I know the slab in slightly unlevel, and this is worst at where concrete that replaced the ground tile is. The worst place is shown where variation goes up to 1-1/4" max from the low spot on the slab (0"). The concrete where the ground tile was installed is at least 3" thick. Framing is directly on concrete using ground contact wood.

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Flooring Plan I am finally getting around to replacing the flooring. The old flooring was carpet on top of old ceramic tile. I wanted carpet (its warmer) but the other half wants laminate. I know I have to keep it flat within specs of the flooring. The plan is to.

  1. grind down the cement that was replaced with the ground tile to 1/4 height above the low spot (or locally where the old slab height is if greater).
  2. Use leveling compound to fill the low spot 1/4 inch and feather the edges to get the slab flat enough for the laminate.
  3. Use a 6-10 mil underlayment to seal the concrete.
  4. Use a 12 mm laminate to keep even with the existing tile in the hallway. I am trying to keep the flattest transition and the slab height is about 1/4" higher than the low spot here.

Existing un-levelness of slab

Concerns/Wish List

  1. I really wish the slab was insulated and wanted the extra insulation of the carpet. Is there any solution to help besides putting in block insulation and new subfloor? I was thinking about putting in 1/4 XPS instead of the self leveling compound but that is probably a bad idea for floor flexing. Will probably end up covering most of the area (non traffic portion) with wool rugs. I am also planning on installing 2 inches of XPS foam with borate to the exterior stub wall with an insect break to 2" under in the future

  2. Concerned about feathering edges of leveling compound, is there a better solution for this?

  3. Cutting into ground tile concrete too deep. I know its at least 3 inches in some places, but worried that cutting 3/4" in the worst spot (in that corner) may compromise it.

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  • The technical literature for many laminates will state that no underlayment is necessary. When you pick something out, be sure to check whether you can eliminate the underlayment.
    – popham
    Commented Sep 28, 2023 at 5:35
  • 1/4" XPS isn't an alternative to leveling the floor. You would still have to do both. The Owens Corning XPS specs a compressive strength of 15 psi, where they define compressive failure as the onset of inelastic deformation or 10% deflection. Your 1/4" deflecting by 10% is 0.025" (about 1/3 of 1/16"), which is a serviceable deflection. The only problem is that your 12mm laminate may not spread out loads sufficiently, resulting in local crushing of the underlying XPS. I would buy a pogo stick and do an experiment. If the laminate alone doesn't do the job, a middle layer of 1/2" OSB should.
    – popham
    Commented Sep 28, 2023 at 5:54
  • What's the negative consequence of cutting into the ground tile concrete too deep? You're getting ready to bury it beneath laminate anyway, so I don't think it's that big of a deal.
    – popham
    Commented Sep 28, 2023 at 5:59
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    I wouldn't worry about the feathered edge. If your concrete is well troweled beneath that texture on the low spot, then be sure to rough it up. I'd use a Portland cement based product and check the technical literature if an adhesion admixture improves the stuff's bond. I vaguely remember some such product that mentioned an adhesion additive. It would have been a Henry product in my case.
    – popham
    Commented Sep 28, 2023 at 6:10
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    @popham There is a cement glue/adhesive product to add more bonding to older cement. Usually a separate product that is added to cement mix.
    – crip659
    Commented Sep 28, 2023 at 10:54

1 Answer 1

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Your plan is sound. You have some concerns based on inexperience which is understandable.

The grinding of the floor and covering with leveling compound is the best method for getting a flat floor. The floor does not have to be perfectly flat. In a perfect world it will be, however the slight edge that will result if you do not cover the entire floor with leveling compound will be inconsequential. ( You can sand the edge slightly if you wish)

Adding the bonding agent is not needed. Again if it makes you feel better you can use it. If the floor has not had oils or paints or such on the surface you can lay out compound just fine.

To have a compromise between your other half and yourself, use an underlayment before placing the laminate that gives you the vapor barrier, moisture resistance and some blocking from temperature transfer. I have used the QuietWalk Plus for this purpose and it worked wonderfully. The pad allowed for any minor unevenness, the plastic was the vapor barrier and the fibrous body gave heat/ cold resistance.

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  • thanks the series you mentioned has a good span of products. I think she wants LVP flooring so there is an option that has a descent R value for its thickness, just need to make sure its ok by whatever we choose and important to get a flat surface. You are correct, I have never leveled a slab, only wood underlayment at my parents house a long time ago, and that laminate is still going 20 years later. I am hoping for a insulating solution that can level, but its probably a pipe dream. I would pay for aerogel lol. 8mm of that and 1/2 OBS would get me a floor height of just under 1" with r-5 Commented Sep 28, 2023 at 21:17
  • @ChucktheDuck, Laminate and LVP are different animals. First , you won't get LVP in 12mm. ( Be aware thickness is important. Do not purchase anything less than 5.5mm.) The manufacturers will make you nervous about an uneven floor, but after doing LVP for about a decade it is not the end all. A dip or rise of about the thickness of a nickel is ok. (That's the lip of your leveling compound.) The Quiet walk is great stuff.
    – RMDman
    Commented Sep 28, 2023 at 21:39
  • Thanks, she hasn't decided on laminate or lvp, they are both around the same price range and I was leaning towards lvp with a 20+mil wear layer. Just making sure the floor is flat as it's the last part that generates dust and I can turn back on the central air after cleaning..... I used a laser level to check floor heights and am checking flat ness with my 6' spirit level, can you recommend something that is 10' flat and cheap to check further out? Commented Sep 29, 2023 at 0:18
  • LVP is more forgiving than Laminate. Just get away from big dips or humps. Leveling compound is your friend.
    – RMDman
    Commented Sep 29, 2023 at 1:20
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    The thickness is more important than the brand. I have used a number of different brands.
    – RMDman
    Commented Sep 29, 2023 at 11:38

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