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I am preparing to install a laminate floating floor over a concrete slab in my basement. The concrete slab is installed in what appear to be 18' x 18' squares, broken up across the dimensions of my basement. The slabs are separated by 1/2" wide control joints.

Do I need to fill these joints in with a foam, concrete, or other kind of patch prior to installing a floating floor? The flooring is the Home Depot Pergo flooring with built-in padding. I am going to install a 6 mil vapor barrier on top of the slab prior to installing the flooring. The flooring will be installed perpendicular to the control joints.

Concrete Slab Expansion Joint

enter image description here

  • I would want to seal it that is huge, I usually run a crack line then fill it when floating the floor. Just about any sealer will work you don’t see much expansion in basement’s once cured. – Ed Beal May 20 at 17:47
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    Nah. Any floating floor product will span half an inch without issue. – isherwood May 20 at 18:59
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As far as the joint is concerned, you could fill it with a self leveling sealant, as represton says. I don't think it will affect the flooring either way, though, as your moisture barrier will keep the humidity away from the flooring (and the room).

And, a point of clarity... This is not an expansion joint, this is a crack control joint. It is intentionally weaker so that as the slabs settle, as they eventually will, the cracks will occur along the joint and be less obtrusive.

But more important for all of this, I feel, is whether or not the slab is flat enough for an unsecured laminate. High spots will allow the flooring to move, resulting either in an uneven feel under foot or noise. You might want to caulk in the joints to keep the leveling compound out, then apply a self leveling cement based product to the whole floor. Have you run a straight edge around to see if that slab is nice and flat, and will the flooring package insert tell you what the tolerances are?

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    Puddles: to answer the last part of your question: no, I have not used a straight edge to check the slab for level/flat simply because it "looks" good - that said, it is a new build and I am giving a lot of unearned and unknown credit to the installers. I think I will, however, take your suggestion and put down a layer of self-levelling cement. I'd hate not to and end up with separation or noise issues as you mentioned. Before I do that, I will check the documentation for the required tolerances. Thank you – 2and2IS5 May 21 at 19:53
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You can fill a control joint with a wide variety of products. A self leveling sealant would be your best bet. An example of this is at the link below.

https://www.homedepot.com/p/Sikaflex-29-oz-Self-Leveling-Sealant-in-Gray-106711/202523824

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  • One can fill the joint, but is it necessary to do so? i.e. will the flooring fail if the expansion joints are left alone? Would one be able to feel the joint through the flooring? I can see where the chipped area in the 2nd photo maybe should be filled, but is it necessary for the long, smooth joint in the first photo? At least, that's how I read the question... – FreeMan May 20 at 17:46
  • I appreciate the response. I am looking for a "need" answer vs. want. I see the joint as a large gap that looks significant enough to warrant a professional opinion. My concern is if I fill or seal it, will it cause expansion problems resulting in cracks after the laminate floor is installed which are then difficult to detect and fix. Reviewing the product you linked to appearz to be what I'm looking for; my concern is whether or not it will cause additional problems down the road. – 2and2IS5 May 20 at 18:10

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