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I'm installing CoreTec One vinyl plank flooring as a floating floor over slab in a three-season porch. I don't think my situation is as bad as that described in How can I prepare uneven concrete basement floor for vinyl planks? The slab is even at a macro level (no visible dips or hills). However, overall the cement has a texture like a lightly-textured ceiling, a bit rougher than most sidewalks. I worry that laying the plank over this is like laying it on a bed of nails with many points of contact but a lot of unsupported area in between. Do I need to do a skim coat of self-leveling cement to even this out before laying the planks?

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You can download the DIY installation instructions here, which contain a lot of guidance on the subfloor requirements, as well as instructions. They say concrete is a suitable subfloor, although they are probably referring to a typical interior floor rather than a broom finished surface.

There are also several caveats for concrete. It needs to be level (i.e., smooth/flat) to within 3/16" per 10 foot radius, and not have a moisture problem (the planks won't be damaged, but the moisture can collect under them and wick to walls and cause damage).

Ceramic tile (with grout lines), and textured vinyl are suitable subfloors, so the the surface irregularities in those surfaces aren't considered a problem (but they provide more support surface, that is less pointy).

The planks are rigid, so irregularities won't telegraph through them. They have a cork bottom layer which should conform to the small points in the surface. The planks don't expand or contract, which would mean that they don't move in relation to the rough surface, which might abrade the bottom, although they still want you to leave an expansion gap around the perimeter, so who knows.

If you are concerned that the surface is rougher and more irregular than a typical concrete floor, the best bet is to ask them directly. They have a toll-free support number, 844-743-7439, that you can call to get their recommendation.

  • Thanks. I wish that helped more. :-( I read the directions and called for clarification and I'm still not confident of what to do. They say that the subfloor must be "smooth, flat, and level" but then seem to use "flat" and "level" interchangeably in their exposition. The floor is flat; if I lay a straight edge on it, there are no gaps. The floor is not level; it pitches about an inch in 12'. The floor is only relatively smooth. The woman I spoke to tried to tell me that the pitch of the floor would somehow compromise the locking mechanism. I can't make that make sense. – Chris Nelson Apr 30 '18 at 14:29
  • It sounds like the porch started as an exterior patio and was enclosed. They're saying that it isn't a good surface as-is. "Flat" ensures that there aren't forces stressing the locking joints vertically or the integrity of the planks. "Smooth" ensures that it won't snag and abrade the cork backing. "Level" ensures that gravity and the motion of walking on it won't make the floor try to move; "walking" out of position or part of it trying to walk and horizontally stressing the locking joints. (cont'd) – fixer1234 Apr 30 '18 at 19:16
  • If you want to use that flooring, you would need to use a self-leveling top coat to raise the low side until the entire floor is level to within 3/16" per 10', and has a smooth surface typical of interior concrete. – fixer1234 Apr 30 '18 at 19:16
  • It is not intuitive to me that a grade which you can't see can compromise the flooring. Of course you can't install it on a roof-like slope but 3/16" in 10' is quite level. – Chris Nelson Apr 30 '18 at 19:42
  • It doesn't take much slope to make flooring want to move. Rugs move on a level floor just from walking on them. Walking on flooring compresses it and then it bounces back. Each step creates a tiny wave that momentarily lifts the flooring away from the subflooring and gives it a nudge in a direction to move. Over time, even a small slope can encourage the flooring to try to walk, either as a unit or a portion. The locking joints aren't intended to hold the planks against forces trying to pull them apart. The company is saying 3/16" per 10' is the maximum. (cont'd) – fixer1234 Apr 30 '18 at 20:20

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