Basement Slab Problem

microxlMay 1, 2008

My five year old basement floor developed a network of cracks as it cured leaving the appearance of many islands of cement. In the process they all moved up or down to one degree or another leaving the floor "looking" level to the eye. When I place a 3'straight edge across the cracks in some places it's fine but in others it rocks from end to end.

My plan was to put down Platon by Armtec (the same material as on the bottom of DriCore) and then assemble laminate flooring upon it. Incidentally, this CAN be done as stated in the Armtec literature. No OSB or ply is required. When I rolled out the Platon (with landscape fabric under it) the Platon became wavey in areas and would not lie flat due to the unlevel slab. Laminate would not take kindly to this situation. So, I have rolled up all my materials and am back at square one.

I have read on DIY sites where people recommend using leveling compound. The problem is that when you read the labels, NONE of it is recommended for below grade use IOW


Is sanding down the joints of my cement islands an option? What kind of sanders are out there? Are there any other tactics that I could use?

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I believe that most leveling compounds are gypsum based, and as you stated, are not for below grade or exterior.

You could use a thinset mortar instead, but it will require you to finish it with a trowel and float. Since you're going to put the Platon on top, you're just looking for level, not smooth and pretty. Level can be accomplished using a long and straight 2x4 to screed the thinset after you trowel it on.

Sanding concrete doesn't sound like a good idea, but if you think you want to try it, an angle grinder with a masonary wheel should allow you to "shave" down the high spots. WARNING: wear eye protection and a breathing mask (an assistant can pour small amounts of water on the blade and concrete to keep the dust down), and use a heavy plastic to protect the rest of the house from dust (including covering any vents in your basement). Concrete dust is not good for you.

    Bookmark   May 2, 2008 at 8:25AM
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If your floor is heaving due to hydrostatic pressure or freeze/thaw, levelling will not solve the problem permanently.

And if I were levelling, I'd knock out the high sections and patch them level.

Here is a link that might be useful: Diagnosing concrete slab problems

    Bookmark   May 2, 2008 at 10:39AM
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I think Worthy's observation that the slab might suffer or at least might HAVE suffered from hydrostatic pressure when it was curing could be right on the money. I doubt it was freezing/thawing though. Our lot DID end up a wet one. We are on a wet, clay/cobble-boulder glacial deposit hillside in NH. So, we had to have a good deal of gravel put in before the slab pour. There has never been any leaking. And, the one-ft-sq poly moisture test has never shown the slightest sign of moisture droplets.

Facing reality, I think we are going to be better off using the Platon with a ply or OSB overlay and then a short- knap, synthetic, commercial carpeting. With carpeting, things don't have to be perfectly level. Should there ever be some future movement the laminate floor could end up wrecked by unsightly joints and cracks.

Thanks for your ideas and time. They jogged me in the right direction I think.

    Bookmark   May 3, 2008 at 6:49PM
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