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Community and Q&A

Question about laying tile over concrete basement floor

Smoke Teff | Posted in General Questions on


I’m getting ready to install tile over the concrete slab in the basement. My question is, how perfectly do I need to make sure the floor is ground level? Aside from the odd small crack (which I plan to patch), it seems mostly quite flat, though I can’t tell if it’s totally level with the ground. 

Do I need to apply self-leveling compound to the whole floor before applying the elastomeric waterproofing compound? 


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  1. Malcolm Taylor | | #1


    The tile will mirror the floor, so as long as you can't see any big imperfections, you won't once the tile is in.

    One worry is the existing cracks. If the slab moves once the tile is adhered to it, the tile will crack too. You might consider some sort of de-coupling membrane to ward that off.

  2. Tom May | | #2

    A level...???

  3. Roger Berry | | #3


    You didn't say what size tile you were going to be setting. I found out the hard way that visual flatness can be a long way from real flatness. Malcolm is quite right that the tile will follow the floor, if the tile size is modest. This may be a saving grace with very slow shifts across a long room.

    18" tile is less "bendy" and finding that one overlooked hollow spot with the intersect of four corners, sure put a spot light on the dip. Worst part was I came up on the roughly 3 1/2 foot soft dip with an active patch of thinset ready to place tile. A quick change from a 3/8" notch to a softly stroked 1/2" notch gave me the height I needed to save my bacon, though feathering back to 3/8 as I spanned the dip gave me plenty of excitement. Lifting an 18" tile from any contact with the thinset is a real challenge.

    If you have a four foot level or some other trustworthy straight edge that is longer, set your self the task of slowly checking the floor now. A flashlight set on the floor a few feet behind the edge can be helpful in seeing dips and humps. Pray you don't have humps.

    I use self leveling fillers on wood subfloors under sheet goods, but I prefer to use the vinyl cement fillers that you can pull to feather edge for cement. I feel they bond better. The 1/32 edge you usually get no matter how hard you try is not going to affect the tile set.

    If the basement was power troweled you may luck out and find it to be quite true overall. Up to 3/32" dips are usually not a big deal with 11" or so tile, if you have marked the low spots. When I am raking the thinset, I will go with a 3/16 - 1/4" notch with trusted surface flatness and 3/8 if it is wavy like concrete can be. I keep the trowel tipped at 45 degrees which makes the height of the thinset less than the width. This allows me to gain a quick little bit of height when I come to a pre-marked shallow spot by troweling perpendicular, thus gaining back the notch height in full. Having a 2-3 foot hardwood piece like a 1x3 that is true on the edges to check the tile faces adjacent in all directions will help you keep the willy wobbles at bay. Also, don't ever criss cross the notch pattern. You will never defeat a trapped air bubble.

    You mentioned elastomeric waterproofing compound. Can you say more about specifically what it is and why you think it is needed. If the floor has damp rising, I might question the long term adhesion. Is it approved for under tile.

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