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I'd like to install slate tiles over a pre-existing plywood floor. The plywood floor is 3/4" and sits on joists.

There's a dip about 3/8" deep by 2 feet wide by 5 feet long. My plan was to fill it with self-leveling compound, before installing the cement backerboard and then the tiles. However, the guys at the tile store seem oddly negative about using self-leveling compound. Is there any reason not to? If so, what are my alternatives?

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Did they indicate why they were against self leveling compound? –  BMitch Jun 10 '12 at 11:27
    
No. Their suggestion was to build up extra layers of thinset, which seems questionable to me. –  Drew Steen Jun 10 '12 at 12:06
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3 Answers

up vote 4 down vote accepted

Yeah not sure I share their opinion. I did it 5 or 6 years ago and everything is still good. Just make sure that you use long enough screws in the area with the dip to hit the subfloor.

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Plywood is not the best substrate for self leveling compound. You can do it but for me its very ify.

I'll only put self leveling compound over plywood if the floor has zero to no flex, give, or movement in the subfloor.

Look at it this way if your floor starts shifting or moves every time you walk on it your leveler will flex and start to crack and break. If you are putting backerboard over the leveler then that is an easy answer: all the screws will just bust the leveler and will not work!

I would simply put down the backerboard with thinset under the backer board then screw, NOT nail it down.

Simply build your thinset up enough so your tiles will be flat all the way across your dip.

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Hello ace. Thank you for your edit. Thank you for adding an alternative answer I am sure future readers will take it into consideration. Please do structure you answers in a more reader friendly way (neater). Welcome to DIY forums! –  ppumkin Jul 31 '12 at 14:08
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The only thing I can think of is: what if your floor is mostly flat (except for the dip), but not truly level? Layers of thinset would allow you to build up the dip so that it's parallel to your floor. By contrast, leveling compound will dry level, but if your floor isn't level, that may not help you.

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Well it would help becuase now it would be both flat and level. But you main point remains, all the OP really needs is a flat floor. Making it flat may be easier than making it flat AND level. –  auujay Jun 11 '12 at 16:34
    
@auujay - yes, if he uses compound on the entire floor it will be flat and level. I read the question as tho he just wanted to bring up the "dip" to be flat with the rest of the floor - my answer could have been a little clearer on that point. :) –  mikemanne Jun 11 '12 at 16:40
    
That's an excellent point. Turns out my floor is mostly flat (except for one dip), but far from level (maybe 1" slope over 6'). I did end up using leveling compound, but not self-leveler. By not worrying about the level I only had to use a small amount of the compound to fill the 'dip'. –  Drew Steen Jun 12 '12 at 20:09
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