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Unread 03-22-2011, 04:38 AM   #7
bbcamp
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Join Date: Jul 2002
Posts: 30,274
It sounds like your floor joists are causing the valleys. It would be best to pull up the subfloor and sister those low or sagging joists to get rid of the valleys.

If you don't want to re-work the joists, look for a floor leveling compound that allows you to go feather thin over plywood. It probably won't be a SLC, though. Ardex's Feather Finish looks like a good choice. A good modified thinset may work, but it is not really intended to be a leveling/flattening product.

3/8" is the minimum acceptable plywood for adding to your subfloor. 1/2" would be much better in terms of decent plywood and floor stiffness. Do any flattening with patching compound on top of this second sheet.
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