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Discussion Starter · #1 ·
I realized while using a laser level for wallboard install that I have a low and a high spot in one corner of a large bath (11'x15') near where a large slipper tub will sit. The low spot is about 3/16" lower than the outer edge of the room and that edge at the corner is about 1/8" higher than the balance of the floor. (Total area of concern is about contained within area of ~4'x6'.) Realizing this, I removed (only in this area) the 1/2" sanded plywood that was sitting atop two additional sheets of subfloor that totals 1 1/4" thickness fastened with screws atop 2x10 southern pine floor joists spaced 16" OC.

I have not had good success with floor leveler so I am retiscent to try that again in this room. And I also know that I cannot use 1/4" plywood under the ditra heat membrane I plan to install. I thought about layering 30 lb. roofing felt sheets various times and then installing 1/4" hardiboard atop that but reading on this site that seems like a bad idea (?) And if I use self-leveler just in that small area, I guess I can add 1/4" hardiboard on top, but I won't be adding thinset beneath the hardi it and my thinset above the hardi will have to be different from that over the existing plywood covering the rest of the room. (modified versus non-modified) - unless I prime the plywood???

How can I most easily assess and correct for level in this area? I will be putting ditra heat down atop the subfloor and beneath 18" x18" ceramic tiles with 1/8" grout lines.
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