I am installing 6x24" wood look porcelain tile in an approx 200 sqft room. It is rectified tile, remarkably flat for long tile.
To prepare the subfloor, I had someone install 5/8" plywood over the subfloor tongue & groove planks that was nailed to joists. He told me the floor was pretty flat using a 7' level; OK for tiling, so I installed backer board over that and taped the seams.
However, I did a final check with an 8' level I just bought, I discovered there is a high spot that's more like an apex near the centerline of the floor (tiles will run lengthwise against that centerline). Meaning, the floor has a very subtle 'tenting' (inverted flattened V) sloping consistently from the apex to each wall. The slope from apex to each wall is about 1/2" over approx 8'.
As bad as that sounds, the slope angle is so slight that when I lay the titles parallel to the axis, and run them with spacers from apex toward each wall,the slope cannot be seen either at the grout lines or across the tiles.
I'm considering screeding/leveling the floor with Versabond LFT thinset (tolerates 3/4" bed), which would be a hassle, but part of me thinks it's not worth that effort, because I could just tile because each slope is pretty flat (just not level).
I was planning on using the QEP Lash system because I don't think it will be a problem even with the slopes. I just wanted to get some other thoughts on this before making the leap.