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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.

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  • I'd take the advice below and go ahead with the project. If you think the ridge will pose problems getting the tiles to lay flat, take a belt sander and coarse belt and knock the peak of the ridge down 1/8" or so. The resulting curve should be very manageable.
    – isherwood
    Jun 12, 2017 at 19:13
  • Not that it matters much now, the "high spot" is called "a crown" and it would be clearer to use "perpendicular to" rather than "lengthwise against".
    – James Olson
    Jun 24, 2017 at 20:01
  • What I ended up doing was using some RapidSet mortar with a bonding agent as suggested by an experienced dept. head and a big box store, and in a couple of areas that were thinner fills LFT mortar good to 3/4" thickness. I got most of the floor close to level and finally just decided to fill the rest with self-leveling cement now that I have the larger deeper areas filled. Home Depot sells a self-leveller hardware kit for $150 which is not a bad price and includes the larger mixing barrel. I'll get a couple of people to help me so I can move quickly when the time comes.
    – clearlight
    Jul 7, 2017 at 22:08

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½" over 8'? Given the size of tiles you have, you should be fine. The layer of thin set should be more than enough to compensate for a variance that small. If you're still worried, try laying the tiles out such that the line at the apex lines up with the grout line.

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    Other people have said that, but it turns out that 3/4" over 8' was more accurate. I have filled it now mostly with a good mortar with high PSI, that can be laid very thick and has very good adhesion. I'm going to finish up the leveling with self-leveling mortar to get a more perfect complete finish on the job.
    – clearlight
    Jul 7, 2017 at 22:09
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I wouldn't worry about it. Thinset will take care of it.

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