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Hi, I am looking for feedback for tiling my bathroom floor.

The floor is approximately 5'x5'. The subfloor is 1/2" plywood over 2x10 joists (16" on center). The joists are around 9' in length.

The existing subfloor is level and in good shape. The joists all seem to be level with no sag. I put a level perpendicular across the bottom of the joists and that is level as well.

When I started the project there was a small squeak in the floor so I screwed the floor down using 1 1/4" durock screws (the same screws I used to hang the cement board for the tub surround). The floor seems very solid and there is no movement to the naked eye. I used a laser level on a tripod when tiling the tub surround. When walking on the floor I noticed a tiny amount of movement in the laser level. If I bounce up and down on the floor there is quite a bit of movement in the laser level.

I was concerned that if I went ahead and tiled the floor that the deflection would eventually cause the grout/tile to crack. I added additional support between the joists and then screwed through the subfloor into the cross support that I added. The support was only 2x4s with metal hangers (there is so much plumbing, duct work, and electrical there is not room for much else). The additional support did not seem to make a difference. I then thought maybe the joists were moving so I installed 2 2x4s across the underside of the joists and then added a jack post to support the joists (the jack post rests on a cement floor). This support also does not seem to make a difference. I now think the deflection is coming from the 1/2" plywood subfloor.

My thought now is to either tear up the existing 1/2" plywood subfloor and install 3/4" plywood or put down another layer of 1/2" plywood over the existing plywood (secured with liquid nails and then screwed to the existing subfloor). I like the second option because it is easier.

Before tiling I plan to install a 1/2" cementboard layer over thinset and then install the 12x12" ceramic tile.

Any thoughts regarding what could be causing the deflection? What do you think about installing 1/2" plywood over the existing subfloor vs tearing up the subfloor and installing 3/4" plywood?

Any help is appreciated.

--Rob
 

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Ceraimic tile requires a min of 1 1/4" of RIGID subfloor. Less that that and you will have cracked tiles and/or cracked grout. At a min, I would add another layer of 1/2 4ply plywood(3/4 would be better), glued/screwed down and 1/4 backer board.
 
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