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This is around a tub/shower. The studs are spaced at 24in, cement board is 1/2in.

We're surprised at how much flex there is between the sheets of cement board. I'm worried that tiles may pop off.

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We're planning to use 12in floor tiles on the wall.

Some options I've considered:

  • Leave it. It's fine.

  • Aim the tile grout lines at the seams of the cement board. Flex won't affect whole tiles.

  • Add blocking behind the seam. PITA.

  • Add another layer of cement board, maybe 1/4", shifted over so the seams don't overlap.

What should I do? If anything.

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My guess is that there is nearly the same deflection if you were push anywhere up and down the mid-line of the bay. It's just more obvious at the seam because the unstressed panel is right there for comparison. This could be tested with a 2' level held horizontal. I'd unscrew the durock to add more studs. –  mike Nov 1 '13 at 1:14
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USG says that maximum stud spacing is 16". Blocking may be necessary. I had significant deflection even at 16", but when I embedded fiberglass into the joints with thinset, it went away. If you're going to laminate the cement board, might as well go all the way and use 1/2 inch material with thinset (1/4" square notch trowel) and screws. That'll be solid. –  Edwin Nov 1 '13 at 4:24
    
It sounds like you want to do this the "right way". Think about how you're going to feel about it when you're done. I bet you'll be more satisfied about it if you add the studs. If you pre-drill angled holes for screwing them to the floor and ceiling 2x4 plates it will make it a lot easier. –  getterdun Nov 2 '13 at 17:09
    
I'd hate it if, in a year or two, tiles started popping off! –  Jay Bazuzi Nov 2 '13 at 17:30
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3 Answers

Since they're up, I agree adding blocking is a PITA.

However, if its an inside wall that you have access to the other side of, use a "BIG GAP" polyurethane foam. Drill a series of 1/4" holes every 12" down the middle of each joist bay. Spray a 3 sec burst in each. Wait 12 hr and try the flex, it should be quite firm now.

You may not want to do this near the plumbing valve, for replacement purposes. It won't hurt it, though.

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Sheet the wall with membrane like Kerdi (Schluter product) or Noble Flex. It will tend to 'unitize' the wall (by spanning multiple pieces of cement board). Be sure to embed with a portland cement based thin set mortar.

As an added benefit, it will completely waterproof the wall.

Its a bit belt-and-suspenders, but I like my shower walls DRY.

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In addition to optionally reinforcing the backer board with more studs or cross pieces where the seams will be, you should definitely fill the seams with either mud or grout. In my bathroom I went with mud, but not the stuff you buy pre-mixed. The stuff you mix yourself, IMO, sets up much more firmly.

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