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Old 01-13-2018, 04:27 PM   #1
GoRoos
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Un-flush backer boards

I'm preparing to tile the walls around a shower. I hung the hardibacker a couple of weeks ago and moved along to another project in the room (I wanted to get all the texturing and painting done in the rest of the room so that I could lay the tile edge just over the paint to create a nice, clean "seam" between the two).

I walked in the other day and noticed that one of the CBU panels (the very bottom one - just above the tub) is no longer flush with the panel above it all the way across - specifically, between two of the studs, it seems to bow away from the wall such that there's a section that is just under 1/4" 'out' from the wall. The remainder of the 5' seam (from that stud to the other side) is flush.

This begs several questions:

1. How did half-inch hardibacker "bow" after being hung? Should this movement scare me? (if it moves after the tile is hung, that seems like "very bad news" for the tile job).

2. Do I need to replace this panel? It is on the back wall, so it would mean a considerable amount of removing side panels to get it out cleanly.

3. Alternatively, can I fix it with thinset when I go to thinset the seams?
(Maybe I should have already mentioned that I haven't taped and thinset the seams yet).

4. The stud-space behind this section of the panel is actually the HVAC return. There's a large opening in to that space on the opposite side of the wall (where the HVAC air filter is). It would be very possible for me to use that to climb in to the space and "add" a stud in the offending area and use that to fix the offending panel - both pulling it back flush with panel above it, and providing an additional anchor between the two panels.

Thoughts or advice appreciated!

Jay
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Old 01-13-2018, 04:36 PM   #2
jadnashua
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It's hard to say why things have bowed, but your experience points out one of the reasons why CBU is not considered structural...it will bend to conform to the substrate underneath.

What is the gap between studs where the board has bowed?

Did any of the screws break out and punch a hole?

Are all edges supported by something?

If you put a straight-edge across it, how much is it out of whack?
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Old 01-13-2018, 10:00 PM   #3
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Was there wet lumber under that hardi backer

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Old 01-13-2018, 10:37 PM   #4
GoRoos
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Jad:

Those two studs are 16" OC. Panels are flush on top of the studs.

No punched through screws (I mean, they are all just barely counter sunk with that little tiny dimple around them from the counter sinking).

There are three panels between the tub top and the ceiling (it's a 9' ceiling, so from the ceiling down, there are two whole 3' panels, followed by a partial one, followed by the tub)

Checking with a straight edge - all the other panels (and these panels everywhere but between these two studs) are within 1/16" of flat, all the way around. Between these studs, the top panel is about 1/8" bowed in to the wall, the middle stud is about 1/4" and the bottom is just barely bowed in at all (that explains the apparent lack of flush - the real problem is that they are all bowed in slightly, but to different depths.)

Shady:

Maybe we are on to something here. When I demo'd the old tile out, I discovered greenboard. Everything was bone dry, though.

Part of the rest of the room involved removing the vanity. Within about 30 minutes of closing the sink valves and removing the vanity, one of those valves failed (I'd never seen the insides of a valve spray out the attached copper, but that's exactly what happened) and sprayed a bit of water in to this corner for about 90 seconds (it didn't flood the room or anything - and I was standing there to stop it very quickly). The gaps between sheets are less than 1/4", so very little water got behind the hardiebacker (I know because I climbed in to the HVAC return to look) - however, the hardiebacker itself was wet to the touch for about 10 minutes.

What's the chance that moisture caused the hardiebacker to bow in to varying degrees? If that's what happened, how should I proceed?
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Old 01-13-2018, 10:42 PM   #5
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P.S. If it matters, the plan is to redgard on top of the hardibacker, after taping and thinsetting the seams.
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Old 01-14-2018, 04:26 AM   #6
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Welcome Jay,

As I was reading your first post my immediate question became: Was there any change in moisture or humidity after you installed the Hardiebacker? Then you confirmed it.

Hardiebacker won't generally swell or move with moisture, but your studs will. One of your studs may have bowed or twisted with the change in moisture. It doesn't take much stud movement to cause bowing in a panel installed across a stud gap.

If it was mine I'd unscrew the bowed panel and reinstall it or replace it. Can you get the screws out? You might find that once you remove all the screws the bow will disappear. Then you might be able to reattach the board in place by screwing to the studs in different locations. You'd want to fill any old screw holes before applying redguard.
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Old 01-14-2018, 11:00 AM   #7
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Thanks, folks -

Warren - your theory is that when the studs got wet/moist, they actually caused the hardibacker to bow. Seems perfectly sensible.

Does that mean that the next panel I put up there won't stay flush, either, as now the stud is out of whack? Or will the new, good backer somehow "pull" the stud back in to whack?

Any concern about the studs behind this panel becoming swiss cheese with all the screw holes?

You ask if I can get the screws out - not sure why I couldn't (I do occasionally have to back one out and redrive it to get a counter sink). I would have to take down all the panels on either side, as this is the back panel, and the edges are covered by other panels (and I made sure that there are no seams with four corners of hardibacker) but not sure why I wouldn't be able to do that.

Because of which panel this is, it does mean taking out pretty much the entire installation and starting over. But if that's what it takes.....
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Old 01-14-2018, 12:41 PM   #8
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So long as your studs are completely dry you should be OK to re-attach.

For the swiss cheese question I wouldn't worry about it unless you previously had dozens of large deep screw holes before you put up the Hardie.

You should first try removing all of the screws in the bowed board, leave that board wedged in place by the other boards, and see if the bow goes away. Then you you can go from there. If the offending board straightens out within reason you should just be able to re-attach it (again with screws in mostly different locations). You should also be able to push on the board from behind to help move back towards straight.
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Old 01-14-2018, 03:28 PM   #9
GoRoos
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Great advice - thanks for the help!

Here's what I did (and it worked!)

- Removed screws from the stud to the right and left of the bow. The gap got significantly better, but still not "great".

- Built a brace that fit exactly between the two studs and gave me an additional "rail" exactly under the problem seam. Attached that brace to the two studs and then attached the hardiebacker to the new rail.

This pulled it EXACTLY flush and the straightedge test passes exactly now.

- Replaced screws. Every still sits great.

Thanks much! (and I know you'll be awaiting my next newbie question...whatever that is... off to thinset the seams now...)
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