Doubling up concrete board for tile shower?

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JonS

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Building my first full tile shower and I just heard (from this forum) about the trick of notching the studs to create clearance for the shower membrane folds. I didn’t do this and I’m kinda Leary about doing this on exterior load bearing walls in an old house. Would there be a problem if I put up a 1/4” Durock board with a rectangle notched out of the corners. And then put a second full sheet of 1/4” durock over top bonded together with thin set? Essentially creating a little pocket for the membrane. It would be a very small area in each corner that wouldn’t be full strength. I could even pack thinset on top of the folds to give the top sheet some strength in that area.
 

jadnashua

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That would probably work, but had you looked into something like KerdiBoard? It’s a tileable, waterproof sheet with a foam core that would be easy to notch out. The hassle with anything that large is getting full coverage as there’s very little PSI you can apply, and the studs are likely 16: OC, and screws won’t really do much in the middle of a sheet, so getting the air out so there aren’t pockets between the sheets can be an issue. CBU isn’t waterproof, but it’s not damaged by being wetted. It works by embedding a sheet on the floor because you can put screws at a much closer pattern than you could ever think of on a wall with studs..

The best place on the web that I’ve found for questions on tiling things is www.johnbridge.com
 

wwhitney

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Would there be a problem if I put up a 1/4” Durock board with a rectangle notched out of the corners. And then put a second full sheet of 1/4” durock over top bonded together with thin set?
That's not a good idea, unless a single layer of 1/4" durock is rated for wall use on studs 16" o.c. (and that's what you have). [I think Hardibacker is so rated, somewhat surprisingly, so it would be OK with Hardibacker.] You can't field laminate two layers of cement board to be as strong as a single layer. So what you'll end up with is something twice as strong as 1/4" durock in terms of out-plane loads applied to it, while 1/2" durock is more like 4 times as strong.

With a foam board like Kerdiboard or goboard, you could use 1/2" material and make a 1/4" rabbet in the bottom back corner.

Cheers, Wayne
 

JonS

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I think what I will do is use 1/2” sheets that gets me down to 15” above the shower pan so for that bottom area I will use 2 layers of 1/4” hardi board with the back layer notched out. I might cut and pre fit the pieces then thinset in between and let them sit overnight with a bunch of weight on them then screw them to the wall after laminating together. It will also have support down there from the backer boards i put in for the shower pan. It’s my own house so I don’t expect I will be kicking the shower walls much.
 

wwhitney

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Reading your post again I see you mentioned "shower membrane folds". So if this is a PVC liner, with a mud bed on top, Hardibacker is not a good choice for the bottom layer of wall board that gets buried in the upper mud bed. As a fiber cement product with wood fibers, it is not rated for the high continual moisture level present in an upper mud bed on a traditional PVC liner shower base.

But I guess you could laminate a rear layer of 1/4" Hardibacker that starts just above the liner with a front 1/4" layer of cement board that extends down over the liner and gets buried in the upper mud bed. [Edit: not sure how well the thickness would match 1/2" cement board above.]

A more conventional approach would be to go ahead and relieve the studs behind the liner, assuming you still have access to them by folding the liner down (a 1/4" notch in a load bearing stud is not a problem, the prescriptive code allowance is for a notch depth up to 25% of the stud width, or 7/8" for a 2x4 stud). Or else to fur out all the studs above the liner.

Cheers, Wayne
 
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jadnashua

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Trying to laminate sheets of cbu is similar to setting large format tile, which requires some special techniques. What must be done is to spread the notched thinset material out into one, continuous layer. Say you weigh 240# and your feet encompass 24 square inches, that's 10 psi, which isn't much and you'll end up with gaps which can accumulate moisture. Notch your studs or use something that can be rabbited. You could use some spacers on the studs above the liner, but that along with your lamination idea would likely make transition to the rest of the room messier.
 
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