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Mortar bed for shower wall

| Posted in Construction Techniques on May 19, 2004 02:42am

I’m helping my friend remodel his bathroom.  My skills are in carpentry, not in tile setting.  The left wall of the shower enclosure over the tub has ended up out of plumb and so the tiles in that corner will not meet, or will require cutting, looking terrible.  He doesn’t want to pull down the wall board and backer board to plumb and line the wall.  My solution is to float the wall. 

Is a curing membrane required between the backer board and mortar bed if the bed is only about 1/2″ thick and do I need lath inside that bed to hold it to the wall?  One person I spoke with suggested a bonding agent on the Wonderboard and then the mortar bed, which he thinks will hold the bed to the wall and also isolate it so that it can cure.  Can I float it with thinset (what we have is polymer enhanced) or do I need to mix actual mortar?

 

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Replies

  1. User avater
    SamT | May 19, 2004 05:10pm | #1

    >>He doesn't want to pull down the wall board and backer board to plumb and line the wall

    Well, he can do it right this time or he can do it right in a little while, but he will be doing it right sooner or later.

    SamT

    Arguing with a Breaktimer is like mud-wrestling a pig -- Sooner or later you find out the pig loves it. Andy Engel

  2. Frankie | May 19, 2004 05:37pm | #2

    Oh my God! Are you serious? Keep things simple, within your strengths and skill level. It is SO MUCH easier to unscrew the boards and sister the studs behind so they are plumb.

    Floating a wall requires more material, time and skill. Making it plumb AND flat ain't so easy.

    What do you mean "He doesn't want to pull down the wall board and backer board"? He has wallboard AND backerboard on the wall? He only needs backerboard WITH a vapor barrier behind it, on top of the studs. When you sister the studs you can correct this too.

    You don't need a bonding agent for thinset to adhere to backerboard.

    A curing membrane?.......... Oh Brother!

    F.

    1. Scooter1 | May 20, 2004 12:45am | #3

      Ditto. What Frankie said.

      Before I start a job, I put a 6 foot level vertically and horizontally across the area. If necessary, I sister studs next to the offending ones and then hang the CBU over the 6 mil poly. Pulling that stuff down and doing it right would be easier than floating a wall mud bed as a beginner. That would take me two days!

      I am a tile guy and simply don't have the skill necessary for floating a wall. If ya'all wanna learn, by all means go for it.

      Regards,

      Boris

      "Sir, I may be drunk, but you're crazy, and I'll be sober tomorrow" -- WC Fields, "Its a Gift" 1934

  3. RW | May 20, 2004 05:28am | #4

    So this guy I know calls me up on Monday. "Can I get a ride. I gotta go to the doc and the car tire is flat."

    "Why don't you just fix the flat and be on your way?" I ask.

    "Nah, I think I just want to try and build me a new car instead."

    "Isn't that kind of a waste of your time, and won't it just cost more in the long run?"

    "Ya think so????"

    "If you pick up a starving dog and make him prosperous, he will not bite you. This is the principal difference between a dog and a man." - Mark Twain

  4. JerBear | May 20, 2004 05:58am | #5

    If you're a carpenter, then do the carpentry.  If you are a mason or better yet a plasterer then yes, you could float the wall plumb.  Do the carpentry.

  5. User avater
    Dinosaur | May 20, 2004 06:05am | #6

    You obviously don't have any idea how difficult it is to 'float' a vertical surface and make it come out both plumb and flat. If you did, you'd already be in that bathroom with a wrecking bar and a bunch of empty buckets to haul out the scrap....

    Listen to Boris--he does a lot more tile than I do, and I do a fair bit. I don't think either one of us would want to try that stunt unless there was no other way out. Save yourself and your friend a lotta heartburn: fix the wall.

    Dinosaur

    'Y-a-tu de la justice dans ce maudit monde?

    1. UncleDunc | May 20, 2004 06:44am | #7

      >> You obviously don't have any idea how difficult it is to 'float' a vertical surface and make

      >> it come out both plumb and flat.

      But if he wants to find out, this is his big chance.

      1. User avater
        Dinosaur | May 21, 2004 07:23am | #8

        It might be his big chance, but it's his friend's bathroom. If he wants to keep his friend as a friend....

        Thinset ain't spackle. I've been known to use the rest of a left-over bag to repair patterned stucco on the outside of a building (it tools nicely for that) but it just sags too much to stay flat on a wall. He'd wind up having to do about 'leventy-seven coats and finally have to tear the wall out anyway.

        On the other hand, if you never try anything, you never learn anything. But he'd be better off trying this sort of thing out on his own place for a first time. (I like to say all my mistakes are in my own house, LOL....)Dinosaur

        'Y-a-tu de la justice dans ce maudit monde?

  6. JerBear | May 21, 2004 02:03pm | #9

    I'm curious to see what you end up doing.  If you do end up floating, you must use a stucco-mortar mix and that can't really be feathered down to nothing.  It should be fortified and yes a bonding agent for concrete (Creteweld) is recommended.  Create plaster grounds or "screed lines" on the top and bottom of your wall and make sure they are perfectly plumb.  Use a full length screed rod for your rough coat and then finish it with a flat bottom float.  I could tell you how but I can't put the skill into your hands.  Good luck and let us know how things turned out.

    1. rarhodes | Jun 13, 2004 07:58am | #10

      Thanks for your reply.

      It's my friend's house and it's his decision.  He didn't want to pull out the tub, pull down the backerboard in the enclosure area, the vapor barrier behind it and then the wall board which was not in the shower enclosure in order to fix it.  When the problem was discovered, (which was largely my fault, since I didn't catch it and I am the one with the experience) he just wanted to get his bathroom back together as efficiently as possible.  He had decided to border the tile with a wood trim strip about 2" wide, so the floated bed did not have to feather to absolute zero in order to look good.  I wish I had seen your reply before we finished it. 

      We used metal lath attached with a few galvanized nails to affix the mortar bed to the Wonderboard.  The thickest part of the bed was about 5/8" and when finished and tiled it looks great.  We did use a polymer enhanced mortar mix to float it with and although it did sag a little (requiring two coats to deal with this), it came out plumb in the corner and floated nicely into the wall, tapering into the green board on the rest of the wall without being more than barely noticeable.  The wall was out of plumb by nearly 5/8" in the corner of the tub enclosure tapering to only about 1/8" out it the opposite corner.  When the wood trim strip was added, we can barely tell it's that way.  Even though we did it and know what to look for, it's still difficult to tell that the wall is anything but puumb and square.

      I would rather have not had the problem, but we learned alot in the process of fixing it.  It's not my first choice in methods, but it has been educational.  My friend says "It sure beats the h___ out of ripping out my whole bathroom AGAIN!"

      1. JerBear | Jun 13, 2004 02:43pm | #11

        Good job.  Even though it was a pain, you did the right thing and have a happy ending.

      2. User avater
        SamT | Jun 14, 2004 04:00pm | #12

        Glad it worked out for ya.

        SamT

        Arguing with a Breaktimer is like mud-wrestling a pig -- Sooner or later you find out the pig loves it. Andy Engel

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