Supervising the experts - backer board install

movingNovember 22, 2011

We paid plumbers to repipe the house in copper and are redoing the tub surround at the same time so the walls are down to the pipes and studs. I decided to pay the 'experts' to put up the backerboard for me before we tile and now wonder how much work I have ahead of me. (The experts being the plumber's wall guy.)

The wall board guy came yesterday and had no backerboard. He returned today with backerboard but started installing it without any vapor barrier. When I told him to stop he said, no, the vapor barrier goes over the cement board. uh. no. I called his boss and he was told to stop work. So tomorrow, it seems, he will return to put up a vapor barrier.

Since I AM now the expert and he's just the muscle, please help me confirm *I* understand the right way he should do the job.

I've got a 3 walls needing backerboard, from tub ledge to ceiling. Fixtures are on one short wall (shower over tub), no cutouts for soap/shelves. My understanding is that he should wrap tarpaper horizontally around the 3 walls starting from the bottom, overlapping the tub, then overlapping each layer by at least three inches working up to ceiling. (tarpaper should NOT be hung vertically, right?) 3mm+ plastic sheeting could also be used?

He left the "hardibacker 500" here so I see that it is .42" thick. One wall from stud surface to tub lip is .25", the other two walls have a .5" gap (fixture and long walls). Those .5" gaps need to be shimmed, yes? or is there some other work around to get the job done and this joker outta my home?! Or should I just run out to Lowes and get some .25" thick strips to quickly shim it tonight?? Then the backer board will sit on the lip of the tub all the way around? or should the backerboard overlap and all 3 walls need to be shimmed?



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"it all depends". If the tile is going to be applied directly to the Hardibacker with thinset, then yes, you need a vapor barrier between the Hardi and the studs. But...if using a membrane based waterproofing like Redgard or Hydroban, then you do not want a vapor barrier behind the Hardi. Only one vapor barrier - you never want to trap materials between two vapor barriers. If you really want to become an "expert" you might want to do some research into a Schluter product called Kerdi. Spend some time thinking about water, the planes of your vapor barrier and where the moisture will go when it penetrates your grout and gravity takes over. I'd highly recommend reading a book called, "The Kerdi Shower Book" before proceeding. Google it.

Having to argue with your contractor about how the work should be done is a bad sign. All of this should be specified before the work starts.

    Bookmark   November 23, 2011 at 8:40AM
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I think I'd ask for a new tiling sub. Are you using a waterproofing membrane? If so, heed dseng's reply.

I'm not a pro but here's what I've read. (I did my own bathroom and have spent a lot of time studying.)

Yes, I'd shim so the backer board and vapor barrier cover the tub flange without bending. Keep them 1/2" off the tub deck. That said, I've read that people will bring only the vapor barrier down over the flange and the leave the board just to the top of the flange but I wouldn't do it that way. Yes, layer the paper horozontally; cover fasteners with the bottom of the layer above. If you use plastic I've read you should go at least 4mil thick.

Mongo, a contributing pro here has a couple of drawings that may help here: (scroll down about 2/3's of the way). The drawings are for a kerdi waterproofing membrane shower but you can imagine a plastic sheet behind the cement board in place of the kerdi layer.

IMHO, you should be able to trust your contractor to know what he's doing and be responsible for his sub's work and professionalism as well. Don't talk to the sub, talk to the contractor. Good for you that you've educated yourself somewhat and caught this.

    Bookmark   November 23, 2011 at 10:29AM
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Sarah, the other kids are pretty much spot-on.

First, the gap between the framing and the tub flange: When the cement board goes on, the face of the cement board needs to at least be even with, or even slightly proud of, the tub flange. What you want is for the cement board to be held slightly above the lip of the tub flange. The tile will hang past the bottom of the cement board, it'll go over the tub flange, and be held just off the tub deck.

So: Installing the tub...I'd look at the manufacturer's installation instructions and make sure the tub was properly set. Ledger board? No ledger board? Cast iron or plastic/acrylic/fiberglass? Some get set in mortar beds, some just get adhered to the floor with construction adhesive. But also check out the detail for fastening the tub flange to the wall framing.

For the barrier behind the cement board, use either horizontally lapped tar paper, or 6-mil polyethylene plastic sheeting.

For a topical barrier (should you go that route instead), you can use roll-ons like Hydroban or RedGard, or a sheet membrane like Kerdi, which dsegn mentioned.

FWIW, you may as well read up on the installation instructions for the hardie board.

The gap between framing and the tub flange could create problems. Excessive tub movement, "creaking" sounds when weight is shifted in the tub, cracking of the flange, etc.

So yes, your idea to furr out the studs is right on the money, make sure the gap gets filed in so there is proper contact between the tub flange and the wall framing.

Do not let them hang the hardie on the studs and float it over the face of the tub flange, so you end up with an air gap between the hardie and the stud down by the tub flange. That "kick out" will throw your tiling off and could cause integrity problems with the wall down the road.

    Bookmark   November 24, 2011 at 11:03PM
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