Shower pan curb construction

OuroDecember 19, 2013

We recently noticed a slow leak on the ceiling under the shower and, after some investigation, hired a contractor to rebuild the shower pan for probably the fifth time. Since we have had several issues with this shower, we decided to carefully monitor the process and unfortunately discovered the contractor screwing cement board through the pan liner to the wood curb on all of the three sides. This technique is obviously unacceptable due to the likelihood of a leak developing, but it is unclear to us what the standard technique is. The guides that we have seen recommend shaping metal lath over the liner over the curb. Our contractor is claiming that this is unnecessary and suggesting putting cement board on the outside and the top and only thinset on the inside. This technique appears to be unreliable, since the thinset may not remain in place and may not bond to the existing mortar in the rest of the pan, which, for some incomprehensible reason, is already built and tiled. The contractor is repairing the punctured liner over the curb by gluing a patch of liner over it. Is this sufficient, or should we insist that the liner be replaced and the rest of the pan be rebuilt? I would appreciate any advice on this topic.

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It's a Band Aid fix, at best. Not to say it won't work....anything is possible, I guess...but I wouldn't do it. Screw penetrations on the inside or top are unacceptable.

Proper construction would be to bring the rubber membrane up and over the curb, securing it on the outside only. Proper corner folds or premade corner patches are glued in, assuring watertightness. Wire lathe can be "overbent" with a 2x4 to clad over the curb. Adding a little lime to the regular "floor mud mix" gives you "fat mud," which will cling to the lathe and can be formed to fit over the curb.

You can build some wood "forms" on the inside and outside of the curb, pack in the mud and screed the top flat....and by that, I mean with a slight tilt toward the shower. Ready to tile.

    Bookmark   December 19, 2013 at 1:04PM
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Here is the "standard technique", courtesy of Harry Dunbar. There can certainly be slight variations in methods. Example, unlike Harry, I don't use an admix in deck mud.

Note the cement board on the walls. There are no fasteners in the bottom 8-10" of the cement board, if there were those fasteners would go through the membrane that was turned up the walls, creating holes in the membrane.

The bottom edge of the cement board will not "flop around" because the bottom edge will be locked in place against the wall studs when he packs in the deck mud for the preslope.

Note his curb. There are no fasteners on the inside face of the curb or on the top of the curb. Only on the outside face.

If your installer perforated the membrane by screwing through the top and inside of the curb, did he also screw the bottom edge of the cement board to the studs, creating holes in the membrane there too?

The beauty of the inverted "U" of lathe that is set over the curb is that the lathe ties the mud on all three faces of the curb together, giving strength to the tile and grout that will cover the three sides of the curb. Smearing a veneer of mud over the membrane on the inside face of the curb, tiling over that, and hoping it doesn't delaminate or that the grout doesn't crack over time?

I don't have confidence in your man's methods. It's discomforting that he so casually discounts and disregards the proper way to do things.

Good luck. I do hope you get things done to your satisfaction.

    Bookmark   December 19, 2013 at 5:12PM
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