Hardiebacker not entirely on top of tub edge

moose186August 22, 2010

I just installed 1/2 Hardiebacker along the stud walls of my tub surround, after putting up a vapor barrier. On two walls, the CBU rests basically on top of the tub lip that sticks up from the edges of the tub. But on the wall with the plumbing, the CBU is just thin enough to fit between the studs and the tub- it doesn't rest on the lip. It's really close, and between the thinset and the subway tile we are going to install, water would roll off of the tile and into the tub. But...

1. Do I need to lay another 1/4 Hardiebacker over this 1/2 Hardiebacker in order to have the CBU rest on the tub lip? Is there another way to shim the 1/2 out a bit to have it rest on the lip?

2. (a little unrelated)- can I use ordinary drywall mud and alkaline mesh tape to seal the seams, or should I only use thinset to do this? If the latter, can this wait until we mix thinset to begin tiling?

Thanks so much in advance!!!


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The CBU should be about 1/8 inch above the tub lip, and the water barrier should curl out from behind the CBU and be in this gap.

It is a lot more than a vapor barrier, it is also a water barrier.

While tile it slef may not allow water to pass, grout does.
Grout and CBU is water proof only to the extent they are not damaged by water.

Water will wick through the grout, and can wick through the CBU also.

You need to look at how water that might make it through to the CBU and water barrier can get back into the tub instead of into the framing.

    Bookmark   August 22, 2010 at 3:05PM
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As always, Brickeyee is correct.

Here's a link with a diagram:

I'm just a DIY'er but here's what I might add:

Reasons why the CBU is above the tub flange top is to prevent water from wicking up the CBU. What I'd do, to further prevent this is to put liquid waterproofing membrane on the bottom 12" or so of the CBU - front, back and edges.

Another reason the CBU should stop above the flange is to keep the CBU surface flat. A flat CBU surface is important for a quality tile job, as are wall planes that are plumb, (so your corners are square and the gaps where the walls meet at 90 degrees are uniform, not wider at the top or bottom). So if your walls are out of whack, use fir strips to get them all plumb and Plane down any bows in studs.

I think there's an issue with an outside wall that has a barrier on the outside sufrace - you'd have two barriers and moisture would be trapped. Maybe a pro can comment on this.

Since your tile backer, CBU, is a bit above the top of the tub, there might be inadequate support for small tiles on that first row, right above the tub, especially for a 1" x 1" mosaic. I'm not sure about how to deal with this though. I think you can fill this space with thinset to provide that backing but it seems you'd introducing a wicking opportunity. Maybe some sort of thin flashing or plastic membrane arrangement is done - like a thin strip of plastic behind the thinset, in a C-shape.

    Bookmark   August 22, 2010 at 8:23PM
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"As always, Brickeyee is correct."

If only it was true.

    Bookmark   August 23, 2010 at 2:37PM
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