Waterproofing bathtub cement backer board

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Devedander

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So the most common advice I have come across when waterproofing cement backer board involves some sort of water proofing solution like Kerdi or Red Guard but something that I am still unclear on is how the backer board is installed slightly above the tub flange...

If you paint a waterproof substrate onto the backer board and water gets through the tile/grout it will run down the backerboard and then it seems to me it the surface tension would cause it to run under the boards lower edge and to the studs/behind the lip of the tub...

Am I missing something here?

Doesn't this issue mean a water proof membrane overlapping the lip is still needed?

Also if the water does run down on to the tub lip where does it then go? It seems like it would have to run all the way around the edge of the tub and dump down the from of the tub into the floorboards/wall?

Thanks for any answers!
 

Terry

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The best practice is for the board to not touch the tub deck, and have a gap that is filled with Silicone.
That means either keeping the backer board above the tile flange, or shimming the wall so that the backer board can come over the tile flange, and still be above the deck. Most of the time, the walls don't get shimmed.
 

Devedander

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Thanks for the reply, so I have 4 screws holding the tub to the studs, if I place the backer board on these screws that will give me about 1/4 inch between the top of the flange and the backer board.

Are you saying the best practice is to fill this game with silicon? That seems kind of difficult considering there is nothing behind the backer board except the studs but I figure I can make it work...
 
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jadnashua

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FWIW, millions of tub/showers are built without surface waterproofing. The CBU is not affected by the water. Any water vapor that gets behind it would be stopped by a vapor barrier installed there, and lapped over the tub's tiling flange. This protects the wooden studs from being damaged.

Now, I do prefer a surface waterproofing. Schluter has a very significant YouTube channel and shows this, as do they in their Shower Manual. When using Kerdi, they use Kerdiband attached to the cbu or other approved backer board and then seal the bottom edge flap using KerdiFix, a silane based sealant that is both flexible and very long lasting (essentially, permanent).

RedGard, you might want to use their reinforcement fabric, then embed it into the Redgard, sealing it to the tub flange.

Most of the companies discuss this in their installation instructions, and they all differ slightly. You need something flexible at the materials interface. There should be something also flexible at the tub/tile junction. My preference is to avoid caulk at all, and use an engineered expansion joint. Schluter, and other companies make some suitable for both at the tub/wall junction and the corners of the walls. Industry standards call for all of those places to be flexible. Caulk is not forever, an engineered joint is for practical purposes.
 
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