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Unread 11-22-2020, 09:07 PM   #1
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How to handle the tub flange.

Hi folks. I used to spend quite a bit of time lurking here but it's been a while as i haven't had a tile project in years. I'm doing a full bathroom remodel and will be installing a new bathtub. The tub fits the 30" alcove space perfectly and i have no plans to fur out the studs The face of the hardiboard will sit a little proud of the tub flange but will not overlap it. I would like to use Redgard for waterproofing and would like advice on how to handle the tub flange to hardiboard transition. The areas on both ends of the tub front that extend to the floor do not have a flange and are areas of concern as well. Thanks in advance.

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Unread 11-23-2020, 07:30 AM   #2
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Welcome back, Bill,

Since you're going to terminate the Hardie just above the tub flange you don't have a lot of options. You might consider using a band of water proof membrane, like Kerdi, to cover that gap. Install it on your Hardie with a somewhat lose mix of thinset mortar,, keeping the edge of the band in contact with the tub deck, then use Kerdi Fix to seal the band to the deck.

The down side of the Kerdi is that you'd then a little bit of a bump down there, If you're using large format tile, necessitating a larger notched trowel, you can likely compensate for the bump by adjusting the amount of mortar. Smaller format tile, using a smaller notched trowel, will be more of a challenge.

Moreover, given the lack of a flange at the front of the tub, the "legs", you'd really want to cut your Hardie as precisely as possible so that it follows the radius of the front of the tub, where the deck rounds over to the front, in order to keep that gap to 1/16th to 1/8th so that it can be effectively caulked with silicone. Or here again you could use Kerdi, cutting it carefully so it conforms to that radius.

When planning your tile layout don't forget about that front radius. Ideally you want to cut your tile to conform to as this fellow did with his in the photo below.
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Unread 11-23-2020, 09:37 AM   #3
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I think a banding, like Kerdi band, is theoretically the best way to handle the tub flange. However, in addition to what Dan mentioned above, I've found the corners to be annoying with banding material. The tub flange is typically inset and you have to cut the inside corners to get them to conform properly.

What I usually do is just use alkaline resistant mesh tape and apply sealant over it. I use either Wedi board or GoBoard for backer board so the sealant that I am using is for whichever product that I am installing. You wouldn't want to use silicone, for example, in this application and smear it on the surface as that would create a bond breaker.

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For the sides, it's best, from a water management perspective, to have a tile "leg" go down the side of the tub. Just like the photo in the post above. It's also best to have tile backer board behind that leg.

If that's what you have then you simply need to seal the gap between backer board and the tub.

Sometimes, I've run into a situation where drywall will be up against the tub. In this situation, I've addressed it as shown in these photos.

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edit: I consider 1 out 3 upright photos a good post!
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Unread 11-23-2020, 06:45 PM   #4
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I was hoping that Kerdi band and Kerdi fix would be an appropriate solution. I'll be using 6X 24 tile so i think ill be able to manage the buildup situation just fine. I appreciate the advice and useful tips guys.

Thank you.
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Unread 11-23-2020, 07:26 PM   #5
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KerdiFIx will work, but it's expensive. You'd need some for the overlap to bond to the tub, but elsewhere, thinset is preferred. For that application, you could probably get by with the smaller tube, but the larger one that goes in a caulking gun may be easier to get a consistent bead. As long as you have enough to spread and embed the membrane, it's fine, though.

Before tiling or applying thinset, make sure to wipe down the cbu, as, especially HardieBacker, it's quite thirsty, and will suck a lot of water out of the thinset making it hard to embed the material. Keep in mind, thinset gets it strength by curing, not drying...it incorporates the needed moisture into its chemical structure...the excess (which makes it spreadable) evaporates.
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Unread 12-01-2020, 08:52 PM   #6
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Should i bond the Kerdiband to the Hardi before applying the Redgard to the Hardi?

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Unread 12-01-2020, 09:29 PM   #7
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Bonding Kerdi to cured RedGard with thinset mortar does not guarantee a waterproof joint. Bonding RedGard to Kerdi does not guarantee a waterproof joint, but at least your joint would be "shingled" in that application method.

Best option would be to use Kerdi for your wall waterproofing.

My opinion; worth price charged.

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Unread 12-04-2020, 02:33 PM   #8
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I don't think my ocd is going to allow me to deviate from what the manufacturers specify so I'll be returning the Hardibacker for Kerdiboard. I think that will allow me to sleep a little better. Thanks for informing me.

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