Cast iron bathtub and backer board

[Boilermaker27]

I finally made up my mind and decided to go with a new cast iron bathtub from Lowes, Kohler and it was on sale for $269, I think they wanted to get rid of it. Anyway, looking at the tub after I took off the wooden crate top I noticed there was not a very large flange around the back and two sides. I know the acrylic bathtubs have a flange that sticks up about 3/4 inch and you put your cement board down to the bottom of the flange and that creates the seal. Being cast iron the flange is a bit thick and I cannot pull the cement board out to rest the cement board on the top of the tub. What is the best way to create a water tight seal between the bottom of the cement board and the top of the flange on the tub?

 

[Terry]

You don't want the backer board to touch the flat surface of the tub. It can wick water up the wall.
We normally set the backer board just above the tub. Most tile setters leave the gap between tile and tub open, and seal it with Silicone.

 

[Sponsor]

 

[Jadnashua]

Since neither the tile nor the cbu is waterproof (they aren't affected by moisture, but that doesn't qualify as waterproof), it's recommended to install a vapor barrier behind the cbu on the wall, then bring it in, over the tiling flange. Cut it off after you get the cbu up and you can seal it to the tub with silicon, or something like KerdiFix is a solid product.

There are techniques where you can apply a surface waterproofing, either paint on, or a membrane. If you use one of those, you don't want a vapor barrier behind. My preference is to use Kerdi from www.schluter.com, but Hydromet, and RedGard (two paint on waterproofers) and Noble make other products that work well.

If you want to avoid caulking entirely, Schluter has some expansion joints that can be used both at the tub/tile wall junction and in the tile-tile corners - never have to caulk ever. To do this, you really need to have the walls nice and plumb.