Shower Tile – Tiling Under Shower for Optimal Waterproofing

1. No. The water leaking would then be expected to sit under shower based and on top of the moisture barrier? This might help diffuse an issue for a couple weeks but you would never know and might have a bigger issue.

2. What is your expectation here? If you put down lets say, a plastic sheeting, under backerboard... Well it will have screw holes everywhere and will leak. If you want your entire flooring waterproof like a big shower, then treat it like a shower and use something like a kerdi membrane or a paint on rubber membrane after your backerboard but before thinset. I have done both in bathrooms but most of the time neither. It depends on the importance of what is located below the bathroom. I personally install 1/2" durarock on bathroom floors. I lay out the backer so that the seams don't match the subfloor and also I try not to screw in the backer to the joists. If you had a 1" subfloor I could see going with 1/4" but this is hardly ever the case.

That is pretty much it except waterproofing. The easiest way to do this is do your plastic**/backerboard first, then drywall, mud/tape/sand/, hit it with waterproof membrane***, then tile last.

** - I don't put plastic behind hardiboard unless I am afraid of water coming in from the other direction. With plastic if backer gets wet you are looking at longer dry times and the water most likely making its way down.

*** - You can use something like redgard. It paints on in about 20 mins and you do two coats. Its $50. Does a tub surround HAVE to have this. No. Would I suggest it. Hmmm. I don't know. This is determined by what is behind tub walls and materials. Hardiboard isn't going to let a small amount of water going through grout hit the other side. Some other brands of backer will (for instance durarock - which I would never use for showers but prefer for floors). If it is an outside wall, especially one that might be cold for a few months, I would be more apt to do the waterproofing. I just did a tub in my house a couple years ago and didn't. It is all inside walls and would have been overkill. (Old tile was on drywall that was in pristine condition after 50 years)

Another tip is to use the .5" hardiboard for tub. You will need to buy 4 sheets for a typical tub. They come in 3X5 sheets. You will simply stand each one up for each end. So backer will extend outside of the tub and 5 feet above it. You will then lie the back piece horizontally, and then cut your fourth at 2 feet to be even with the others. This advice is given you have standard American 5 foot tub. If you don't just make sure backer extends a few inches outside tub and goes about 5 feet higher than tub. For tiling I always recommend trying to tile past backer board so you have seamless transitions.