There is a gap, approx 1" from the tub rim to the wall board. I am tiling the shower walls, but it seems like a bad idea to leave a gap behind the tile. If I need to fill this gap, how can I and what can I use? I've seen the Kerdi-fix products, but the pictures show something that will stick out under the tile. To be clear, the gap is betwen the top of the tub, not the flange that comes up, and the wall board.