Questions & Answers

So, I finished the first shower and dutifully kept the cement board out of the mortar bed leaving a small gap between the cement board bottom edge and floor tile which was then sealed with caulk prior to setting the (large) wall tiles.

I can't say that I really like this solution because:

1. It leaves the bottom 5-6" or so of the cement board unsupported (no screws). Fine but not perfect.

2. It requires sealing the resultant gap at the bottom of the cement board which is messy and time consuming.

3. It reduces the available surface for thinset adherence of the bottom row of wall tiles which is a bigger issue when using smaller tiles.

4. It flys in the face of a technique that has been used for decades (planting the cement board in the mortar bed)

So, I'm thinking of doing this the old fashioned way and embedding the cement board bottom edge in the mortar bed!

Some thoughts here:

1. The cement board is to be fully Redgarded right down to the floor tiles

2. the lower portion of the cement board is separated from the bottom of the wall studs and blocking by 10" of PVC

3. Any wicking up the cement board therefore has to migrate up 10" from the bottom edge just to find any bare wood.

Now here are some thoughts on how to possibly counter any wicking tendency:

1. fully coating (front and back) the bottom 10" of the cement board with Redgard before packing the mortar bed

2. using a plastic barrier over the studs that overlaps the PVC liner a bit at the bottom

Honestly, it's just hard for me to believe that this a is a problem particularly if the two measures above are taken.

I am open to opinions on this.

Steve