I am about to install USG Durock® Brand Cement Board with EdgeGuard onto my bathroom frame where the toilet is (I'm using HardieBacker for the shower FYI). This is new construction. According to USG's directions, I need to use a "latex-fortified mortar" to fill the joints between boards.
I'm looking all over Google and the Home Depot website. I see a product called Thinset Mortar Admix, which can be mixed with PremiumPlus, MasterBlend or CustomBlend thin-set mortars. They say to use that latex additive instead of water when mixing.
However, I already have purchased Porcelain Tile White 50 lb. Fortified Thinset Mortar (Custom Building Products) from Home Depot.
Question:
Since the above porcelain thinset is described as "fortified," is that as good as using regular thin-set mortar with a latex additive mixed in? Or should I return the porcelain thin-set mortar, and buy the cheaper normal thin-set mortar and use (instead of water) the latex additive when mixing it?
Best Answer
The cement board instructions are for a wet area. Since there is no need to waterproof behind a toilet you should not be worrying about this.
I would buy cement board tape and stick it across cover both boards. As you trowel the mortar for your tiles feel free to push as much into the mesh that it can hold. That is your install.
If you try to set your seams first and you let them dry inevitably you will have a ridge. That ridge can cause a lot of depth issues when applying mortar to the wall and tile (backbuttering). I advise not cover your seams in non-wet areas and apply the mortar in one go.