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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?

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    You can call the tech line at Custom Building Products for recommendations. Apr 6, 2021 at 20:26

2 Answers 2

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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.

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As you might imagine, there are various levels of latex additives and you can control the mixture yourself if you use latex additive instead of water. Most thinsets on the market today are "modified" or "fortified" and these terms are fairly interchangeable; unmodified thinsets are marked as such ("unmodified" means its bonding agent is just portland cement, which does a fine job by itself).

If you look on HD website for that product, it includes a link to the SDS (safety data sheet, listing what concerns there are with chemicals used) that lists "vinyl acetate - ethylene copolymer" as 1-5% of the total mixture.

Practically speaking, thinset is thinset and just about anything will do the job of filling in joints between cement boards. There's a fiber mesh tape to help bridge the gaps. Most professionals mix their own thinset instead of buying the already-sludge stuff, because this gives them the ability to add additives or customize the water ratio a little to achieve the desired consistency, and they'll typically bulk-buy and use the same thinset for multiple jobs, mixing just what they need to exactly how experience tells them works the best.

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  • So should I take the stance that thinset is thinset, and skip the latex additive? BTW the porcelain thinset that I have is a bit more expensive than the regular kind, so I guess that would mean that it's stronger than the regular stuff, right?
    – Mark
    Apr 6, 2021 at 19:56
  • If you look at the Home Depot listings, yes it's "higher quality" and thus more expensive, but theoretically its formula is slightly different and better suited to its purpose (large porcelain tiles). If you're going to use some of it to lay large porcelain tiles, you might as well use it for joint filling as well. If you're going to use drywall mud to skimcoat and paint, or use a tile that's not large porcelain, you might consider returning the thinset you have and get one more well-tuned to joint-filling purposes. Apr 6, 2021 at 20:49

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