Please critique my plan

[kanne]

Hello,

I came across a recommendation to check out the John Bridges website for tiling advice over on Reddit. I have reviewed tons of posts, the Liberry and purchased the Tile Your World book. I'm hoping all of my research has paid off but I was still hoping to get an opinion on my plan. Any and all help would be greatly appreciated.

Background: My home was built in 1908 and is located in the midwest. The house has a lot of the original details which we are in the process of restoring. Right now we are focusing on our upstairs bathroom which is the only full bathroom in the house. It is about 70 sq feet and has a clawfoot bathtub. We have a shower conversion kit on the bathtub so it is completely surrounded by a shower curtain.
We will only be tiling the floor. The tile I have is a marble mosaic and comes on 12x12 sheets.

Plan:

Right now we have 3/4" plywood subfloor. We pulled up the subfloor to look at the condition of the floor joists and confirm the deflection load. We are rated for natural stone tile.

The plywood seems to be in good shape but I am planning on adding another layer to make sure the floor is rigid enough for the marble. I plan on purchasing 11/32" plywood to accomplish that.

I was then going to use Versabond White fortified thin-set mortar mixed with Custom Building Thin-Set mortar admix (to get a latex p.c. mortar) on the plywood then install a Sun Touch radiant mat.

On the Sun Touch radiant mat I will again use the Versabond White fortified thin-set mortar but I will not add in the mortar admix. After that I will add Ditra.

On top of the Ditra I plan on again using Versabond White fortified thin-set mortar without the admix. I will make sure all the waffle spaces on the Ditra are filled and let this set for a day. I plan on doing this because my tile is a mosaic marble and some of the tiles are less than the 2" minimum required by Ditra. I understand doing this will void my warranty with Ditra.

Finally I will set my tile as the final layer. I am unsure whether to again use the Versabond White fortified thin-set or if it would be okay okay to use Custom Building Products mortar for Natural Stone. I realize Ditra calls for an unmodified thin-set mortar but I wasn't sure if the Natural Stone mortar would work better for my tile.

Once the tile is set I will use Polyblend non-sanded grout.

Again I am completely open to any suggestions or critiques. I just want to make sure everything is done correctly.

[cx]

Welcome, Kara.

It is very, very unusual for a floor joist system to meet the L/720 requirement unless natural stone tile was specified in the planning. You're confident of your calculation?

Without seeing your tiles, I would recommend against the use of Ditra as your underlayment if any of your tiles are smaller than the required 2x2" specified by Schluter. There are other sheet membranes you could use if height is an issue, or a CBU if you have the room.

While Schluter does allow the use of nominal 3/8ths" plywood as the second layer of subflooring, I'd recommend you use at least nominal 1/2" exterior glue plywood regardless the tiling substrate you select.

Not sure why you'd want an additive with your Versabond. The Versabond is already modified and meets the requirements of ANSI A118.11 for use in bonding to plywood. Using it over the Ditra voids the warranty, but it will work fine. You shouldn't need an additive for any of your intended uses.

Non-sanded grout would be my last choice unless your grout joints are all less than 1/16th-inch. The industry allows un-sanded in joints up to 1/8th-inch, but unless your stone surface is prone to scratching I would use sanded grout.

[kanne]

Thank you so much for taking the time to read and respond!

I will absolutely double check the calculations on my floor joists to be on the safe side but I'm pretty confident it is correct. The joists are pretty large and most span about 10 feet. Someone also added cross supports between the joists that look like they have been there for quite awhile. I did not account for the cross support when completing my calculations. I just assumed since there is the clawfoot and tile someone made sure the floor was strong. I was really worried about the deflection value because I bought the tile before doing all my research so I was pleasantly surprised when I pulled up the subfloor to measure.

Do you have recommendations on an underlayment other than Ditra? I was really excited to use it based on the Tile Your World book and everything I've read on these forums. It seems like such a great product. I was really disappointed to read they do not recommend use with smaller style tile but I understand why. I read that Ditra reps have stated one can use it against manufacturer recommendation if they add the thin set and then let it cure before tiling but it does void the warranty. I am open to using another underlayment but would like to avoid CBU due to the added height.

I will go ahead and upgrade to the 1/2' plywood. I was debating between 3/8" and 1/2" mostly due to the height concern but in the grand scheme I'd rather be safe than sorry.

The grout joints on my tile are 1/16" so that is why I was leaning towards the unsanded grout. Would it still work to use sanded with that size? The tile is marble as opposed to glass so I don't think scratching should be too much of an issue.

Thanks again! I convinced my husband we should redo our bathroom before our pending baby arrives (T-minus 18 days until due date).

[mark999]

Kara,
I prefer to use statamat vs ditra as an underlayment. It allows the use of modified thinset below and on top. I like to set it and fill in the waffles one day and set the next. There are lots of choices for grout these days. I would recomend one that requires no sealing of the grout. Some type of epoxy grout would work well but has a learning curve to install. A quality cementious type grout like tec power grout or maipai fs would be a good choice.

[kanne]

Thank you for taking the time to read and give me feedback, Mark.

Does the strata mat work okay with tiles that are smaller than 2"? Or is it similar to the Ditra and that is why filling the waffles then waiting a day to set is a good idea?

I read a little about epoxy grout but thought I'd stay away from it since I heard it is not great for beginners. The idea of not having to seal the grout is extremely appealing but I'd rather take the time to regularly seal the grout then mess up trying to use epoxy grout. Any insight you could give on this would be greatly appreciated!

Do you prefer the Tec brand over the Home Depot brand? I have both stores near me so picking up products from either place is not a big deal.

Also I see you are in St. Louis. I'm located in Kansas City, Missouri so not too far away =)

[mark999]

You can call Laticrete 1-800tech support and they can advise on their stratamat.After filling the waffles I would not have a worry about setting that tile on it. Power grout by tec requires no sealing. Some people have had issues with it not hardening but I have not. Since hearing that I always do a test board a day or two before to make sure. Not as familiar with the fs by Maipai but I think It also requires no sealing. Make sure you use a high quality thinset. I like Laticrete 254. Wash it off your hands or wear it off. Sticky stuff.

[Tool Guy - Kg]

Marble is softer than glass. If your tile is polished, beware of your average sanded grout, as it can scratch. While most folks like sanded grout for floors as it’s strong and durable, you’ve got a rather stiff structure and your tile won’t be subjected to much deflection. In addition, the gaps are pretty tiny and therefore not subjected to as much relative physical abrasion as larger joints. So, unsanded grout will perform just fine if you want to use it. However, if you want sanded grout, gravitate toward sanded grout that contains grains of sand that are relatively smaller and more rounded than average sanded grout, as they’re less likely to scratch your tile (if it’s indeed polished)...like Laticrete Permacolor. Plus, Permacolor is rated to be used in joints as small as 1/16”. At the end of the day, it’s your choice.

As far as your tiling substrate, I see only a liability in using uncoupling membranes with air channels for this particular job with tiny mosaics and concentrated loads of the tub’s claw feet. Whoever said that pre-filling waffles on Ditra helps in the support of tiles smaller than 2” is wrong. It will aide in installing them flatter, but you’ll still have the same fundamental lack of support where the small tiles hang out over the edge of an air channel.

And coupled with your plan to embed the uncoupling membrane over a hand-troweled bed of mortar on top of a heat mat and you’re very likely to produce a wavy tiling substrate. You DON’T wanna install mosaics over a surface that isn’t flat.

For that reason, I’d reconsider your plan. I’d install the heat mat per manufacturer’s instructions (they vary, so don’t assume anything). Then pour a few bags of self leveling cement over the heat mats to give yourself a nice flat surface to start with. Then, install a real, live crack isolation sheet like Nobleseal CIS. You’ll be left with a tiling substrate that is flat enough to install those mosaics with a relatively small notched trowel. And that is IDEAL for mosaics. Not only does this give you flat finished results, but you’re not dealing with excessive messy mortar oozing up through all the joints and giving you a migraine.

[kanne]

That is great information! Thank you so much for taking the time to share.

I just ordered NobleSeal CIS membrane to use as that seems like the perfect solution to my problem.

Using a self leveling cement also makes perfect sense because I did have some concerns about my floor being flat since this is an older home.

I feel much more confident after making these changes to my plans. I will report back with how it goes and I might have to check in with a couple more questions that pop up.

I cannot stress enough how much I appreciate everyone taking the time to share their knowledge so I can make this project (hopefully) a success.

[kanne]

I laid my plywood and heat mat, prepped my floor thoroughly then poured my self leveling underlayment.

Unfortunately I was a little short of SLU as you can kind of see the outline of the heating mat. Everyone is closed so ill have to wait until the morning to add more.

My question is will I need to prime my SLU before I add another layer and is there any chance I can lay down my Noble membrane to start tiling tomorrow or will I need to wait until Monday? Thanks in advance for any assistance you can provide!