Hello!

I'm new to the forum and I have some questions about a bathroom remodel project.

I have gutted my bathroom and will be installing 12" x 24" white carrara marble tile on the floor and some walls. The area is approximately 11' x 12' (minus 3'6" x 6' for a steam shower). The house was built in mid-80's and has a 3/4" OSB t&g subfloor on 2 x 10 joists (#2, Hem Fir), 16" OC spanning 14' 6" over a crawl space. I have some questions about the flooring:

1. Question: While I plan to install 1/2" ply underlayment over the subfloor, should I install any additional blocking and/or sister boards to the joists for structural integrity and/or deflection needs?

My plan is as follows:

A. Install 1/2" exterior grade (AC) plywood underlayment over subfloor. Screw-down using deck screws, drive screw heads slightly below the surface. Screw 4" oc around perimeter and 6" in field. Screw from center of sheets to edge. Leave 1/8" expansion gaps between sheets and 1/4" around perimeter. Then use floor patching compound with latex additive over seams and screw heads. When compound dries, sand smooth w/electric sander.

2. Question: should I use 1" deck screws to avoid puncturing through to crawl space?

B. Install two NuHeat mats using modified thinset with 1/8" v-notch trowel. Flatten as much as possible using a wooden float. Allow thinset to dry.

3. Question: should I use extra thinset - - to 1/8" - - for areas not covered by NuHeat (i.e. around toilet/bidet, under vanity/cabinet, etc.)?

4. Question: is there a special modified thinset brand and/or additive I should use?

C. Use modified thinset to skim coat across top of NuHeat. Allow to dry.

D. Install Ditra using unmodified thinset (Hydroment Ditraset) with 11/64" x 11/64" square-notch trowel or a 5/16" x 5/16" v-notch trowel. Flatten with float.

E. Install marble tiles using unmodified thinset with 1/2" x 1/2" square-notch trowel with 1/16" spacing between tiles.

5. Question: does Ditraset come in white (to use with white marble tile)?

F. Grout with unsanded grout mixed with latex additive.

G. Seal the grout lines.

6. Question: what is best sealer to use and what is best method to keep it off marble tile?

If any other parts of my plan look incorrect, please let me know.

Thanks!

Welcome to the forum.

Are you sure the clear span is 14'-6"? In my area that does not even meet minimum building code let alone supporting a tile floor. The current deflection rating for the specs you have provided is only about L/345. You need a minimum deflection rating of, L/360 for regular tile and a minimum L/720 for natural stone. As it is your not good to go for either tile or stone.
Maybe you can double check the span and get back to us.

We'll try and answer your questions anyway.

1. Depends on verification of your existing span as to how much extra support your going to need.
A. You do not need to do any floor patching or sanding.

2. Yes

B. Yes

3. For the large format stone you plan on using your going to want that floor as flat as possible. I would suggest using self leveling cement to cover the floor area after the heating is installed.

4. To do what?

5. Don't know about that product as it's not available in Canada. An alternative is Mapei Kerabond in white.

F. If that's what the grout manufacturer is suggesting, then ok. Otherwise most normal grouts are already modified so ther'e no need to add any additives. Who's brand are you planing to use?

G. You should be sealing the stone and the grout.

6. I like Aqua Mix or Stone Tec products. Depends on what's available in your area. Why do you not want to seal the stone?

Randall:

Thanks for your response.

The home I'm working on was purchased last year and, after all remodeling jobs are complete, my wife and I hope to make it our retirement dream-home.

The clear span is confirmed at 14'-6". The entire width of the home is 29' with a 6" wide steel I-Beam positioned in the middle to carry that load. While I don't know what the building codes were in this area (Northwest-Lower-Michigan) back in the mid-80's, I assume the home passed inspection then.

If my current situation will not meet the minimum deflection rating of L/720 for the marble I want to install, what would you recommend I do? Should I create some new concrete pads in the crawl space and then use jacks to hoist a new beam in place - - essentially cutting that clear span under the bathroom area in half (from 14'-6" to 7'-3")? Would that give me the L/720 rating I need? Or, is there some other method that would work better?

You also mention that I don't need to do any floor patching or sanding after installing the 1/2" plywood underlayment. Does that mean that the 1/8" expansion gaps and 1/4" perimeter gaps would be filled with thinset at the time of installing the NuHeat mats? Or should something else, like urethane caulk, be used?

Is there a special modified thinset brand and/or additive I should use to install the NuHeat mats?

Do you have a preferred brand of grout you use and/or is there one best suited to use with marble tile?

With regard to sealing the stone and grout, you're right, I missed a step - - that's why you guys get paid the big bucks! What I plan to do, as I've read on other websites/forums, is to seal the face or top of the marble first, in order to keep the grout from getting in the porous surface and causing a hazy look. After that, I planned to seal the grout lines. My question was how to keep any "excess" sealer off the marble tile edges - - thinking that might cause a problem. Would double-sealing the entire marble field be ok?

Thanks again.

Steve

Your home probably passed inspection because the floor joists just met the minimum building code for your area 14'9". Although the deflection for the existing structure meets the minimum L/360, it just not good enough for a tile installation. The dead loading of the floor coverings(tile) needs to be bumped up to get a proper deflection rating. Because your already max'ed out, a tile installation is out.

Splitting the existing span with a support beam would almost double the deflection requirements for stone. If that's easy for you to do, then I would add the beam.

There is no industry requirement to fill plywood underlayment joints and is not really necessary when there's other isolation membranes being applied, such as Ditra. It certainly never hurst to do some overkill by caulking or 3/4" masking tape to stop mortar from filling the expansion gaps.

Any modified thinset will work to set the Nu-Heat mats. There's no need for extra additives.

There is no specific grout that works better for stone. All the manufacturers have really good grouts nowadays. Installers usually have personal preferences based on past experience working with a particular brand. What I use and like, someone else may not. All I can say is, buy what's available in your area and read the instructions thoroughly.

For a marble install I do not pre seal any stone before grouting. I install everything and wait however many days the sealer manufacturer suggests before sealing. You just have to do a good final cleaning (wash) to remove all grout haze, let dry 24hrs and then seal.

Sealer excess on the edges can be a problem if your not really carefull applying before grouting. Thats why I don't do it that way.

1. Just use regular old sheetrock. Wipe it down with a damp sponge immediately before applying thinset to remove dust and to keep the sheetrock from drawing the moisture out of the thinset.

2. No vapor barrier is needed for the reasons you mentioned.

3. Notching the studs is necessary only when a conventional PVC liner is being used. Since you're using Kerdi, this step is not necessary.

You're using a completely different system than the conventional preslope/liner/slope method. Many of the steps you read about refer to the old system, but not the Kerdi system. Don't get the two confused.

You might check out some of Harry's tutorials with pics on Kerdi installs elsewhere on this site.

More questions . . .

The project I'm working on is a complete remodel of the master bedroom and bathroom. The bathroom - - roughly 12'x11' - - is divided into three areas; 1. Toilet/Bidet room, 2. Steam Shower, 3. Main Tub/Vanity room.

As mentioned earlier, I had been working on getting the bathroom floor "beefed-up" so it can support the 12x24 marble tile I intend to install. That involved installing a new crawl space beam, cross-bracing, strapping, and blocking. With that done, I purchased a laser-level (really cool) that could help with straight grout lines. Upon turning the unit on and walking across the floor, I noticed a significant amount of movement in the horizontal beam. Uhg.

So I'm now back to trying to "solidify" the floor. In the mean time, all of the walls have been built (for the three areas) and wall board was installed. I went with regular drywall for the master bedroom, Denshield for the steam shower, and a combination of Denshield/Greenboard for the rest of the entire bathroom.

My plan is to use the same 12x24 marble tile on the walls (in addition to the floor). In most areas this will come up 3' high off the floor and topped with pencil liner, feature tile, and another pencil liner. On a couple of walls, the marble will go to the ceiling. In all cases, the tile will be installed to Denshield with latex modified thinset.

After all of that, here's my question: the guy who installed the wallboard taped and mudded (pre-mixed drywall compound) the joints where the Denshield and greenboard meet. Can I use latex modified thinset over this drywall mud? Or, do I need to seal the mud first? The joint is in the exact area where the top of my pencil liner will end with the greenboard above to be painted.

Am I understanding correctly? You have Denshield fron the floor up to about 3 feet all the way around the bathroom with greenboard above, running to the ceiling? This is the joint that you are worried about - the horizontal one?

This being the case, I wouldn't worry about the drywall mud. You'll be fine to go over it as it is. The only concern that I would have is how smooth the transition is between the two boards. If they don't quite line up, then you may have issues getting everything to line up properly.

I would look at a white thin set with the marble. With the size of the tiles, you'll want something that will be sticky and grab reasonably fast. I've found that the Laricrete 4-XLT is good for larger tiles on the wall, or you may want to look to one of the lightweight mortars.

The other thing to think about is waterproofing. If your steam room is actually a steam room, as opposed to simply a walk in shower, then I would strongly recommend a waterproof membrane. Steam is different from water, and grout and cement are not waterproof (dispite popular belief). Moisture WILL eventually get through. Take your time and make sure it is installed properly because your tile is heavy and will be bonding to the waterproof membrane. If the membrane comes loose from the wallboard, then the weight of the tile can pull the whole thing down.

Hi Steve,

I wish you hadn't changed your plan in regards to the type of backer in your steam shower, since now you are not in compliance with the way it should be done. You obviously don't realize it, but DensShield is not recommended for use in a steam shower.

When we last spoke you had decided on DenShield in the steam shower, however you were going to install Kerdi too. What happened to the Kerdi idea? This changes everything.

Two problems with Dens in your case, Georgia Pacific says;

A. "DensShield Tile Backer should not be used in commercial saunas, steam rooms, around fireplaces or areas where prolonged exposure to heat exceeds 125°F."

B. How are you going to follow diagram DS002 for shower installation instruction in this link? http://gp.com/build/product.aspx?pid=4684 Click link, then click installation, then click how to install........ go to page 3.

Study to way they want you to do the floor/wall intersection. That is silly. It's easy to draw a sketch, but how are you gonna do that? Notice they want the deck mud to also go up the walls so Dens is not buried in the deck mud. The reason they don't want DensShield buried in the deck mud is they believe it'll turn to mush in time.

Jaz

I can see by the responses that I wasn't clear - - sorry. Here are some photos that will, hopefully, illustrate where I'm at:


Photo 1. Inside master bedroom. To the right of window will be a built in closet. To the right of that is a small vestibule with master bathroom beyond.


Photo 2. Stepping into master bathroom. Wife's jetted tub will be under window. Wife's steam shower is to the left. Note the steam shower and wall surrounding the window are all clad in Denshield.


Photo 3. Steam shower will have 3' glass on short walls.


Photo 4. Steam shower glass door will have transom window at top with high efficiency in-line exhaust fan in front. Note - shower ceiling is sloped 1/4" per foot with vapor-proof light in middle.


Photo 5. Looking back from tub area. Left door (pocket door) leads back into master bedroom. The right doorway leads into toilet/bidet room. Note that Denshield was installed from the floor up to the greenboard.

My main question in the last post was with regard to the Denshield to greenboard transition. Those horizontal joints were mudded (and completely smoothed) using regular pre-mixed joint compound (not setting compound). After reading a variety of opinions on this (in other forums), I thought I'd ask the experts here. Some suggest sealing it with a paint primer, some say to rip it all out and use the same modified latex thinset that will be used to set the tile.

Note too, that I'm only talking about the walls in the toilet/bidet room and the tub/vanity area - - not in the steam shower. The steam shower will still be "Kerdi-fied", complete with Schluter drain, using Kerdi-Band, Kerdi-Fix, Kerdi-Corners, etc. I called Schulter and made sure, before buying the Denshield, that it is approved by them (it is). Only the steam shower will get Kerdi, not any other walls.

I also called Georgia-Pacific about using Denshield in a steam shower and was told that it can be used, provided it is less than 48 square feet and you use a waterproof membrane (which I am). The complete technical guide for Denshield tilebacker outlines the use for Residential Steam Rooms (refer to DS013 in their PDF File - Page 11). In addition, I asked if Denshield could be embedded in the mud pan. I was told that the boards themselves can be embedded in a mud base provided the bottom edge and entire face of the boards are coated in liquid waterproof membrane (like Redguard).

That seemed silly to me - - especially since I'll be using Kerdi throughout the whole area. Still, like Jaz mentions, the Denshield would likely turn to mush over time if placed in a mud pan without using Redguard. And, if a product like Denshield with its waterproof face and mold resistant core would turn to mush, I believe any wall board other than CBU would turn to mush as well.

That said, I had the boards glued and screwed into position such that the bottom edge will be roughly 1/4" - 1" to the top of my mud pan. This gap area was going to be a question for another time, but I guess I'll bring it up here. While I understand the need for some spacing around the perimeter of these boards for expansion and contraction, I also know that filling those gaps with silicone may cause problems with thinset adhering to it. I suppose I could glue down some 1/2" foam backer-rod and fill the rest of the gap with the same unmodified thinset (Ditraset) that I'll use to put up the Kerdi. Any advice here would be appreciated.

While on the subject of Denshield, I'd like to offer my reasons for choosing to use it. Like a lot of "first-time DIY's", I've read the pros and cons of using Denshield online. For its use, inside the steam shower, I like the fact that its fiber face and mold resistant core is a step up from regular drywall or greenboard. While that might not matter since I'm going to "Kedi" the entire inside of the shower, it does matter, to me, that the other side of the board is working against any possible mold and moisture intrusion from the outside walls.

I also like that its lighter weight and easier to install than cement board, which is important to me, since I'll be installing fairly heavy 6x12 porcelain tiles on the walls and ceiling. It's the same reason I continued to use Denshield on all the walls where I'll be installing heavy 12x24 marble tile. Note on photo #2 (the tub-window wall) will have that marble tile all the way to the ceiling.

Don't get me wrong, I'm not trying to sell Denshield, I could just as easily have gone with Durok or Wonderboard, but Schluter doesn't recommend it and since I went with Denshield for the steam shower, I simply chose to continue with it for all other tiled areas in the bathroom.

So after all that diatribe, the question still remains: can I use latex modified thinset over joint compound to safely and securely adhere 12" marble pencil liner? Again, the marble pencil liner will run all around the entire room. In fact, there will be two rows with a glass tile feature strip in-between.

All other Denshield joints that haven't already been mudded (outside of the steam shower) will get alkaline resistant fiber mesh tape embedded with latex modified thinset - - the same thinset I'll use to install the marble tile, feature strip, and pencil liners. The steam shower joints will simply get Kerdi-Band and Kerdi-Corners using unmodified thinset.