Flooring Tile - Floor strength for marble tile

Hi there. I have some white carrara marble tile, 3/4" inch thick, that I want to install in my bathroom. I'll be doing the installation with the help of a handyman.

The floor is about 5.6' x 4.5'. The joists are 2" x 7.5" and placed 14" apart. Do I need additional support under the floor? Regarding the subfloor, I have seen 1 1/8" and 1 1/4" specified as the minimum thickness for natural stone. Would 1/8" really make a big difference? Because I'm cutting it close there and would probably have to plane the doors to achieve 1 1/4" subfloor.

Thanks in advance!

It depends on what that 1 &1/4" is made up of. That thickness guards against a deflection problem between joists. It cannot have any vinyl and or luan as part of the buildup. What is on the floor now and how much plywood is there now?

Sounds like you have 2" x 8" joists 16" on center. What is the unsupported span of the joists?

Tileguybob, right now there is very old, 3/4" ceramic tile on 1/2" of mortar with no plywood at all. The floor is from the '40s, I think. The surface of the tile is awful but it sure is firmly installed! Anyhow, I guess will have to plane the doors and increase the height of the threshold no matter what, won't I?

HeresJohnny, the span between the joists is 14", so that is 16" from center to center.

Like a lot of posters here, I had no idea what I was getting into when I bought the marble, but it's non-returnable so I need to make it work. I appreciate your help.

marble/natrual stone has the least resistence to breakage and therefore needs a stong foundation. What heresjohnny was asking you is how long are your joists without anything holding them up from the bottom. This is important because sometimes it is nescessary to double up or "asister" the joists for added strength. When you see the 1 1/8" rating that is the total structual layer. That does not count your cementboard (which I would use for that) or anything else above the plywood. If the floor is a little higher then the transition point into the next room so be it. You can get/cut a threshold to accomodate it. Cutting a little off the bottom of a door to get it to close is also easy and not really a consideration.

Vanlee

There has to be something under the 1/2" cement layer to support it over the joists. Maybe some 1x6" planking or similar.

Oh, yes Johnny there is. I'm not sure how thick those planks are. I have some pieces of the floor that were cut out to install heat vents, so I can measure it. I'll post again when I've done so.

Buildpro: I'll have to take another look under the house to get the length of the joists.

Thanks to you both.

V.

OK, I checked it out. The subflooring is 1" thick hardwood. I'm not sure of the width. There are no posts of any kind supporting the joists under the room. It's the bathroom, and the tile would butt up against the bathtub.

So, the specs are:

Floor is 5.6' x 4.5' (not counting bathtub)
2" x 8" joists 16" on center, unsupported
1" plank subfloor
Currently 1/2" of mortar under 3/4" ceramic tile, to be replaced with 3/4" marble tile

Is it doable?

If so, any chance I could leave the mortar as is and tile on top of that? So far, it seems to be sound, but I've removed only a few of the ceramic tiles.

Also, I'm wondering if you have any suggestions about what type of thinset to use. It would be great if we could use a quick-dry thinset, which I know is awful to work with, but it's the only bathroom in the house and I'd rather not spend too much time camping out at a friend's house.

Thanks again :-)

So far no.:thumbdn:

Need to know the unsupported length of those 2x8 joists.:confused:

The entire length is unsupported, so it's 4 1/2 feet. The only support under the house besides the foundation wall is down the center, and the room is not on the center of the house.

So I need to get the length of the joists from the exterior wall to the center of the house where the supports are? Is that right? I'd say it's about 11 feet, but I'd need to verify.

So I need to get the length of the joists from the exterior wall to the center of the house where the supports are? Is that right? I'd say it's about 11 feet, but I'd need to verify.That's the idea yes. At 11' + you are looking at needing additional support either by asistering your joists (doubling) or adding a support beam in your basement which in an unfinished area would be the easiest to do by far.

OK. Thanks, BuilLDPro, for your help.

VL

Well, I thought I had decided to forget the marble tile for now but I guess I haven't. So...adding a support beam under that particular spot in the crawlspace would be a problem for reasons too ridiculous and complicated to describe, so I think I'll go with asistering the joists, which my carpenter friend will take care of.

Also, I realized that I've been saying I have 3/4 inch tile, which is so not true...it's 3/8". Ridiculous of me..but anyhow, my questions are back to the underlayment.

With 1" wood planking as the subfloor, I still need 1/4" plywood plus some type of cement board set in thinset. Is that right? Or do I just need 1/4" cement board?

Also, is there any real difference between Wonderboard and Hardibacker? Any recommendations about which to use?

I am also wondering what to use as a vapor/moisture barrier. Would the cement board function as a vapor barrier? What about a product called Ditra?

My final question is, what type of screws should I use on the plywood, if I need it, and the cement board?

I know that's a lot of questions, but piecing all this together is daunting! I would appreciate any advice. Thanks a lot.

VL

Not sure how your going to go about asistering the joists if you can't get to them from the bottom. You can do it from the top if you can get beam to beam. If you remove the floor of your bathroom can you add a joist to the existing ones the full length? Let me know that and I can go on to your questions.

Hi BuiLDPro. I can get to the joists from the bottom. It's really a matter of preference. The previous property owner had dug out a large portion of the dirt in the crawl space, braced the dirt walls with a wooden retaining wall, and installed the hot water heater on the dirt down there. Also threw about a ton (literally) of every kind of debris in the hole. I just got the area cleared of junk, reinforced with a concrete retaining wall, had a furnace installed, and a concrete pad poured for the hot water heater. The bathroom is located just over the spot where there is a sloping transition between the two levels in the crawlspace, which is just in front of the water heater and furnace. I don't want to put a post there in that spot because it would partially block the easiest point of access to the lower level of the crawlspace.

I bought the house through probate. The previous owners were supposed to clear out the junk but that turned into a fight and they only took some of it. Now that everything looks so clean and open under there, I'd rather just have the reinforcements to the floor up and out of the way. It's probably a visual/psychological thing more than anything else.

VL

OK just wanted to check before I told you much else siince it might have made a difference. With the 1" planking I would add an additional 1/2" ply instead of 1/4", screwed down with 1 1/2" drywall screws (aviod the joists). Then a layer of 1/4" cement board. Brand is of no real importance. The cementboard should be installed with a layer of thinset (1/4" notched trowel) and usiing the special screws that are usually sold right by it. Some people prefer the square heads. Makes no real difference to me. An additional moisture barrier is not required. I have never used the ditra system so I can't speak to it though it looks a little gimmicky to me. I would stick to the tried and true. Another important element here is to use a good modified thinset. Don't get the home depot kind but go to a reputable tile store and look for either Tec Fullflex (preferred) or Maipei Ultraflex. Post back when you have more questions. Good Luck!

Use deck screws instead of drywall screws for the second layer of plywood. They will hold better. The drywall screws will strip out and heads will pop off.

I personally like the square drives for the cement board. They are easier to set flush with the cement board.

I like Tec thinsets myself but have used custom thinsets from HD and Laticrete and Mapei. They are all good products.

As to ditra, I have used it a couple times with no problems. Use a modified thinset under the ditra and an unmodified to set the tile. Give it a little extra set up time before grouting.:)