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-   -   Slate Thinset Advice (https://www.johnbridge.com/vbulletin/showthread.php?t=53777)

scienceguy 09-08-2007 12:18 PM

Slate Thinset Advice
Well, I thnk this is just the place to get the advice I need. I am a novice home-owner, involved in an extreme makeover editon of renovating just about everything and learning it all as I go along. I got myself 260 squ. ft. of slate tile on craigslist for a mud-room/stairway-opening/hall-way/corner-jog/and another hallway stretching from one end of my house to another. I didn't realize at the time just what was involved in laying slate tile, and of course the floor is eneven in places and there are about 8 doorways to get around. But it is very rewarding, despite the amount of work, and is coming out beautifully.

Unfortunately, my main source of information has come from books and advice from mega-chain home improvement stores late at night (when I can get there). I started with Versabond, did about 65 squ. ft., ran out. I couldn't get the same thing, and was recomended to Megabond. I've done another 24 squ. ft. (with 1/2 of a 50 lb. bag, mixed with the additive), but it is much harder to work with and I am not as happy with the way things are coming out. Now someone recommeded Flexbond instead.

Does anyone have experience with these products, and can you give me a little guidance on the best was to proceed? It is in an entryway/mudroom/hallway that everyone coming into our house will use, so It will get a lot of traffic. While I am concerned about the price, I do want to use the best possible product for the space and slate. I did put down 1/4 inch hardi-backer-board, with about a billion screws all inset, so the floor is pretty level and solid.

I don't have a picture right now, butwill try to post one later to give you a feel for what it looks like.


ddmoit 09-08-2007 12:30 PM

Welcome, Scienceguy. :)

Please give us a first name to call you by.

Before we get to the thinset, please tell us more about your floor. Since you put down cement board, I assume the floor is joists and layers of plywood. For a natural stone material like slate, your subfloor has to be pretty stiff. Have you checked your joists with the Deflecto ? Additionally, stone usually requires properly installed subfloor layers equaling at least 1 1/8" thick.

Tell us about your cement board installation. Did you use thinset between the subfloor and the cement board?

Sorry to bombard you with so many questions, but you'll want to make sure you're on solid footing before you proceed further. No thinset in the world will help if you're not.

Scottish Tile and Stone 09-08-2007 01:40 PM

Iffin everything that Dan says is a go, you will want to use a Medium bed thinset. It will say something like marble and granite on the bag. Regular thinset wont hold the slate up very well. Also, after you trowel the medium bed on the floor, flat trowel the backs of each stone before setting into place.

scienceguy 09-08-2007 09:48 PM

More Info
Thanks for the responses.
I'm Steve, by the way. I will re-do my signiture if I can figure out how.

Thinset under the backer-board? :uhh: Unfortunately, not. :cry: I did use it in one area that was not level to level it off, but otherwise no. I do only have about 1/2 of the project done with backer-board, and 1/4 done with tile so far, so I can pull up some of the backerboard and put thinset under that, and use it for the rest of the project. the areas are, from right to left across the front of the house, a roughly 8X10 entryway, 5X15 hallway, narrowing to 4X10 hallway, doorway to 4X6 and 4X12hallway. I don't know the joist information underneath, just can't get to it to see it. I am pretty sure it is 3/4 inch plywood, so that only gives me 1" with the 1/4 inch backerboard.
I did back-butter the second set of tiles I did, but not the first set.
I will move on to thinsetting the backerboard from now on, and pull up the ones I can and add it there.

I went and bought more versabond for the backerboard, and flexibond for the tiles. I guess I will see tomorrow how my tiles so far are coming out. If I screw the backerboard down with thinset, do I need to wait the recommended 24 hours before laying the tiles? I am doing this in stages so as to keep the house useable while I block off sections.

Thanks for your advice,

ddmoit 09-08-2007 09:59 PM


Originally Posted by Steve
I am pretty sure it is 3/4 inch plywood, so that only gives me 1" with the 1/4 inch backerboard.

I'm sorry, Steve, but cement board doesn't count towards the structural height requirement of the subfloor. Cement board merely serves as a suitable substrate for the stone - it adds no structural strength.:(

The purpose of the thinset under the cement board is to eliminate any voids that would allow deflection, which is bad for tile - especially slate.

You're in a situation where your joists and subfloor are not up to industry standards for natural stone. It doesn't mean that your installation is guaranteed to fail, but the odds are not in your favor.

scienceguy 09-08-2007 10:14 PM

floor thickness
How can I accurately determine the thickness of the floor?

Also, is it mostly a problem in the wider areas (8X10), but will the narrower hallways (4 or 5 feet wide) be ok?

I will go down to the basement and look again to see if I can figure out the joist size in any parts of the hall.

Hamilton 09-08-2007 10:18 PM

I couldnt read any information that spelled out a specific deflection
rating for your floor steve, but it is a major concern hereabouts as
we like to see folks build things right the first time. However your
question was which thinset to use. Your versabond is a basic modified
thinset and will hold your slate ok. Probably for years. Your Flexbond
and Megabond are both topnotch thinsets and are more than suitable.
If your slate is guaged you will be fine with any one of these. If it
is an ungauged slate and varies dramatically in thickness you should
consider finding a medium bed mortar to install the rest.
For the rest of your cbu, definitely get some thinset under it and follow
the manufacturers install directions. Typically thinset over the wood subfloor
and either nail or screw it down. Its best to follow their particular instructions
for any warranty that may be involved. Keep us posted, and if you can
get some pics we love em. :nod: Best of luck

scienceguy 09-12-2007 03:37 PM

Thinking and Planning...
Here is some more information about my space, and maybe this will help all of you experts to see it better. I can't send any pictures right now, but will keep working on it.

I basically have three areas..
1. The mudroom, 8.5'X11', built above the floor in an old barn/garage.
2. A long 5'X16 then 4'X10 hallway, which is the edge of an addition between the house and barn
3. The 4'X18' hallway in the main house, 120 years old or so.

The long connecting hallway, the newest part of the house, at about 40-50 years (5'wide by 16'long then 4'wide by 10'long), has the best floor, with 9" joists running perpendicular to the hall, every 16 inches. these are covered with 3/4 plywood, to which I fastened 1/4 inch backerboard (screws set every 6-8 inches). Two thirds has no thinset below it, and is already tiled with slate. I am removing the other third of backerboard to add thinset, and then will rescrew it. I did the calculations for this and they seemd to come out alright.

The main house hallway (4' X 18') has 6-1/2" X 2" joists every 16", running the long way along the hall, above 6"X6" beams, every six feet apart. This has 3/4" plywood, but 1/2 of the hall has old liimoleum above that, and 3/8 plywood above that, and newer-old linoleum above that. Someone in the past sloped the floor between these two levels over a three foot section, and the whole thing was previously covered in disgusting carpet.

The mudroom floor (8.5'X11') has, from top to bottom, 3/4 inch plywood (exactly), above 5-1/4" floor joists (of unknown thickness and spacing), above 1" think old wide-pine boards (the finished floor in the room next door, and unfinished floor in the rest of the barn), above a network of 6"X6" and 6"X8" beams (every 3 feet), above series of triple 2X7's every 10 feet, in an overall 30X30 foot barn space, above stacks of cinderblocks and lolly-columns. I have put down backerboard and tiled over 1/3 of this space, and will remove about 1/3 of the backerboard to put thinset under it, then replace it.

Confusing? It is to me!

I am planning to use versabod to attach the backerboard, and flexibond to attach the slate tiles. Once the slate is down I will seal it, then grout with spectralock mini units, then reseal.

Being a complete novice at this, I only just learned (through reading in the forum) about leaving an expansion gap around the edges. There are some small gaps, but some places in the 5 foot hallway where the tiles go right up the the wall. I will now leave a space along the two sides for the remainder of the work.

My original thinset question is answered, but now that I have learned so much more, I wonder about the long-term life of my work. I really don't want to stop, but can't tell from the multiple levels of support if my floor is strong enough. I know that the professionals here are very conservative, for good reason, and I really admire the level of expertise that you all have. So I am wondering...

1. How will my two unusual floor spaces hold up in the long run?
2. In my center hallway, which is along the edge and part of an overall 19'wide X26' long room, does it matter that it is a hallway with walls on both sides? Can I use the 5' wide space for my calculations, or do I have to use the overall dimensions of the floor, even thought I am only putting slate tile on a 5' and 4' X 26' long part of the room?
3. Will the 14' long X 5' wide section I already tiled, without an expansion gap, be alright?
4. Will my 3/8" gradual sloping rise in the hallway pose a significant problem, or will thinset, backerboard, screws, and then troweled thinset under the tiles be alright for that part of the installation?

If more clarification is needed let me know, or if you really need pictures or a diagram to visualize this maze let me know that as well.

Thanks for your help,


scienceguy 09-13-2007 09:14 AM

Bumping Myself
Just bumping up my thread...If the answes aren't there yet, no problem. I'll just hang around and keep checking in. Still no pictures, sorry. A defunct digital camera.


scienceguy 09-14-2007 12:45 PM

new thread title, please...
Hello to any moderators out there...
Could you rename my thread? I would like to call it,
"Steve's Slate Surprises",
since every time I read a new thread I discover some new surprising thing about tiling with slate that I didn't know. It is amazing to me the wealth of knowledge that is out there, that I wish I had known before I started this job, but am also glad that I didn't, since I probably never would have started if I had known what I know now. I am back to work after being scared off for five days, and am now (in order)
1. removing all the baseboards
2. undercutting all the doorjambs
3. thinsetting the backerboard
4. back-buttering the slates
5. leaving 1/8"-1/4" expansion gaps along the walls
6. Re-thinking one end of the hallway I am tiling, as I believe (but do not know yet) that the deflection rating is no good there

Did I miss anything? I think I may write a book...It will probably rank right up there with that kerdi-book I hear so much about...
"Things I Didn't Know about Tiling but Wish I Had!"

I am going to make sure my wife knows that if a professional tiler ever comes to visit our house, I be forwarned, so I can prepare for the laughter. :o

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