Subfloor and underlayment? - Tile Advice

[RtKris]

I’m planning on tiling a 140 sqft room and obviously my biggest concern is cracking. Currently my floor is 23/32” exterior osb over 2x10’s 16” oc across a 12’ span from foundation to beam. I’d like the finished height to meet the 3/4” wood flooring that abuts it and I’m planning on using 6” x 36” x 10 mm ceramic tile right now. I’m not sure about subfloor requirements, I’ve only tiled on concrete. I’m thinking about screwing the existing osb down more, then using a Ditra or laticrete stratamat underlayment, then tiling. It seems like this will get me close to the 3/4” height requirement. Will this be enough though? I’m leaning towards the laticrete since it’s cheaper and I don’t know any better. Any other suggestions? Thanks

 

[Spare Tool]

Jeez it's hard mentally timesing all your measurements by 25 to metric, but if you've 3/4's to make up Inc a tile then would have thought a six mm (1/4") cement board glued and screwed will suffice..

 

[RtKris]

Sorry, yeah the tile is listed as 2/5” thick. Seemed kind of odd. So that’s about 0.40”. 1/4” backer board would be enough over one sheet of plywood eh? Good to know, I was thinking I needed 1/2”.

 

[RtKris]

Jeez just realized I’m in the UK, no wonder. Not sure how I missed that. Well this will be confusing I guess

 

[Old Mod]

Jeez just realized I’m in the UK, no wonder. Not sure how I missed that. Well this will be confusing I guess

No, not really confusing, but it does mean we actually need to engage the grey matter, but it can get a little difficult at midnight on a Saturday evening 😁

 

[Spare Tool]

Sorry, yeah the tile is listed as 2/5” thick. Seemed kind of odd. So that’s about 0.40” 1/4” backer board would be enough over one sheet of plywood eh? Good to know, I was thinking I needed 1/2”

Can you get 6mm or 12mm hardibacker cement boards over there?

 

[RtKris]

Can you get 6mm or 12mm hardibacker cement boards over there?

Yes 1/4” and 1/2”, though I think the 1/2” is less, more like 7/16”? Sounds like I should be good with that. I think I may skip the cement board and use a decoupler membrane like Ditra or the laticrete. It just sounds easier and it’s a little less height. Anybody use those?

 

[Spare Tool]

Sprinkle some fairy dust under the ditra and should be ok...I'm out, good luck 🤞🤘

 

[RtKris]

Sprinkle some fairy dust under the ditra and should be ok...I'm out, good luck 🤞🤘

Okay great I’ll try that but I didn’t see that at the Home Depot here. Who makes it? Leads me to my second question, should I get the modified or unmodified? Thanks you’ve been so helpful!

 

[Old Mod]

You’ll want a modified thinset if your tiles are porcelain.
I can’t imagine them to be ceramic
if they’re 36” x 6” and a specified floor file.
(I could be wrong of course)
Ultimately it’s going to rely on the amount of
deflection in your floor.
Here, we would not choose to tile on to OSB, exterior grade or not, we would want it replaced with a more suitable material or it would be overboarded with an inert flooring board,
(glued and screwed) Hardiebacker or cement board equivalent, or XPS type board like wedi.
Timber, no matter what type, is a poor choice of substrate on which to tile.
It’s subject to far too many variables as to whether or not your install will have any kind of longevity. (Expansion, contraction and moisture)
Timber is not a stable material.
A matting like Ditra 25 or your laticrete mat will only assist with lateral movement of your substrate, not deflection. (Side to side, not up and down)
Hence our course of action would be to exchange the OSB for something more suitable like 55/64” hardifloor not herdiebacker.
Failing that, the overboard route would be taken.
A quick deflection test would by to place a brim full glass of water in the middle of your floor and see if you can make the water spill by stamping on your floor in close proximity to the glass.
Better still a horizontal lined laser, and watch for the line to vibrate when the floor is stamped on.
If it vibrates, then there is too much deflection present.
The only other way to make your floor more stable without the extra height, is to add more cross braces between your joist, this should help reduce the deflection.
Ultimately, the choice is yours, especially as you’re installing your own tile, a professional should refuse in its current condition.
If you can remove all deflection and mat over the OSB with Ditra or similar using something like Ardex AF200 adhesive, then you may have a floor that will work.
But with a 6” x 36” tile, there is a high possibility it will have problems if your substrate is not the best it can be.
Now the choice, really is yours.
Good luck.

 

[RtKris]

You’ll want a modified thinset if your tiles are porcelain.
I can’t imagine them to be ceramic
if they’re 36” x 6” and a specified floor file.
(I could be wrong of course)
Ultimately it’s going to rely on the amount of
deflection in your floor.
Here, we would not choose to tile on to OSB, exterior grade or not, we would want it replaced with a more suitable material or it would be overboarded with an inert flooring board,
(glued and screwed) Hardiebacker or cement board equivalent, or XPS type board like wedi.
Timber, no matter what type, is a poor choice of substrate on which to tile.
It’s subject to far too many variables as to whether or not your install will have any kind of longevity. (Expansion, contraction and moisture)
Timber is not a stable material.
A matting like Ditra 25 or your laticrete mat will only assist with lateral movement of your substrate, not deflection. (Side to side, not up and down)
Hence our course of action would be to exchange the OSB for something more suitable like 55/64” hardifloor not herdiebacker.
Failing that, the overboard route would be taken.
A quick deflection test would by to place a brim full glass of water in the middle of your floor and see if you can make the water spill by stamping on your floor in close proximity to the glass.
Better still a horizontal lined laser, and watch for the line to vibrate when the floor is stamped on.
If it vibrates, then there is too much deflection present.
The only other way to make your floor more stable without the extra height, is to add more cross braces between your joist, this should help reduce the deflection.
Ultimately, the choice is yours, especially as you’re installing your own tile, a professional should refuse in its current condition.
If you can remove all deflection and mat over the OSB with Ditra or similar using something like Ardex AF200 adhesive, then you may have a floor that will work.
But with a 6” x 36” tile, there is a high possibility it will have problems if your substrate is not the best it can be.
Now the choice, really is yours.
Good luck.

Yeah I originally had no intention of putting tile on this floor because I didn’t trust it, then my wife found a tile she really liked and I couldn’t find a wood floor I liked.

I toyed with the idea of ripping out the osb and replacing it with something else if I had to. We have panels here called advantech at our local store (we do have a Home Depot, but sadly still no fairy dust there). Its still osb but supposedly much better. Your Hardifloor sounds like good stuff, I haven’t seen it though. I was surprised that the Ditra website says that a single layer of osb over 16” joists is suitable, but I know little about Ditra. I do have new construction so that may help I guess.

I have read that cement board offers no structural support but I do find that hard to believe, no doubt there is some. I am sure it’s more than Ditra though.

The tiles I’m looking at are indeed ceramic. I’m guessing long ceramic tiles are not in my favor to avoid cracking. I haven’t had a chance to actually see them and I’m not sure on the quality so I’m a bit worried they have a lot of bowing and warping but that’s another issue I guess.

I’ll see if I can get my hands on a laser, or at least a glass of water. That sounds like some good advice to me. You’re right, it is my floor and it’s not the end of the world of it fails. I’ll just rip it up and do over but it would be real nice to avoid that. Sure if it’s somebody else’s than you want it 100%. I don’t believe we have Ardex, I’ll have to look into that.

Thanks for the advice, definitely something to think about.

 

[Old Mod]

Schluter Usa is slightly different in its advice to Europe 🤷🏻‍♂️ Perhaps check with their tech line.
If you can get hold of Sharon Huffsinger (pretty sure that’s her name) she’s on the money, and you can def trust her.
[automerge]1569825737[/automerge]
Don’t know what states she deals with tho, sorry

 

[RtKris]

In fact Schluter says I should be good with my current osb plywood with 19” oc floor joists. Go figure. I guess we are just loose cannons here in the us.

 

[Old Mod]

In fact Schluter says I should be good with my current osb plywood with 19” oc floor joists. Go figure. I guess we are just loose cannons here in the us.

Well there you go Kris, all good.
You see you guys have specific Schluter thinset for mats, we don’t.
There’s a number of differences tbh.
Glad you’re sorted! 👍