Hi folks,
I’m about to install a tile floor in our family room addition and would like some advice about underlayment. The room is 20’x35′. The floor consists of 2×10’s 16″ IOC, spanning about 12 feet at the max, with 3/4″ tongue and groove subfloor. The tile is 16″x16″ Italian tile. I don’t know if it matters, but we’ll be using radiant heat in the floor.
Here is what I’m thinking based on the advice I received at the place where we’re buying the tiles. They recommended a total of 1.25″ underlayment so I plan to add another 1/2″ and glue and screw it to the 3/4″ subfloor. I’ll leave a 1/4″ gap between the sheets so I can get some mortar in there to avoid having the tiles crack at the joints of the underlayment. I’ll use thin set mortar to adhere the tiles to the floor.
Does this sound like a reasonable approach or is there a better way to do it? Thanks in advance for your advice!
Roger
No longer aka Nostra Dumbass…
Replies
Questions:
Where is your radiant heating going?
What type of system is it?
What underlayment for the tile over the plywood are you using?
Is the tile marble or porceline?
Unless I am reading something into your post that I shouldn't be, you are intending any underlayment, except regular plywood. This would be a mistake. I recommend only CBU's like Wonderboard or Hardibacker or a mortar bed, unless you want to use some the new German technology. Your complete answer is appreciated before I can go any further.
Boris
"Sir, I may be drunk, but you're crazy, and I'll be sober tomorrow" -- WC Fields, "Its a Gift" 1927
I, too have a lack of understanding what your plan is for radiant location so this deals only with the floor without it:
The tile installers recommendation of thickness is most likely a rule of thumb without any consideration for engineering, type of materials, spans, etc. The goal is to provide a substrate for the tile that, A) won't flex so as to crack tiles under load or allow them to separate from the subfloor, B) has a similar amount of thermal movement as the tiles.
Your t&g has a lot more movement than 3/4"cdx or advantech subfloor material so you are already running behind. (You might stiffen that up with pouring gypcrete to house the radiant tubing but then you add load that you have to figure for in the joists) Adding 5/8" cdx would help stiffen the whole substrate if you glue and screw it down. Now you still have a material that will move differently than the tiles under thermal changes. Adding cementious backer board will give you a good base.
Now you might be ahead of yourself already, having not figured out these elevations before framing the walls because all this build up might have made the door frames too short.
I guess I've touched on half the variables you should have considered but something to think about.
If you want to put the tile directly onto the plywood then you will have to use Schluter Ditra. The good thing is that with the Ditra you can put it right on top of the existing plywood.
If you don't want to use ditra then you have to use either a CBU or float the floor with a mud base.
Don't bother adding more plywood and don't put the tile directly on top of it.
Thanks, guys. I didn't get the sense that the salesperson was very confident in her advice so I'm glad I checked here. Here are some answers:
The radiant system is hot water running through WIRSBO tubes attached to the underside of the subfloor with metal plates. There will be insulation in the bays under the tubes.
The tiles are ceramic (Is that the same as porcelain?). They aren't marble...
To be precise about the subfloor, it consists of 4'x8' sheets that are stamped with the following: AMA, The Engineered Wood Association, 23/32 Inch, PS1-95 Underlayment. There are a few other markings but I can't really make them out.
The floor joists are 2x10s 16" OC with a span of 11.5 feet.
The original plan was to use engineered wood (radiant friendly) for the floor but the the wife decided that she'd rather have tile. I've laid tile in smaller spaces before (bathrooms, kitchen) using thin set over luan over 3/4 plywood and never had problems with cracking or coming apart but I imagine this is a different ballgame.
Thanks for your help! Please let me know if you need any more info...
Roger
No longer aka Nostra Dumbass...
Forgive me, when you said, " 3/4" tongue and groove subfloor." I was thinking t&g 1x8 boards instead of plywood.
You've got a good start. I would glue and screw cement backer board and install the tiles over that. Plenty of installers should be glad to do the backer board for you because it's part of the tile job and reflects on it. They may want you to add 1/2" underlayment, glued and screwed first.
Excellence is its own reward!
Thanks, Piffin. Sorry that I wasn't clear about the subfloor in my first post. I greatly appreciate the help!
Take care...
Roger
If ignorance is bliss then apathy is sheer heaven...
Hey piffin -
Pardon a dumb question from someone who's "flooring challenged". I know almost as much about flooring as I do about building spec houses............(-:
Are you saying that the guy may need 1/2" underlayment, then cement backer board, and THEN the tile ? Seems like that would make the floor really thick. Isn't that backer board stuff 1/2" thick too?
If people concentrated on the really important things in life, there'd be a shortage of fishing poles.
Here's a similar discussion on the John Bridge forum.
http://www.johnbridge.com/vbulletin/showthread.php?threadid=2595
BTW: I did a fair amount of research before tackling a tricky shower pan job. The John Bridge forum was the best source of tile information that I found. There are some very knowlegable guys over there and they are usually very quick to answer any quesitons. If you have any tile questions, this is the best place to ask them.
>>The goal is to provide a substrate for the tile that, A) won't flex so as to crack tiles under load or allow them to separate from the subfloor, B) has a similar amount of thermal movement as the tiles.
I have never been able to figure this out and everybody I asked couldn't provide a satisfactory answer so I decided to ask the wise piffin.
With an isolated mud bed I can see how it works but with any other methods I have problems understanding. Of course most of the times they work. Even Michael Byrnes suggested it may be O.K. to set tiles directly on plywood provided you leave space between the underlayment and use modified thin set. Now the plywood and the tiles will have different amount of thermal/moisture movement, while the space between the plywood is filled with thin set and the tile bonded the the plywood. What would happen when the plywood and the tiles move differently? Would the flexability of the thin set compensate for the different movement in the plywood and tiles? Same as with cement boards. If you thin set the cement boards and screw them to the plywood then set tiles, what happens when the plywood and the cement boards start moving due to change in temperature and moisture? Or in this case the cement boards act as a buffer so the plywood moves more than the cement boards and the cement boards move just a little bit more than the tiles? Remember the cement boards are thin set and screwed to the plywood.
Tom
What would happen if...
I don't know all these answers but I know that the more layering you do the better off you are. The reason I suggested that the installers would have the call on adding 1/2" underlay PLUS backer baord (Yes it can get pretty thick, which is why you need to plan for it al from the beginning to avoid trip steps of 3/4" at ransitions to other floors) is because these are 16" tiles. If they were 8" or quarry tile shapes, I would expect the grout joints to absorb a certain amt of the flex and show microscopic cracks in the grout lines. On my own home, I have framing @19-3/8"OC with 3/4" T&G subfloor, overlaid with 1/2" underlay glued with PL 400 and nailed @6"OC. The 12" ceramic tiles are installed over that with thinset. After five years, I have four cracked tiles. Two of those we can discount because my wife cropped a cast iron frying pan on the spot while trying to hang it from the ceiling hook but the other two are in a high traffic spot and shouldn't have craked except from some movement under them. Larger tiles do require a more solid base and it is the installer who will catch the flak so he should be the one to make the call.
Excellence is its own reward!
I can see more layers would reduce the flex but there is still the movement issue. Still no satisfactory explanation, thanks anyway, may be I shouldn't ask any more. Another question, is mud bed the only way to correct an out of level floor? I am talking 3/4 in. over 8 ft.
Tom
is mud bed the only way to correct an out of level floor?
You might consider jacking the house back to level and correcting what caused the sag to start with.
If the thinset will allow a modest amount of lateral movement for thermal/moisture reasons, then using thinset to apply the backerboard plus thinset to apply the tiles, you get double the movement buffer so the layering adds more than stiffness, theoreticly at least. And the cementious backerboard is similar in movement properties to the tiles so the greatest amt of separation would be at the plywood to backer layer, rather than at the backer to tile layer.
Personally, if I were a humid day trying to pop tiles loose with all these layers and screws and glues to fight against, I think that I would just give up and blow out to sea, round up a few of my hot humid friends and start a hurricane.Excellence is its own reward!