subfloor preparation for ceramic tile

Hi -

I am preparing to put a ceramic-tile floor in a 4-season porch, which has dimensions of approximately 17 ft long by 6 feet wide. This porch was apparently an add-on, built some time ago by the original owner. The flooring that was there (I just ripped it up) was carpet-over-pad-over-vinyl tile-over 3/4 plywood-over-sheet vinyl-over-tar paper. All of this was on top of 1X8 pine boards that are laying on the joists. The joists run lengthwise, and are unfortunately on 20-inch centers (like I said, the guy did this himself). What remains right now are the 1X8 pine boards on the joists.

My plan was to screw down a layer of plywood of some thickness, then put down 1/2-inch cement backer board on top of thin-set mortar, and tape all the seams.

The question is when I go to lay down the plywood, what thickness should it be? I ask this because I know from reading that the subfloor under tile should be at least 1-1/8 inches thick, and this includes the thickness of the backer board.

An additional question: is the subfloor thickness measured from the top of the joists to the top of what's laying on the joists? If it is, then do I already have 3/4-inch subfloor already there, due to the thickness of the pine boards?

The thickness of the floor is of concern because if the top of the tile gets too high, I wont be able to put a door mat or runner at the inside of the back door, which leads into this porch.

Long-winded question, but if you can make it through, I would appreciate any help with this.

Thanks very much,

Dave

 

You need at least ¾" plywood over a moisture barrier.
That subfloor needs to be stout!

The cement board adds no strength to the installation.
An uncoupling membrane such as Ditra, works better then CBU tile panels.

 

Thank you for the response.

By telling me to put 3/4" plywood as a subfloor, do you mean I should put the plywood OVER the existing 1X8 boards that are now laying on the joists?

Also, is CBU short for "cement-board underlayment", and what is Ditra?

Thanks,

Dave

 

yes to the 1x8's run the plywood opposite the direction of the 1x's and glue them with construction adhesive then screw every 2 inches on the seams and every 4 to 6 in the feild sound s like overkill but you dont want this thing to move.If you have a crawl space you can put 6 mil plastic on the dirt in the crawl space.Ditra is a waffle faced membrane that you stick to the floor with mirtar before laying tile ,it will ad about another 3/8's to the floor

 

With that joist spacing I would use Ditra.

 

Thanks for the replies.

I ask this only because I may have a height problem: I STILL have to put 3/4" plywood down over the existing 1x8's?

If there indeed is a height problem, and I decided to rip out the 1x8's down to the studs, what would you suggest for a subfloor construction, layer by layer?

Thanks,

Dave

 

First off, Ditra (http://www.schluter.com/english/prod...601-index.html)

will only add an 1/8" to the finished height if done by their specs.


If you remove the 1x8s You will then need 2 layers of ¾" plywood running opposite direction of the previous layer of ¾" that gets installed on the joists that are 20" apart.


You want that floor to be Stout!

Going under to porch and installing more joist, between your 20" span with some cross bracing, will give you the stout floor, without the extra layer of plywood.

 

ok with the 1/4 notch on the trowel and the1/8 actual thickness of the ditra about 1/4 to 5/16 ,kinda like building a rocket hugh?If you are taking down to the joists add some cripples in there to support the floor use 3/4 inch plywood a stout one like c.d.x. the span is only 17' so the cripples should be plenty screw it down and use 1/2 rock as an underlay you will be good to go

 

Thanks for all the help.

We have decided, because of the effort involved, not to go down to the bare studs.

Due to the height issue, I may further investigate Ditra. I am, of course, a novice when it comes to tile floor, so I am in the dark as to why Ditra (being reffered to as a "membrane") would add rigidity to the installation (does it?) as opposed to the cement-backer board, which I was told in the first reply, does not add rigidity to the installation (which REALLY confused me as to why it doesn't).

I am going to start a new thread, because we have another problem with this floor - it is way out of level (in the 6-foot dimension), probably by 3/4" to an inch. I was told of a floor-leveling product, sold in big bags, that was like a mortar, only it was "mixed up, and then poured on", and made a floor level.

Thanks,

Dave

 

Cement board doesn't add rigidity, it uncouples the subfloor so that the wood can expand without the tile cracking. Ditra does the same thing.

 

Kim -

Thank you very much, that little piece of information was extremely helpful in my understanding of subfloors and materials.

Dave

 

Hi, Floorman -

This is a silly question, but in your last reply, you said if we went down to the joists, then 3/4-inch plywood and "1/2-inch rock", and you are "good to go". What do you mean by "rock"? And, is this layering all under a top layer of Ditra? I guess I don't understand the lingo.

By the way, the joists in this porch are so old that they are "true" 2X8s, that is, they are 2 inches thick, by at least 7.5 inches wide. Would this make any difference to influence my subfloor?

Thanks,

Dave

 

Floorman, Ditra has square pockets for the thinset to go. It doesn't sit on top of the ridges. You press down and I use my 30# carpet seam weight to rub the ditra into the thinset. I have seen people roll it with a 75# roller too.

Even though it does bond the Ditra to the subfloor, that isn't what the thinset under the Ditra is for. Same for the thinset under CBU.

Remember what uncoupling means.

 

CBU also uses nails or screws So the thin set is just to fill the void between the layers. With Ditra the thin set is all there is to hold it down.If its not bonded it would be Unbonded ,and if that was good why bother bonding it in the first place??.

 

Uncoupling means unbonded!


You want the subfloor beneath the Ditra to be able to move at different rates. Wood expands faster then mortar and stone!! Movement kills a tile or stone installation.
That is why Ditra specifies no latex in the thinset, under the Ditra, and latex modified thinset to set tile on top of the Ditra.


This is also the reason Ditra has less failures then CBU panels.

Ditra simulates a sand "mudbed", which we all know from the old days, a mudbed isn't stuck to the wood subfloor. The screen holds it together.

 

Are you tring to say that you don't want the Ditra bonded?If so why bond it at all? Leave it unbonded from the start and you would have a floating floor.Why not set tile to felt,news paper,sand, ect.There are standards that must be met.I would be willing to bet that on a percentage rate that CBU out performs it. Most mud beds are 1.25" to 2" not 1/8" of unbonded thin-set as reported.

 

Woody. Have you ever seen much less worked with Ditra?

Sounds like you know more about it then I. Tell us your take on the subject.

Then I'll get Dave Gobis in here to set us both straight.


But until then, here is some of Daves information...


The Tile Council of America handbook has this year added the term uncoupling
membrane. The definition is a system that separates the finished surface from the
substrate to allow independent movement between the two and prevent the transfer
of stress to the tiled surface. There are several manufactures of these systems.
While a sheet type membrane, they typically have different performance properties
than other systems. Among the advantages, the system is they allow for
elimination of the control joint relocation required with other systems. Uncoupling
membranes also allow for vapor transmission that can have a negative effect on
most other systems. They can be used either for crack suppression or
waterproofing with an additional seaming fabric. They also may be used as a
substrate in lieu of additional underlayment or over green concrete. These systems
are installed using a suitable thinset mortar.

 

Mr.bruno lets cut to the chase take the floor down to the joists,add some supports to the joists by adding 2x4's or 8's in betwwen the joists for added protection from deflection.Next add 3/4 inch ply screwed and glued to the joists next put in the durock laying the sheets perpendicular to the plywood using thinset screw or nail these sheets to the plywood every 2" on the seams and every 4to 6" in the feild use durock mesh trape and tape all joints in the "rock" using thin set and the flat side of a trowel then tile you will have no problems with the job failing.If there is a crawl space put some 6 mil plastic on the ground for a moisture barrier

 

no offense perry but stick to what you know ,carpet stop confusing the issue with egos and what you think you know

 

I am pretty familure with the Ditra system.And yes Gobis also is, but I would agree with Floorman in the statement that reading a article does not make you a expert.Some of what you were saying is half truth you just dont have the facts .Before handing out installation instuctions you should read them again .
Uncoupling was not added this year it was in last year.You seem to have just read the article in FCI that was written by Schluter so do not confuse it with the TCA.And yes, bond it with a suitable thin set not as you stated earlier.

 

ee3
All I did was paste a snippet.
It is not only what you read. It is what you do with the knowledge gained, when taken into the field and apply it with your own hands.


I don't claim to know everything, please correct me and direct me where you found that information, if I'm wrong.

I suggested Ditra because of this statement made in the original post.

>>>The thickness of the floor is of concern because if the top of the tile gets too high, I wont be able to put a door mat or runner at the inside of the back door, which leads into this porch. >>Ditra (snip) will ad about another 3/8's to the floor.>>First off, Ditra (http://www.schluter.com/english/pro.../601-index.html) will only add an 1/8" to the finished height if done by their specs. >>ok with the 1/4 notch on the trowel and the1/8 actual thickness of the ditra about 1/4 to 5/16 ,kinda like building a rocket hugh?>>Floorman, Ditra has square pockets for the thinset to go. It doesn't sit on top of the ridges. You press down and I use my 30# carpet seam weight to rub the ditra into the thinset.>>If its not bonded it would be Unbonded ,and if that was good why bother bonding it in the first place??.>>Uncoupling means unbonded!


You want the subfloor beneath the Ditra to be able to move at different rates. Wood expands faster then mortar and stone!! Movement kills a tile or stone installation.
That is why Ditra specifies no latex in the thinset, under the Ditra, and latex modified thinset to set tile on top of the Ditra.>>stick to what you know ,carpet stop confusing the issue with egos and what you think you know

 

If I offended you sorry.Ditra(Schluter)Has recently stated to use dry set to bond porceline ,you state to use a modified without knowing what tile.Ditra is most often bonded with a modified you said dry set. The mfg. of thin set have their own ideas of what bonding agent to use.. If uncoupling is unbonded that is not the way to go.A commetee was formed for Crack isolation membranes and they all must pass a min. 50psi shear test and that would require they be bonded. This is one of the problems with all forums half truths.People know enough to talk about a product but not enough that they should be speaking for a mfg...Everyone should go to the source for the facts and not take someone who dosnt work for a Co. advice.I have read more misinfo givin out as it were the fact and its only opinion.. In your first reply you state that CBU offers no strengh are you implying that the unbonded Ditra does.Ditra is a choice and a good one ,that is fine, just the BS part ????Remember when you were little and someone whispered in your ear and you in someone else ect.by the time the storey gets back its a differant story!!

 

Here is a quote from the tile man himself, Dave Gobis. Posted at FlooringInstaller..






Snipped from the installation specs for Ditra




Note on modified dry-set mortars:


There are dry-set mortars on the market that have been modified with "flexible" synthetic additives.
Depending on the type and material of the synthetic, these mortars are either gauged with water or with
a synthetic emulsion to a paste-like consistency and only form a complete film-like layer (harden) when
all excess water has been released from the dry-set mortar.

Flexible rapid-setting adhesives have the advantage of drying faster and bonding more mixing water,
which results in less excess water and better "film-formation" of the synthetics. Therefore, dry-set
mortars which meet the standards of ANSI A-118.1 (those that do not contain flexible synthetic
additives), meet all requirements for weather-exposed tile coverings in combination with
Schlüter®-DITRA. Tiles should be laid in such a way that 95% coverage is achieved to attain a securely
bonded covering with a minimum of hollow spaces. Curing time of the mortar must be observed per
mortar manufacturer's requirements.

Note: At the time of applying the dry-set mortar, the DITRA mat must be completely dry; i.e., no water
should be contained in the square, cut-back cavities.

The combination of such dry-set mortars and DITRA is sufficient for bonding porcelain tiles. Since
DITRA uncouples the covering from the substrate, it compensates for deformation stresses caused by
differential movement between the substrate and the tile, as well as moisture-related stresses occurring
in the substrate below, thereby reducing the bond strength requirement.

The DITRA system has been proven with these dry-set mortars for more than 15 years. Synthetically
modified dry-set mortars should only be used if they are specifically characterized as resistant to water,
frost, and weather in the product data sheets provided by the manufacturer, or if the manufacturer
guarantees these properties upon request. Naturally, the tile materials used for the covering must also
be resistant to frost and weather.



Note on exterior installations

Because of the relative difference in expansion between DITRA and certain substrates, it is
recommended that DITRA be allowed to adapt to ambient air temperature before installing. Additionally,
if large fluctuations in temperature are expected during the installation process, it is recommended that a
rapid-setting, thin-set mortar be used to install DITRA.

 

Hi All -

I am very appreciative of all the help and suggestions on this question that I had. However, I really am sorry to see people getting annoyed with each other via written forum.

I have learned a great deal about this subject, so I am trying to sort out what my wife and I should do.

Above all, this is true with any project I take on, I want to plan it so it is either done correctly with future improvements in mind, or perform the work in a manner such that a bad problem doesn't get worse for me or the next guy down the road.

Which leads me to the response to Floorman: you are absolutely right, and I could not agree more, that the definitive answer to this is to get down to the joists, and start over. At that point, I could solve my sloping/unlevel issue, and maybe even use 5/4 plywood, because I would have the room. The problem is in the constuction of this room:

As I said, as much as we can tell, the original owner of the house built this 4-season room himself. He extended the basement with cement-block walls, poured a floor, and put up his rim and interior joists. (under the room is actually part of our cellar - there in a double-door walk in, we have a sink, fridge down there, and it is a useful area). Fastened to the bottom of the joists are Homasote panels, which make a crude ceiling in the cellar.

Well, on top of the floor joists, the guy nailed/fastened the pine boards AND THEN ON TOP OF THAT constructed his stud walls. So, on the outer three walls, the bottom plates of the walls are on top of the pine boards.

You can see where I'm going: in order to remove the pine boards, I would have to get some sort of flush-cut power saw, cut the boards out, and then sister in new joists to extend a fastening surface for the new subfloor.

I don't have enough vacation days left for this.

One idea I had was to cut the pine boards slightly away from the walls with a circular saw, and sister in enough joists to support both these little "tag ends" of boards and the edge of the new plywood. Since the ends of the boards would be well away from traffic areas (along the walls), I'm thinking this may work since there would not be a lot of people walking along the outer 4 inches of the room. On top of this would be the durock or Ditra and then the tiles.

The other idea, which my wife is lobbying for, is to put down 3/4 plywood over the boards as correctly as possible (again, trying not to make anything worse than it is), and prepare the surface for vinyl tile. The sloping problem (and I've done this before with moderate success) would be "solved" by scribing in the baseboard piece such that the top of the baseboard is level, creating the illusion that the floor is OK.

The suggestion of getting up underneath the floor, and adding cross-bracing is a very good one, except for the fact that I cannot, even with removing all of the ceiling in the basement, access the entire span of the joists. The part I really can't get to is below the back door, where the traffic is most heavy.

As far as the headroom problem at the back door, depending on the thickness of the tile we choose, and the thickness of the pile in the rug runner we pick out, it could still be an issue, but not as much as I thought. This is due, incredibly, to the fact that the floor slopes downward from the hinge side of the door. It is the hinge side of the door that has the 1-1/2" clearance from the top of the pine boards to the top of the back-door threshold. So, any rubbing of the door sweep on the runner may depend on where the runner is placed.

So, long winded once again. If I go through the effort of getting to the joists, my wife may have a bird, even though her favorite floor by far is ceramic tile. Maybe I'll bang in sick for a couple of days, scratch my head a little, take a deep breath, and have at it.

Thanks again,

Dave

 

Ok lets try this--PER Ditra lit."To bond Ditra use a bonding mortar that is appropriate for the substrate" -Perry- Call the mfg. of the bonding agent and ask them what is appropriate!
"Ditra must be fully engauged in the mortar to provide a mechanical bond to the substrate." This would lead one to beleive that it is bonded.In your oringinal post you said "That is why Ditra specifies no latex in thin-set under Ditra" Now you post "appropriate for substrate" ????That is my point you were close but not correct in your statement. Before info is givin out ask your self if you are willing to warrent and stand behind your statements.If someone listens to you, and it fails are you going to step up to the plate with dollars. Can claims made be backed up??"less failures then CBU" who sayes??

 

That was just the part for using modified thinset to bond porcelain tiles to Ditra!!! LOL!!



They threw in the specs for installing in the rain, during a down pour too, didn't they?




Here is the kicker in there



Sure, I could mix up some dirt and water and make anything bond with mud. The problem is when it dehydrates it turns to dust. Unmodified will hold its form, but doesn't stick like modified, does it? Then remember uncoupling. Then look uncoupling up in your webster.

 

Never mind Perry..You dont need to explain the Therory to me I am very familar with it after being on the commettee that wrote the standard and did the testing for the TCA inclusion.We are currently working on a standard for ISO. This standard also requires BOND of 50 psi, if not met, it fails. Dale Kempster from Schluter was also on the commettee and would most likly tell you your off base.I have also discussed bonding Ditra with the tech. departments of Mapei,Custom,Bonsal,Lati Crete,C-Cure ect.My point is information givin out on line needs to be what is written and warrented by the mfg. not opinions or interpretations by you. Not that your answer was all wrong but .....

 

Let's solve this once and for all.

DITRA is bonded to the substrate using a suitable bonding agent (thinset or otherwise) for the substrate. The bonding strength requirement is much less (~50 psi) than for a 'regular' tile installation because of DITRA's uncoupling and vapor pressure-releasing abilities.

When installed correctly, DITRA and the bonding agent is thinner than 1/4". It's not necessary to use a roller to press the membrane into the mortar, but some people have the roller with them and do it. You have to use a trowel to get into the corners and along walls anyways, so we just use a trowel throughout. It really doesn't require much pressure to set it in the bonding agent.

The preferred method of installing porcelain tile over DITRA is with dry-set mortar.


Dave Bruno: Thanks for the plug on the other thread. We're changing our shipping prices shortly. What part of MA are you in?

 

The standard for ANSI is/will be min. 50psi shear, for both waterproofing and crack isolation.

 

Hi -

I live right outside of Boston in Burlington. I found that United Distributors in my town carries Ditra. They had a sample installation on a small piece of plywood, which really helped visualize. They also gave me a 10-minute video which was pretty informative.

Dave

 

Hi to anyone who is interested -

There are 2 rim joists and 3 interior joists in this room. I have decided to remove as much of the pine boards as possible, and replace them with 3/4-inch exterior plywood. But, before I do that my plan is to level the room, to the best of my ability, but cutting down those portions of the offending joists. By doing this, my headroom problem goes away, and the unlevel-ness of the room is no longer visible (anyone can see it right now). I will then sister in a couple of full-span (16 feet) 2x8's, especially along the joists that were cut down. In between, I will probably add some cross bracing. Because of my decision to remove the pine boards, I can access the whole span of the joists.

After repacing them with the plywood, I will add another layer of 3/4" plywood, laying it perpendicular to the first layer. On top of this will be Ditra, not because of the headroom issue, but because it looks much easier to work with than the cement board.

By the way, today when I went to the local Home Depot to order the supplies, the guy at the Pro Desk basically would not let me order what I wanted, because he said it was "way overkill". He then proceeded to have the stock boys look for a few things that he thought would work better. I tried to tell him all the questions, research, etc. that I've asked and done, but I left there without ordering anything.

If he only knew what this has been like for the past couple of weeks ........

Thanks,

Dave

 

to good is, like, to much money!! Unless the guy at home depot is warrenting it tell him to shove it ........in the back of your auto like you asked. David and I have disagreed , he sayes Schluter Ditra I say Noble Co. Nobleseal CIS- BUT I THINK HE WOULD AGREE THAT SOME OF THESE BIG BOX FOLKS ARE DANGEROUS.

 

Although he's not talking about DITRA or CIS (he's getting his lumber, etc. at HD), I totally agree that trade experience at the big box stores rarely exists.

 

Hi -

What makes me feel somewhat confident is that you guys are commenting about the guy at Home Depot, instead of writing "Oh, my God, David, what are you doing???" in reference to what my plans are for this work.

Dave

 

I think you also know that if you run into problems, we're close at hand.

(Only two hours on I-95 anyways )

Actually, I'm up in Amherst, NH quite often. Only a 5-iron away from the turnoff to 3.

Send me an email - I have to ask you something.

David

 

Dave,

Thanks for settling on the Ditra. It's a fine choice.

I'm glad all you guys have come through this still friends (I hope. ) I have an interest in Ditra and in Schluter Systems, also. Some of the info that is being discussed here was originally released through our web site.

If Ditra is bonded to wood, a modified thin set is used, one certified by its manufactuer to go over wood. The fleece backing on the Ditra is what engages the thin set. The upper polypropylene mat does not have to engage the thin set. A bit of it does just in the course of things, but it's the fleece that holds it down, so everybody's right.

When the product is installed according to directions, it does not consume more than an eighth inch of space in most areas. You may get a littlle more mud in some spots that will cause a small rise, but it's insignificant and not worth arguing about. You should NOT apply more thin set, i.e., use a bigger notch trowel and cause the mat to reside higher off the floor than is recommended. Filling the underside of the square grid of the matting actually works against the uncoupling principle by making the squares rigid. They have to remain flexible.

Having gone through this whole post, I'm still not sure the subfloor and framing system are going to be adequate for ANY tile installation. At the minimum you've got to get everything up to L/360, and more is better. What goes over the subfloor, be it backer board or Ditra or Nobleseal, does not add diddly to the structure.