Photo 1
I am not writing this to tell you why your tile is cracking or why your grout is cracking – I have other posts that may tell you that. (Click on the pretty little links 
) If you happen to have Schluter Ditra as your substrate, this post will tell you why either one of the above may be happening.
While Ditra is my preferred membrane for floor tile installation (as well as countertops and tub decks) it absolutely needs to be installed correctly. The two main techniques for this are fairly simple:
- Make sure the cavities (waffles) are filled correctly
- Install it over an approved substrate (and with the correct type of thinset mortar)
Photo 2
There is a lot more to ditra than those two items but if either one is incorrect I can nearly guarantee a failure. See photos 1 and 2 there? The tile was cracked and it was a direct result of a) not getting the waffles filled correctly and b) improper coverage on the tile. Now b may be due to not backbuttering the tile, an improperly-sized trowel, letting the thinset skim over or set too long before installing the tile or simply incorrectly mixing the thinset. All three of those things will cause any tile installation to fail – whether you use ditra or not.
Not filling the waffles correctly, though, will cause the tile to not be fully supported and/or not ‘locking’ the tile into the ditra. Because it is not correctly locked into the ditra you will lose the mechanical bonding properties of ditra and you may as well install it directly to particle board at that point (That was sarcasm – don’t do that!). For more specifics about exactly how ditra works you can check out Provaflex vs. Ditra wherein I describe exactly how the mechanical bonding process works – and rant about a particular jackass. But the mechanical thing – that’s what you want to concentrate on.
You need to use the flat side of your trowel and spread thinset in every direction over the ditra to ensure that all the little waffles are full. Since the cavities are dovetailed (that means they go down and away from the opening) you need to ‘force’ thinset into the bottom corners of the cavities. Simply running the trowel over the ditra will not do this. Simply running the trowel over the ditra did that (photos 1 and 2).
Photo 3
Installing ditra over an approved substrate is much, much easier. In fact, nearly every bare substrate you find in a modern house would be considered an approved substrate – shiny linoleum is not one of them (Photo 3). While there are thinsets that ‘say’ they will bond to linoleum (and some of them will) apparently the jackass who installed that particular floor was not aware of that. 
See photo 4? I lifted that up with my pinkie – literally! It was not attached at all. He may have had correct coverage beneath the tile and all the little waffles filled – I have no idea. There was not enough stuck to get enough leverage to tear one off and find out.
Photo 4
Most any plywood (even osb 
) is an approved substrate for ditra. And if you use a thinset approved for that substrate, there are no problems at all. Photos 3 and 4 had an unapproved substrate and, apparently, incorrect thinset (and a shitty tile job, but that’s a whole other post). It was nearly guaranteed to fail.
When you buy ditra for your installation every roll comes with a handy little instruction booklet. You can go to Schluter’s Ditra Page on their website and access the instruction booklet (This link is a PDF!). They even have a flash video about the proper installation technique. You can leave a comment below and ask. You can email me. You can send up smoke signals – I’ll answer.
Given the 17 ways to acquire correct ditra installation information above there is absolutely never a reason to do it incorrectly. Ditra, in my opinion, is the best membrane for most floor tile installations. The only time I’ve seen it fail is due to incorrect installation. And that isn’t just the common BS everyone accuses failures on. Me, personally, every one I’ve seen fail is incorrectly installed.
If you use ditra, and if you have an approved substrate, and if you have the correct thinset mortar, and if you fill the waffles correctly, and if you use the proper trowel and get proper coverage it will not fail. Yes, that’s a lot of ifs – when you read it. In practice it really is not that many things to get right. It’s just common sense, mostly.
So here’s one more if: If you have any questions at all about correctly installing ditra and using it for your tile installation please, for the love of all the marble in the Sistine Chapel, ask me below in the comments. I WILL answer you. I’m just super-cool like that 
I just installed the Ditra mat and noticed that there is a slight ridge in the mat, following an apparently high joist raising the sub floor. Apparently the floor was not level from the builder. About an 1/8 inch high point, which is situated in the middle of 13×13 tiles. My question is can I remove the top of the “waffle”, the high point to level the mat? Like take a razor and slice off the high point, it is only 2 ridges wide. Or would that ruin the uncoupling properties. I am thinking the removal of two ridges wouldn’t hurt the stability or strength.
Hi David,
Yes, you can. I’ve done it and it works fine. If you tell anyone I told you that I’ll send Guedo the killer elf over to your house!
Hello.
Just a questions?
1. have removed old vinyl floor tiles of plywood floor, have scrapped all the old glue or what ever they used in the 50’s on the floor off.
We then painted the floor. ( but know have decided to tile)
will it be ok still to put Ditra on the plywood or do I need to get rid of paint first ?
Hi Dickie,
You need to get rid of the paint first.
Hello!
I lifted my existing porcelain tile (to install new porcelain tile) and I am not able to get all the mortar off the 3/4 inch subfloor that is liquid nailed to the original subfloor. Can I float the floor with mortar over the existing mortar to make level and then install the ditra as directed? the bathroom is 5×8. Please help! Thanks! Tom
Hi Tom,
Yes you can.
I am helping a friend to install ditra and tile in a bathroom over subfloor. I’m not worried about the floor and spacing of the joists. She painted Kilz (oil based) over the subfloor because of possible smell coming up. From what I see, that wasn’t needed. My question is if I still use a modified thinset down on the Kilz before the Ditra?
Hi Tami,
Yes.
Will any linear drain work with the Kerdi membrane? Or do I have to use the Schluter linear drains?
Hi Dan,
You can use any TOPICAL drain. Several other companies make them also, including laticrete and pro-drain.
Hi – I’m getting ready to put DITRA-HEAT-TB over a concrete floor in my basement but the concrete is painted with likely epoxy or oil pain (it’s definitely not latex). Any issues with that or do I need to grind the entire floor first and expose the bare concrete? Thank you.
Hi Jakub,
Unfortunately that will need to be removed – thinset (unless it’s epoxy) will not bond to that.
A quick follow-up: I grinded the epoxy paint away (awful job, took me 2 days…) and then used unmodified thinset on the bare concrete and the DITRA-HEAT-TB holds super strong!
Hello
I have a concern with my installation. Over a layer of self leveling concrete I’ve installed a Ditra Heat mat with unmodified thinset, inserted the heating cable, and used the rest of the thin set as a skim coat filling all the divots to keep it in place.
Now two days later I’m getting ready to lay my tile and the is a crunching sound in some sections. Did the Ditra mat not bond to the floor? Did the skim coat not fill all the divots? I’m thinking something didn’t work right and I need to ripe it out and start over. Can you confirm? Also, if I take it out do you think the heating element will still be good? Thank you and have a great day. Eric
Hi Eric,
It depends on whether the crunching is from beneath the mat or not. The hollow top of the pillars in the mat will come out before you tile (after you pre-fill), it may be that. It will likely be fine provided you DO NOT see ANY movement in those areas when you push on them. Installing your tile will solidify all of that if it’s just on the surface.
I just read your post. Ironically, I had the same problem with my install which took place on Halloween day. Initially I was instructed to use the Unmodified thinset for the OSB. This resulted in the same problem you just described….a failed install. I ripped it all out, ground off the residual thinset with an angle grinder, and used a modified thinset to bond my ditra heat to the OSB subfloor. This resulted in a rock solid install. Although my stomach sank and I felt nausous when I knew I had to ripe out the mat, Im really happy I did. The mats now are bomb proof solid after the use of the modified thinset! Im very happy!
Hi Ryan & Roger,
Thanks SO MUCH for posting your experiences! I’m about to undertake a 750 ft^2 diy ditra heat job of my own. I too was told the mortar I was sold was one size fits all… I was suspicious enough to go digging for answers and found your site. It turns out I need the latex modifier to meet ANSI A118.11. Schluter’s instructions are well written, but your site and shared experience really drives the points home.
I have red guard over top of a wooden floor. I would like to install Ditra over the red guard what type of thin set should I use. Should it be a modified or unmodified thin set .
Hi Richard,
Modified.
Considered Ditra but different type application, not sure it would, or would not help. Not tile but slab marble/granite (2 or 3cm) on second floor wood joists (16″ oc). Had considered metal mesh on sub with self-leveling poured to wall with 1/4″ expansion joint at perimeter. Then waterproofing applied, unmodified thinset, then lay slab into thinset. Different type of fracture and flexural issues. Recommended method, would Ditra apply? Thanks Jan
Hi Jan,
Yes, it would apply. It would separate any movement in the framing from the slab itself.
Hi,
I am going to tile a living area that has had a wall or 2 removed, thus enlarging the floor space, so there are several types of flooring to cover over.
Part of the floor is flagstone (8×11 area) butted up next to old concrete (~8×11 area), those two materials butt next to Terrazo (old Florida terrazzo with several hairline cracks). Visually it’s like 2 squares alongside a rectangle.
1. Is Ditra the right product, or even necessary?
The flooring is rock solid, but there is a very slight build up in height needed that the Ditra would accommodate.
2. Do the hairline cracks in the terrazzo need the membrane fixed before using Ditra?
3. Where the different materials meet (terrazzo to flagstone, flagstone to concrete, concrete to terrazzo), do those places where the materials meet need to be membrane fixed also? Or can we just Ditra over the top?
4. Would we use the same adhesive on all 3 floors to adhere the Ditra to those 3 flooring materials?
The thin Ditra over the terrazzo, and the thicker Ditra over the flagstone and concrete will bring everything level.
I’m using 24″ porcelain.
Appreciate your guidance.
Thanks, Mimi
I’m planning quite a large tile project (for Me) at my cottage in Northern MI. It’ll be 350 square feet using porcelain 6″ x 36″ wood look tile. Never worked with tile of this length so I’m concerned over how level the floor has to be? Substrate is Plywood over a slightly damp crawlspace. The ditra sounds great vs laying down backer board. I realize I need the correct thinset for plywood, any suggestions? Will it work with the tile leveling wedges? Also where can I find Ditra ? are there different grades?
I know I sound like a complete newbee but I have done 5 or 6 successful tile projects over my storied 30 year career! haha
Hi Greg,
The specs for floor flatness for a large format tile (any one side over 15″ long) is no more than 1/4″ in ten feet and no more than 1/8″ over two feet. There are three types of ditra – regular ditra, which is likely what you need, ditra XL, for extra height (it’s 1/4″ thick) and ditra-heat, used with in-floor heating cables. If you use ditra any good modified thinset will be fine for installing over plywood. I have no idea what retailers are near you, but google works well.
Just type ‘ditra’ with your town and suppliers will likely pop up for you.
Do I need to use the Kerdi band in the corners and edges around a bathroom floor with a traditional bathtub?
Thanks
Hi Kara,
No you do not.
Roger,
Very gracious of you to answer questions for all of us. So thank you in advance. I live in San Antonio where temps range from 30-100. I am installing 6’x24′ porcelain tile on a flat sidewalk and porch (approx 1000 sq ft). Since I have already had a failure due to improper installation, I am now doing tons of homework prior to round 2….
The surface is concrete, 7 years old, had bead blasted to make sure I have a clean surface to bond to.
Sidewalk has control joints every 3-4 feet with expansion joints every 20 feet or so (sidewalk is~100 feet long)
Porch is elevated 6 inches and part of home slab. approx 15′ x 50′. No control or expansion joints (must be pretensioned or something) and has only a few hairline cracks after 7 years.
Sidewalk has grass that is watered on both sides
My questions (that schluter cust service reps cant seem to answer).
1) I know I cant tile over expansion joints, but are control joints ok as long as I use soft grout joints (silicone grout) around tiles at control joints?
2) Any issues with ditra outdoors in these conditions that you know of?
3) Given much more exposure to temp variations is ditra best bet? My other possible install solution is red guard
4) For red guard, I need to use crack isolation membrane where cracks exist, for ditra I am assuming no need?
5) Last (whew sorry…), do I need to “kerdi band” the seams with outdoor install? I know it will rain… but is waterproofing needed? I dont know if sealing the seams is to protect the mortar bed or to simply waterproof the install for indoor applications.
Thanks Again
Hi Tom,
1. Yes.
2. Not that I’m aware of, provided you have the proper expansion joints where needed.
3. In my opinion yes, ditra would be my choice.
4. Correct, provided you honor both the expansion and control joints up through the tile installation with soft joints.
5. No, it is not needed. The only thing water will do to mortar, once it’s cured, is make it stronger.
Concerning your porch with no expansion or control joints: IF it is pre-tensioned slab you can lay tile over it, however, you STILL need to install the soft joints. Considering it gets direct sunlight at least part of the time, I would install one down the middle (50 feet long) as well as one every 8-12 feet in the short direction.
I’ve read a bunch on Ditra installation. I’m considering it for my custom shower. I am a bit leary of installing this over plywood. 1. Will it work in this type application?
2. Is any other water proofing required (other than sealing the Ditra joints)? The information says no, but…..
3. How should corners be sealed?
4. Lastly, it says in the information to use min of 2″ square tiles. Is that really the smallest size for tiles on this material?
Thanks,
Dan
Hi Dan,
You’re confusing Ditra with Kerdi. Ditra is an uncoupling mat used for flooring tile installation. It can be waterproofed if need be, but it is a flooring underlayment mat. Yes, 2″ is the smallest recommended tile to be used over ditra. It can be used over bare plywood.
Kerdi is a waterproofing membrane used for showers. It is a solid sheet membrane. Yes, it is to be used to waterproof your custom shower. It can not be used over bare plywood. Your floor needs a proper mud bed over it, then the kerdi membrane is installed over that. Provided you have a 2″ overlap on all seams and corners there is no further waterproofing required. Bonding that 2″ overlap with thinset waterproofs the seam – even in the corners.
I have a manual available in the Library which will walk you through the entire process of waterproofing your custom shower with the kerdi membrane if you’re interested.
Roger,
Thanks.
In the Ditra installation manual is says that Ditra can be used in showers as the waterproof membrane as long as the seems are sealed with the Derdi-band. They go on to talk about using their Kerdi-drain or Kerdi-line as part of the installation of the Ditra subfloor.
I’ve never used Ditra or Kerdi.
From your note above, it sounds like the best thing would be to use the Kerdi in the shower and Ditra in the bathroom. Since I’m going to go with a curb-less shower, I was hoping that I could take the Ditra right into the shower.
Another question. I’ve installed wire based floor heating several times. Lowes has this infrared TEC product that is extremely thin and looks easy to work with. Do you have any thoughts on that product?
Thanks,
Dan
You can take the ditra into the shower floor if you want, however, I always completely cover it with kerdi when I do that. One little pinhole can screw up your whole day if you don’t.
The tec product works very well, just be sure to pay close attention to the installation instructions and follow them closely.
Roger,
Thanks,
One more question.
My plan has been to have a zero clearance shower (no curb).
I have engineered joists (I beam shape). Before I knew this, I was planning on cutting part of the joists out and build additional structural support. As I researched engineered joists, it says to not notch out the top. Do you know of any way to modify engineered joists to make the shower floor lower to have the affect I’m trying to build?
Thanks,
Dan
You can not with engineered joists. The entire joist – every inch of it – is responsible for the deflection ratio of the joist. If you remove or alter any of it the joist effectiveness goes down dramatically. The only ways to do it, given your joists, are to either place the linear drain across the entrance to the shower and have the floor sloped up to the back wall, or raise your entire bathroom floor to the level you need the height of the mudbed.
Roger,
Thanks a bunch. I suspected as much, but found conflicting information on the internet.
Like everyone else on here, we all really appreciate the time you take to help us.
More questions!
My space for the shower is 40″ wide and up to 90″ long (I say up to because I’d like to make it shorter but I don’t know how long it has to be to have an zero clearance door and keep the water in). The shower head will be on the 40″ wall. The exit will be on the last 30″ on the side. My first question: How long does a shower like this need to be to prevent water from coming out.
In order to reduce the height requirement of the shower, I’m planning on cutting out the floor. I’ll be adding floor supports to the sides of the current I beam joist at the correct angle for the floor. after placing some tar paper over the opening, I would place 1/2″ concrete board on top of that, then the Kerdi plus the narrow Ditra on top. I’d fill any gaps between the concrete board with thin set. My thought would be to have a horizontal drain about 60″ from the shower head perpendicular to the 40″ wall. I’d have a reduced slope from the end of the shower toward the drain for any over spray. I would use thinset to get this slope (then Kirdy and Ditra). With the thicker Ditra outside the shower, I can then have a slight slope into the shower with little change. I know this is complicated. Generally speaking, I am very detailed and don’t mind the extra effort required to get the affect that I’m looking for. What do you think about this plan?
Thanks for your advice!
Dan
You’re putting concrete board over bare (with tar paper) floor joists? That will not work. The deflection strength of backerboard is essentially nil. Just use 1/2″ plywood, it has much greater strength. I don’t see ANY mention of deck mud at all – what exactly is your plan with the construction of the shower floor – backerboard only? You cannot achieve a slope in that manner. Thinset is not used to create any type of slope, it needs to be deck mud. Ans why are you using ditra on your shower floor? You can, but there’s really no reason for it.
Your layout sounds fine, it’s the construction details that are incorrect. There is no set ‘length’ needed for a zero-clearance shower, provided the floor is properly sloped to the drain water will run in rather than out. You need to have plywood over your joists, tar paper or a cleavage membrane of some sort over that, then deck mud, then your waterproofing. You are attempting to build your shower around your existing bathroom floor for a curbless installation – that’s backwards. The shower is where the water will be, it needs to be built correctly and completely waterproofed, the bathroom floor needs to be constructed to meet that criteria.
Hi Roger,
Enjoy the feedback on your site. Thank you!
My question: Bought 1/4″ ditra to be installed over plywood subfloor. Also got Mapai unmodified thinset for the install. However, my tile man insists using mastic is better. Schluter does not mention mastic and I do not know the differing characteristics of each. Could you give me your opinion on using mastic?
Thanks!
Hi Marilyn,
Mastic is ABSOLUTELY NOT to be used, in any capacity, with ditra. Ever. He has no idea at all what he’s doing.
I pulled up vinyl and some of the backing won’t come up. Can I put the thinset over the backing left behind? Then put Ditra over it? Or lay 1/4″ plywood first?
Hi Bill,
You should put plywood over it. The glue that is holding it down is water-soluble, so if (when) it gets wet it will release the backing. If your ditra is bonded to it then your ditra will no longer be bonded. I learned that lesson the hard way – like everything else.
Hi do I need to install another layer of subfloor or can I just use the ditra right over my existing subfloor thanks ron
Hi Ron,
I have absolutely no idea.
I need a bit more information, like the joist specs of your floor, the thickness of your existing floor, and the type of tile or stone you plan on installing before I can begin to answer that question for you.
They are 2×8 16 on center and just porcelain tiles 12×24 and I think the sub floor is 1/2 plywood
You should be able to go right over that with the ditra then.
Ok great thanks
I won’t need to put down any dura rock board or anything?
Yes, the thinset can be stained. Aquamix (as well as a few other companies) makes a grout colorant. It will color both the grout and thinset to a uniform color. You can also get a grout saw and remove the thinset down to at least 2/3 the depth of the tile and regrout it. Read through this: Regrout your tile
Nope, you can go right over the ply.
Ok thank you
hi what is the best way to water proof a shower ceiling i dont want to do tile and if i do the kerdi on the ceiling can i use regular Spackle over it and than paint it if so what would you recommend for the best paint brand to use thanks ron
Hi Ron,
Unless you have a drop header over your shower (where vapor and steam can get trapped in the ceiling of the shower) then just using a good latex paint will work just fine. No real need to waterproof it at all. Drywall mud will not last long-term over any type of waterproofing. If you’re that concerned about it you can install cement backerboard on the ceiling, then prime and paint it.
hi i am installing ditra in my kitchen and putting in new kitchen cabinets as well and i am putting tile down my question is should i tile the whole floor before the cabinets or after and if i do tile the whole floor even under the cabinets what happens say 10 years from now we want to change the tile than im stuck with the old tile under the cabinets and also my kitchen is going to have an island what do i do in that case how do i secure the island cabinets to the floor if it is tiled already and can i penetrate the ditra with screws to secure the cabinets to the floor thanks ron
Hi Ron,
You can do it either way. IF you plan on ever changing the tile – then put the cabinets in first. Yes, you can drill through the tile in order to fasten down the island.
Ok thank you
I am doing heated bathroom floors using Ditra heat membrane, which is 1/4″ thick. The heated floor is to meet up with non-heated floor of the same tile. I am using regular old Ditra uncoupling membrane, which is 1/8″, leaving a 1/8″ differential between the heated floor Ditra and the non-heated floor Ditra. What would be your advice for the best way to deal with this height difference? Thanks.
Hi Dave,
If ditra is the only thing you have available then you can install two layers of it – one right over the other. If you have access to ditra-XL, it is the same height as ditra heat. You can also use a trowel that is 1/4″ deeper for the regular ditra (that will give you an extra 1/8″ beneath that tile).
Thanks Roger. I already have the regular ditra, but to avoid doubling the cost of it (using 2 layers), I think I’ll take the deeper trowel route.
I ran out of time whilst installing over ditra and I have a number of the ditra squares full of mortar but didn’t install the tile over it, will it be OK to just go back and install the tile over that section or should I try to remove the mortar from the ditra first?
Thanks
John
Hi John,
The mortar is just fine in the ditra. I pre-fill mine often and install the tile the next day. It’s fine.
Help please! The tile guy installed ditra directly on the plank subfloor. He then floated the top with a layer of thinset. The plank floor is 1×8 laid on the diagonal with about 1/8 inch Gap between each board. No tile is down yet. Along each joint between the floor planks, the thinset and ditra has raised to a peak with a hairline crack in the thinset following the length of the seem. Looks and feels like a failure to me. The tile guy and the contractor swear this is fine and there will be no failure.
I’ve been doing some research and learned that a 1/2 inch layer of quality exterior grade plywood should have been put down first. Also I’ve Been Told the ditra was not appropriate anyway considering the tile is 2×2 hexagon Carrera marble and would need a mat with smaller dimples.
1. With the extra height of the floor in mind, can the plywood be placed over this mess or do I have to have it all torn off and apply the plywood directly to the plank flooring?
2. Either way should it be glued and screwed and should it be screwed directly to the joist or just the planking with the 6 inch spacing pattern around the edges and 8 inch spacing in the field?
3. Finally, with the 2×2 marble hex tile in mind, what are the pros and cons of applying quarter inch Hardie vs ditra or similar membrane with smaller dimples? and any other advice you can please provide.
Much Thanks
Hi Kurt,
1. It will need to be removed and the ply needs to be installed directly to the planks.
2. Just screwed to the planks only (NOT into the joists). 8″ oc spacing.
3. Ditra is not approved for that, you may end up with problems. I would actually go with a backerboard in that situation.
Thank you for the info. We removed the plank down to the joists and are going to start again with T&G then hardi. Hopefully we sre on the right track. W
e are trying to keep the tile even with the wood floor.
Can you use Schluter-Ditra mat as the shower subfloor? I would put it in a 1/2″ mud bed over plywood subfloor. And then use the white plaster/bond stuff to fill the cells. And then use a 4″ strip of the orange fabric at the bottom perimeter of the shower going up the wall 2″ and then kerdibacker up the shower walls. Would that be ok? I’m actually just checking up on my tile intstaller..
Hi Paul,
You should not. It still creates little pockets of thinset and water which, in a shower floor, would NEVER drain or dry out. The proper installation is a mud bed with the kerdi sheet membrane over it.
Hello Roger,
I am building a new house is Florida. The living area is on the second floor and includes a covered deck and Florida room on the outside of the living area but over the lower floor (garage and workshop). I am planning to use ditra as a waterproof membrane for the deck and Florida room floor to prevent leakage into the lower level. The deck and Florida room at times may be exposed to driving rain and sun. Is it appropriate to treat these areas for construction with the same specifications as used when tiling a bathroom?
Hi Ken,
Yes, and you need to install kerdi-band over the ditra seams to make it waterproof.
Matt: I was talking with a schluter field rep today and asked that question. He said he most certainly does NOT recommend adding backer board. Note: i am building a shower. More: call Schluter customer service and they will give you contact info for a field rep on your area.
Roger:
I have an old house with structural plank floors. There are high and low spots in the floor, and to level it out, my tile guy is suggesting only putting plywood over half the floor and using a leveling agent on the other half. He would then install the ditra system over the entire floor. The end result would be that the high end of the floor would have the ditra system installed directly over the structural wood planks, which has me concerned. I’ve read the ditra manual, which states that a minimum 3/8″ plywood is required over the planks. He insists that the tile/grout lines won’t crack and that gluing 3/8 plywood strips to the bottom of the wood planks (basement side) will give them extra strength and prevent fluctuation. He’s the tile guy, so I don’t like to second guess him, but does this sound right to you? Thank you.
My subfloor is 3/4″ plywood on manufactured trusses at 24″ centers. There are some squeaks, so I was going to tear up the subfloor and add reinforcement, but realized that the edges are sandwiched between the truss on the outer wall and the stud wall for the upstairs. I was going to use Ditra but squeaks mean movement, and movement means trouble. So i was thinking about adding a layer of backerboard instead to make it rigid, but would then lose the advantages of the Ditra. Should I use the backerboard or stick with the Ditra?
Fellow DIYer here,
The first thing I’d want to check is the deflection of your trusses to make sure they are ok for tile. I think it’s a pretty good bet that they are, but you’d want to make sure they have a deflection of L/360 or better for ceramic tile. (If you are considering natural stone, the deflection requirement is double at L/720, and your trusses are far less likely to meet that requirement)
Assuming the trusses check out, I’d be inclined to keep your existing subfloor and add a second layer to it, assuming it’s an appropriate type and in good condition. You want T&G plywood with no face grade lower than C (CDX or anything called “sheathing” isn’t up to the task).
If your existing subfloor meets those requirements and you can make the squeaks go away by adding more of the correct type of fasteners and you are comfortable that the squeak fix will last), then you can then add an additional layer of plywood.
(Ditra XL (thicker) is specified for use over a single layer of 3/4″ subfloor over 24″ joists, but that scares me a bit)
If you can’t fix the subfloor you have, you’d have to remove and replace with the appropriate material – you can glue the new subfloor to the trusses with construction adhesive if you like, but be sure to screw them with the appropriate type of screw and spacing.
In either case you need a second layer of plywood to get the strength (between joist deflection) up to snuff for regular Ditra. The Ditra handbook specifies 2 layers of plywood/OSB totaling a minimum of 1-1/8″ thick (3/4 + 3/8) for using Ditra over 24″ joists. I can’t speak to it myself, buy my understanding is that you are really best to avoid the 3/8″ material that you will find readily available on account of it not being very flat.
Having said all that, a layer of 1/2″ CC grade (or better) with exterior / exposure 1 rating (needed so the plywood glue doesn’t soften up when exposed to the wet thinset) is probably the best route to go. DO follow the directions in the Ditra handbook when installing that second layer.
In particular:
– The second layer of plywood should be installed with the strength axis (long dimension) perpendicular to the joists (as with the first layer)
– The panels should be placed so seams don’t overlap with the original panels – 1/2 panel offset laterally, and the end joints should fall at appx 1/4 point of the joist spacing (6″ past a truss, in your case) (page 17 of the Ditra handbook)
– The screws should *not* penetrate into the trusses, only into the subfloor layer.
– You generally *do not* want to attempt to use glue between the two layers – it’s not that it can’t be helpful, rather it is difficult to pull off unless you know what you are doing.
– You DO want to use the appropriate type of fasteners and the appropriate spacing. Hint: it’s probably a lot more screws than you think. I recall that it is 6″ spacing on the perimeter and 8″ spacing in the field, but you’d really want to verify that.
After you are done with all of that, it’s time for the Ditra. If you chose plywood for your top layer, you just need to pick a decent quality modified thinset to adhere the Ditra (paying close attention to the Ditra instructions and the wonderful elf tutorials about such things)…if you chose OSB for your top layer, you will find yourself in the unenviable position of having to locate a modified thinset that is approved for use over OSB. I have tried and failed on a number of occassions…not saying that many won’t work, but they don’t specify it.
That’s what I have to say about it. I get some things wrong from time to time, and if I have misspoken here, hopefully Mr. Elf will step in and straighten me out.
I meant to address the CBU issue directly. It does not add any meaningful stiffness to your floor structure. Really. An additional layer of plywood is really the only way to make the floor stiffer…Eliminating the squeak (movement) is job #1, though.