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Old 06-13-2011, 01:23 PM   #1
stevemar
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Tile over plank subfloor (again)

OK, so I will start by apologizing for asking what seems like a common question. I've read through the forums on a number of applications for tiling over plank subfloors, but still feel I do not have the answer to what I should do in my project. Worse, the guy I have helping me do the job (who has a lot of tiling experience) is recommending a different approach. I'd appreciate any clarification you could provide.

Here is my situation: I am retiling a kitchen with 12x24 tiles - the kitchen is small, around 140 sq/ft but is in the center of the house (actually an apartment). It is in an old building (110+ years) with 1" thick plank subfloors laid perpindicular to massive floor joists spaced about 18-20" apart (the building is stone/brick). The floor is solid. It is in the middle of the house and connects to a hallway/dining room with 1/4" plywood and 3/4" hardwood floor. I have an inch, so if possible I'd like to minimize the difference between the tile and the hardwood (I guess that is sort of a given).

Now my question: My expert help (he does work as a contractor and installs a fair amount of tile) is telling me that I can simply put a 1/2" or 5/8" cementboard/wonderboard on top of the subfloor and tile away. That doesn't seem to foot with a lot of what I have read independently. Based on advice from here and other areas, I was thinking of the following alternatives (1) 1/2" plywood + ditra or; (2) 3/8" plywood + 1/4" cementboard or; (3) just to put down 5/8" plywood. So does anyone have any input on what my best option would be? Key issues:
- I have seen many Qs with thinner plank subfloor, but could it be the fact that mine is 1" thick that is leading to my friend recommending just the cementboard? Is this OK?
- He has never laid ditra before, so is a little cautious of it - should I push forward with that option? Is it overkill here? Will I run into any issue where it meets the HW floor if I do not use ditra's edging strips?
- How about my other alternatives?

Thanks for your help - my head has been swimming with the options, but I definitely want to get this right.

Steve
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Old 06-13-2011, 02:49 PM   #2
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Welcome Steve.

First off, lets check the structure. What are the size, spacing and maximum unsupported span of the joists? Plug your info in HERE and tell us what you get.

Can you see the planks? Are they in good shape? and are they tongue & groove? How wide are they?

If you're structure will support a tile install, then you do have a number of good options for tiling over planks. Just installing cbu over the planks is not one of them.

For transitions to other flooring materials, a small height difference is common and can be easily handled with a wood, stone or metal threshold/saddle or edging if planned properly. Best to make sure the structure and substrates are up to task, then handle transitions as needed.
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Old 06-13-2011, 08:49 PM   #3
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extra info

Thanks dhagin.

Here is the info from the deflecto calculator - the deflection calculated is 0.125 inches, this translates to a deflection of L / 958. (This is being somewhat conservative on the measurements). The planks are 4.5" wide, tongue and groove and 1" thick.

So I cannot just do CBU - I guess I sort of knew that given what others have said and despite my friend's assistance. Could I get away with 3/8" plywood and 1/4" CBU or am I asking for trouble? (I ask because there are 3/8" flooring plywood panels at my local lowes - I cannot find 1/2")

Thanks for the info.
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Old 06-13-2011, 09:57 PM   #4
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If the planks are in good shape; not all bowed, cupped, twisted, cracked or splitting; then go over the whole floor and renail/rescrew all the planks. Then install 1/2 AC, BC or CC exterior glue ply (EGP) like THIS.

Problem with that 3/8 you're talking about, is it's likely 3-ply and not APA rated AC, BC, or CC EGP. So, no good for under tile. If you can find some 3/8 Halex or equal, then I think that would be fine too. Make some calls, someone has or can get some decent plywood for you, besides blue box store.

Over the plywood, you need a "tile friendly" layer like NobleSeal CIS (1/32" thick) glued with NobleBond, Ditra (1/8") or cement backer board (1/4").
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Old 06-13-2011, 10:17 PM   #5
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Welcome, Steve.

What Dana said.

My opinion; worth price charged.
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Old 06-14-2011, 04:46 PM   #6
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Thanks + two more newb questions

First off, thanks dana for the comments & insight - incredibly helpful and I think I am finally getting clarity around what to do. It is really appreciated! FWIW, I did spend some time calling around looking for Halex today, but no-one seems to know what it is.

Q1: Any chance anyone knows where I can find Halex in or near NYC?
Q2: If not, to save the room I will do 1/2" plywood + ditra and try to persuade my friend that it is the way to go. I read the article on plywood placement (perpindicular to joists) - does that apply the same if you are laying over plank flooring? I am assuming the plywood grain still goes perpindicular to the joists, but this makes it parallel to the planks. Just checking...
Q2.5: OK, I'll sneak one more in - if I am laying ditra do i have to do anything special at the walls? (I watched the video and read the guide - it shows wall trim, but can I just tile to 1/4" from the wall and/or right up to a wood transition and be OK? Do I need metal transitions?)

Thanks again for everyone's help.

steve
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Old 06-14-2011, 05:22 PM   #7
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1- I dunno

2- Ignore the diagonal boards and lay the plywood perpendicular to the joists.

3- Stop the ditra just shy of the wall / fixed surface. Nothing else to do unless you are waterpoofing the floor.
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Old 06-14-2011, 08:40 PM   #8
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1- To find good ply in NYC, might make a post in the "Pro's Hangout" asking for some help. Something like "Where to buy 3/8" Halex or 1/2" AC/BC/CC EGP in NYC?"...

2- What Paul said. I also try to get ply edges to land in the middle of a plank and not between planks. Might need to rip some of the ply down, but this is better than the ply & plank edges lining up.

2.5- Transitions between floors can be wood, metal, stone, lots of options there. Leave 1/8-1/4" between the tile floor and adjacent floors and caulk the gap to allow for differential movement.
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Old 06-14-2011, 09:13 PM   #9
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1. Good luck. Even if you find it I'm not at all sure it meets the requirements for the second layer over a sawn board floor. But I really don't know.

2. Don't know why Paul wants you to ignore the diagonal boards since you don't have any. Don't know why Dana agreed with him.

You lay all subflooring with the strength axis perpendicular to the joists regardless which layer or what kind.

2.5. What they said. Even thems what answered question #3. You'd think it was Friday hereabouts.

My opinion; worth price charged.
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Old 06-20-2011, 12:49 PM   #10
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Thanks

Just wanted to thank everyone that replied to this. I went with 1/2" plywood and ditra on top. Although my friend was a little leery of the ditra at first, by the end he was a convert - claims he will use it moving forward on work jobs. The tile floor went down beautifully and is in its 3rd day of drying as I type this (the only wrinkle is that I could not find good quality un-modified thinset for between the tile/ditra - others on this board felt it was OK to use versabond, so I am hoping it does not cause dry time problems... either way, I am staying off it for 3-4 days).

Thanks again - the advice here was of incredible help to getting my project completed.
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Old 06-20-2011, 03:21 PM   #11
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Modified thinset over ditra - dry time?

I installed (or had a friend help me install more accurately) porcelain tile using the Ditra underlayment this weekend. Under the Ditra (to the plywood), we put latex modified thinset. Unfortunately, I could not find any un-modified thinset other than Custom Blend (reportedly crap) at the local HD, so after reading several threads here I went with Versabond for bonding the tile to the Ditra. Unfortunately, I should have read a little further because I now see that people recommend not grouting if you are going to do that until later. We grouted the next day (18-24 hours later).

OK, so that is done and hopefully I am not going to rip up the floor. I'd also like to wait until it is dry to move anything on to it (er, like my fridge ). Can anyone give recommendations on 1) whether I have created a major issue for myself and 2) how long (assuming the issue is just longer dry time) I should plan on waiting? Any guidance here would be great - it was put in 48+ hrs ago but will wait as long as I need to.

Thanks. Steve
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Old 06-20-2011, 03:52 PM   #12
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It could take weeks - or longer...are the tile porcelain? If so, both the Ditra and the tile are nearly impervious (the Ditra is for all practical purposes, the tile is nearly so). If the porcelain is glazed, it's worse. How big are the grout lines? The only 'good' thing, is that Versabond, while a good modified, isn't a highly modified thinset, so it doesn't have as many fillers that need to dry as the stickier premium stuff.

Got a dehumidifier and fans?
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Old 06-20-2011, 04:32 PM   #13
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Steve, please keep all your project questions on one thread so folks can see what you're working on and what's been previously asked and answered. We can give it a more generic title any time you'd like.

It's best not to grout for at least 24 hours after setting tile in any conditions, actually. An extra day wouldn't have hurt with the Versabond over Ditra but it may not be a problem at all. Usually what you run into is blotchy grout color because of uneven drying.

But the Versabond will cure just fine under there, I believe. I still wouldn't recommend heavy traffic on the floor, such as your refrigerator, for at least a week (7 days) regardless what you'd set the tiles with.

My opinion; worth price charged.
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Old 06-20-2011, 08:43 PM   #14
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OK - makes sense. Did not know the preferred approach on the forum... Now I do.

The tile is unglazed (good, I guess) but is 3/8" thick and has 1/16" grout lines (bad, I assume).

Appreciate the guidelines on the dry time cx - that makes me feel better that I have not just created a mess of mud that will never dry. I'll give it a week before moving anything heavy on it. Thanks all.
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Old 06-21-2011, 10:25 AM   #15
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The way a modified thinset works is the modifiers actually coat the cement particles (when mixed properly). It's the modifiers that are primarily the sticky stuff and the sand and cement are the support mechanism. It's the modifiers that need to dry out, not the cement (or the inert sand) as the cement will cure regardless (and the cement plays a much smaller part of the actual bonding). In an unmodified thinset, all that is required is for the cement to cure - it doesn't need to dry at all, and the cement is the bonding agent. If there is excess moisture, that will evaporate eventually, but there shouldn't be much, if any, when mixed properly (excess water weakens it, actually).
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