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Unread 10-16-2020, 06:38 PM   #1
1902BrickFlat
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My First Home, 1904 2nd floor kitchen reno - underlayment requirement help

Hi!

I'm tiling the 2nd floor kitchen of my 1904 brick flat and confused about underlayment/substrate requirements. Thank you for any education

Tile = 12x24 porcelain
Mortar = Mapei Ultraflor Plus

Joists = 10in high x 2in wide x 10.5 feet long, 16 inches apart... my deflection calculation is L/933

Current subfloor = 3/4inch tongue and groove nailed directly to the joists, and a level of luan that is has been very secured to the floor by the previous owner.



Question 1.
Since my deflection is above requirements for my tile, do I still need to build up the underlayment to 1 & 1/8inches? Or can I just add 1/4 hardie board or ditra for a smooth substrate?

Question 2.
Since hardie board and ditra require a thinset backing, does the luan need to be removed? Or will it be alright since it's underneath either an uncoupling membrane or a hardie board that shouldn't expand/contract? (With the luan the floor is very level and it is incredibly secure, so I'd like to avoid removing it)

>> Follow-up to question 2:
a. OR could I use hardie board without thinset?
b. OR exterior grade plywood as substrate and adding mapei keraply (latex mortar additive) to mortar and tiling directly to the plywood? (this is what the tile store recommended, but I've heard negatives about tiling over plywood)


Thank you so much for the help! This is my first home and I've been working on it for years. I love learning so much!
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Unread 10-16-2020, 06:55 PM   #2
cx
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Welcome, Nancy.

Quote:
Originally Posted by Nancy
Current subfloor = 3/4inch tongue and groove nailed directly to the joists...
3/4" T&G what?

1. There is no such thickness requirement in the ceramic tile industry.

2. Yes, you must remove the Lauan. That's a requirement of all manufacturers of tiling substrate products and the ceramic tile industry.

a. Absolutely not. Please download the installation installation instructions for Hardiebacker if that's the substrate you intend to use.

b. There is no exterior grade plywood. There is exterior glue plywood and that's what you want for any part of your subfloor structure and it must have no face of grade lower than C. The exposure rating of EXT or Exposure 1 in the grade stamp is your indication of exterior glue.

We (TYW) do not recommend tiling directly to plywood. There are very particular requirements for the subfloor structure and depending upon your answer to my first question you may or may not be able to meet the first of those requirements at all.

My opinion; worth price charged.
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Unread 10-16-2020, 07:29 PM   #3
1902BrickFlat
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Thank you for the reply!

I believe the T&G is douglas fir.

Based on below, would exterior glue plywood over the lauan, then thinset, ditra, and tile work?

Or if I remove the lauan, would I be able thinset the hardie directly to the T&G hardwood underneath?

Or do you have another recommendation?
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Unread 10-16-2020, 07:38 PM   #4
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The question about "T&G what" was to determine if you had sawn boards, plywood, OSB, or something we've never heard of.

Again, the Lauan plywood must be removed.

If the subfloor is sawn boards, you have only two options: You can use a reinforced mortar bed a minimum of 1 1/4" thick or you can add a minimum of nominal 1/2" exterior glue plywood with no grade lower than C and then a tiling substrate of your choice.

No, you cannot install any tile installation substrate (other than the mortar bed above) over your sawn board subfloor. Well, you can, of course, but not if you follow any substrate manufacturer's recommendations and not if you want to comply with ceramic tile industry requirements and not if you want your tile installation to last.

My opinion; worth price charged.
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Unread 10-16-2020, 07:47 PM   #5
1902BrickFlat
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I think they're sawn boards? Definitely plywood or OSB. not They're douglas fir tongue and groove 3/4 inch hardwood nailed directly to the joists without anything below them. In other areas of the house they're the finished hardwood floor. The house was built in 1902.

Okay, so my plan will be:
1. must remove lauan
2. add nominal 1/2" exterior glue plywood with no grade lower than C
3. ad hardiebacker to plywood (with thinset) prior to tiling

And that should do it?
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Unread 10-16-2020, 09:16 PM   #6
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Lets get a couple of pictures of your floor. Hardie backer is a thirsty substrate for tile, I am not sure why anyone would use it but there are installers that love it. I would recommend durock or permabase or the ditra cause it is less work to put down. I try to use stratamat(laticrete's version of a uncoupling membrane) on every job that I can. You may not be able to use a uncoupling membrane because you are going to be removing layers of floor and your door jams and casing may be to high for that but you could build it back up with plywood
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Unread 10-16-2020, 09:28 PM   #7
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That plan will work, Nancy.
Quote:
You may not be able to use a uncoupling membrane because you are going to be removing layers of floor and your door jams and casing may be to high for that but you could build it back up with plywood
I'm not understanding that comment from Shawn. I don't know what your previous finished floor was, but seems to me you'll be adding more with the plywood and underlayment and tile than you had previously. Perhaps I'm missing something?

My opinion; worth price charged.
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