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Old 06-07-2009, 10:01 PM   #1
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Subfloor and Underlayment Questions


Hello, I've been reading previous posts and am now ready to ask my own questions. I understand from cclarlisle in a previous post that there are four acceptable methods of tile installation:
(a) with thinset, on two layers of 5/8" plywood, or
(b) with thinset, on one layer of 5/8" plywood and 1/2" CBU, or
(c) a 1.5" thick mortar bed+metal lath combination, or
(d) with thinset on one 5/8" layer of plywood plus an uncoupling membrane.

This is my floor - 2x10 joists, span of 12 feet, unknown wood type, probably spruce, old but in excellent condition - probably closer to a full 2". Spacing is 16oc. Above that is 5/8 plywood. I have screwed the plywood down every 4-6 inches. Tile will be Porcelain, 18X18. ONE HITCH: half the floor has very old tile linoleum, glued with mastic. It is EXTREMELY difficult to remove. Like nearly impossible. It is very thin - 1/8th maybe.

First question: There is ONE inexplicable soft spot. That one I screwed down every 2 inches. It still moves. The joists are strong - they do not move. I've stood under while large men walk above. Just the plywood flexes in one area - about 3x3. Assuming I do not put another layer of plywood down before tiling - how do I strengthen this spot?? - it is accessible from below.

Second question: I am a very experienced DIY. However a proper mud underlayment system seems more than I want to tackle, and a larger floor height than I want. I have exactly 1 " to spare. Is there any reason why, given my facts, I can't just put Ditra down and tile it?

Third question: Can I leave the old linoleum? ( I know you guys normally recommend removal).

Fourth & Fifth questions: If I put another layer of plywood (exterior grade), am I right that I do NOT screw to the joists? If I put this second layer of plywood (5/8ths), do you still recommend Ditra, or can I go without? Cost is a factor.

Phew! Oh! I forgot - can I get Ditra in Canada?

Thank you for any advice you can give me.
jtz


Last edited by JulieZ; 06-07-2009 at 10:03 PM.
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Old 06-08-2009, 11:50 AM   #2
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Subfloor and Underlayment Questions


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Originally Posted by JulieZ View Post
Thank you for any advice you can give me.
jtz
How large is the area?

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Old 06-08-2009, 04:32 PM   #3
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Subfloor and Underlayment Questions


Quote:
Assuming I do not put another layer of plywood down before tiling - how do I strengthen this spot?? - it is accessible from below.
Reinforce it by installing another structural wood member at the center of the offending location. If there is one soft area there is likely more.



Quote:
Is there any reason why, given my facts, I can't just put Ditra down and tile it?

Quote:
Can I leave the old linoleum? ( I know you guys normally recommend removal).
Ask Schluter Systems.


Quote:
If I put another layer of plywood (exterior grade), am I right that I do NOT screw to the joists?
Correct!

Quote:
If I put this second layer of plywood (5/8ths), do you still recommend Ditra
Absolutely.

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Phew! Oh! I forgot - can I get Ditra in Canada?
That's six! YES, all Home Depots have access to DITRA.
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Old 06-08-2009, 08:30 PM   #4
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Subfloor and Underlayment Questions


Much appreciated. I'll contact Ditra.
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Old 06-09-2009, 03:59 AM   #5
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Subfloor and Underlayment Questions


run a support beam perpendicular to the floor joists from below and that will give you added strength.
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Old 06-09-2009, 08:22 AM   #6
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Subfloor and Underlayment Questions


Julie:

Those four thinset methods I outlined in a previous post were for the installation of tile on panels over a wood joist floor system, where the joists are no more than 16" apart o.c. and which conforms to the Building Codes...Now supposing the flooring is to code, and that the specifier of tiles has done the preparation groundwork of deflection issues etc, then those are the only construction details of that system - it doesn't gives details on whether there's linoleum, other tile, cork, rubber, bamboo - or any other element you may think of - because those elements aren't suitable.

And "suitable" means 'problem-free' too. I mean there's no law that says you have to follow those guidelines it's just that there have been many problems reported if you don't...and what are problems? well, shifting, delamination, cracks on the tile and on the grout are just some...

Some people can live with those problems but in general tile pros can't...and get paid for it.
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Old 06-17-2009, 09:56 PM   #7
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Thanks everyone for your replies. Just to be certain - because this linoleum is really difficult to remove - can I not put the second layer of plywood over the linoleum? (it's the hard stuff).

Also, do you know of less expensive membranes than Ditra?

I should be able to prepare the subfloor soon!
jtz
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Old 06-17-2009, 10:13 PM   #8
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You must have the world's toughest linoleum. I removed two layers of vinyl tile over linoleum over plywood before I installed my stone tiles. The vinyl came up using a flat pry bar, which also got most of the linoleum up, the rest of the linoleum came up easily with a very sharp, wide chisel. There was a relatively small amount of mastic stuck to the plywood, which I left in place. I installed 1/8 inch electric floor warmer, modified thinset, ditra, and unmodified thinset over ditra for the tile. All the instructions are on the Ditra website, very good, very thorough. At $1.20 a square foot for the ditra, it represented about 5 percent of the total per square foot cost, I thought a relatively modest investment for peace of mind.
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Old 06-19-2009, 06:47 AM   #9
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Can anyone quickly tell me whether T&G plywood should be used instaed of butt edges? I had the wrong plywood delivered (Not T&G) and am going to call this morning to have it changed. Thanks
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Old 06-19-2009, 08:20 AM   #10
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Quick answer: Typically t&g is used over the floor joists and is the subfloor. Square-edged ply is used for underlayment (if it is the right material).
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Old 06-19-2009, 10:13 AM   #11
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Subfloor and Underlayment Questions


Quote:
If I put another layer of plywood (exterior grade), am I right that I do NOT screw to the joists?
Correct!


Hey guys, noob question. Why wouldn't you want to screw this new layer of plywood to the joists? I would think a screwed floor would provide a more sound foundation for the tiles.

Hey Julie, Be sure to post up some photos.
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Old 06-19-2009, 03:16 PM   #12
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Hey guys, noob question. Why wouldn't you want to screw this new layer of plywood to the joists? I would think a screwed floor would provide a more sound foundation for the tiles.
....and you would be incorrectomundo!!!

Initially a subfloor is fastened to the structural floor joists using adhesive AND fasteners of some description be it nails, screws, staples, etc.

Along comes a tile installation requiring the substrate to be more substantial than what is offered by just the subfloor. So...another layer of plywood is added. This layer must be 3/8" or greater. This layer is NOT GLUED and is ATTACHED ONLY TO THE PREVIOUS LAYER. It is not to be attached to the floor joists. This is to aide in creating some isolation qualities between the structure and the tile. Tile is rigid and unforgiving. A wood structure is not so rigid and very forgiving.

Besides...What's a dirt biker know about anything?
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Old 06-19-2009, 06:05 PM   #13
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Subfloor and Underlayment Questions


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Originally Posted by Bud Cline View Post
... This layer is NOT GLUED and is ATTACHED ONLY TO THE PREVIOUS LAYER. It is not to be attached to the floor joists. This is to aide in creating some isolation qualities between the structure and the tile. Tile is rigid and unforgiving. A wood structure is not so rigid and very forgiving.
OK, learned something new today. What is the best way to attach it to the previous layer?


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I know dirt!
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Old 06-20-2009, 04:26 PM   #14
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OK, learned something new today. What is the best way to attach it to the previous layer?
Screw it!

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I know dirt!
...and abrasions I'll bet.
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Old 06-23-2009, 08:04 AM   #15
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Ok Gentlemen, I've installed another layer of plywood to the original, (screwed every 4 inches perimiter and 6 in the field, avoiding joists) and repaired the soft spot. I now have over l/720. I would have sistered the joists also, but thought that was overkill.

Deep Breath - I want to proceed with method (a) and install the tile to the plywood. We do not have incredible humidity here. I cannot afford Ditra, Protega or Nobleseal CIS. I have Mapei's premium ultra contact modified mortar, which is spec'd to install over plywood. BUT to be safe - will a liquid membrane like Redgard or Blueseal add any measure of lateral protection? This is my main question.

Thanks Guys.

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