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Unread 07-28-2007, 12:06 AM   #1
mondo
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sub floor question

i am in san diego doing a job at my son's new old house. we tore out down to the main planking. 1X6 fir on a 45 degree angle nailed to 2X6 on 16 in centers with the span between supports every 7 ft. we need to keep it as thin as we can and i was hoping you guys would give me the ok for: 5/8 t&g plywood glued with panal adhesive and screwed to the planking. then 1/4 hardipanel put into a bed of modified thinset and nailed with roofing nails in the recommended nailing pattern and then a 18 in cermic tile in a modified thinset put down with 3/8 X 1/4 notched trowel and back buttering the tile also. thanks in advance. mondo
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Unread 07-28-2007, 03:35 AM   #2
Jeff_kitchen
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I'm not a pro, but regarding "keeping it as thin as possible", you might want to consider using Ditra underlayment vs cement board. I know that generally these folks don't recommend using 1/4" cement board for floors. Ditra is thin and will give you better support and decoupling than cement board (so they say). I use it for all my tile projects....great stuff. If you're not familiar w/ Ditra, look around this site and you'll see plenty of logos for "Schluter Systems"....just click on one of those. Plenty of articles about Ditra on this site too; just search for Ditra.
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Unread 07-28-2007, 03:22 PM   #3
jadnashua
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I'd go with 3/4" T&G, and a membrane. 5/8" is too close for comfort...a little more and the benefit of the superior decoupling of the membrane should give you a superior floor over the 5/8" and cbu. BTW, 1/4" cbu on a floor is fine, there's no good reason to use the thicker stuff unless you are trying to match heights.
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Unread 07-28-2007, 04:42 PM   #4
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I think maybe our friend Jim (jadnashua) misread this one, Mondo.

The 5/8ths ply over nominal 1" board subflooring is fine. The minimum requirement is actually half-inch ply, but more is better if you can afford the height. You can reduce the overall height and improve the uncoupling characteristics by using the Ditra, as has been suggested.

I recommend you do not try to glue the plywood to the board subflooring. And if I did, I'd sure recommend something better than the Panel Adhesive version from LN. I do, however, strongly prefer screws for the attachment of that second layer. After, of course, adding screws as necessary to the board flooring.

Use of the 1/4" CBU would be fine, too, as the guys have said.

My opinion; worth price charged.
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Unread 07-28-2007, 08:35 PM   #5
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Yup, thought you were tearing out the planks...still like the Ditra over cbu...much easier to carry around, don't like all the screws or nails required on cbu. Don't glue to the planks, but make sure they are well attached to the joists first, though.
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Unread 07-28-2007, 08:41 PM   #6
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no Screws?

Why not screw sheet material to the floor planks or joists? I am confused?
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Unread 07-28-2007, 08:49 PM   #7
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Welcome, ultradouglas. Please give us a first name to use.

Search around about subflooring questions on here and you'll get a better feel for the way subflooring should be done for tile installations.

We're not telling him not to fasten his plywood or CBU into the existing board subflooring, only not to fasten into the joists. Fastening the second layer of subflooring only into the first layer gives some small measure of isolation from the joist structure.

Here is a good article on some of the ramifications of subflooring installation to get you started.

The other tiling substrate being discussed here is called an un-coupling membrane. It's installed without mechanical fasteners at all. But the plywood under it still wants to be fastened only to the first subfloor layer.

My opinon; worth price charged.
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Unread 07-29-2007, 09:04 AM   #8
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good morning everyone. Thanks for the replies. Unfortunately when I checked yesterday before I got started there were no replies. We put the T&G plywood down to the 1X6 NOT T&G boards with panel adhesive and only nailed into the floor joists. We used #8 ring shaft nails. My thinking at the time was the 1X6 fir boards are 53 years old and being on a 45degree angle which makes them span more then 14.5 inches and having some flex in them was to make them more of a part of the plywood. We used 10 tubes for 6 sheets of plywood. The 1/4 inch hardibacker we are going to put down we are planning to use 1.5 in roofing nails which i feel will hold good and not hit the floor joists. by the way I would have taken the 1X6's out if they didn't go under 2 outside walls. cx what did you mean "Panel Adhesive version from LN"? Is there a big advantage to using Ditra over ¼ cbu in this application? mondo
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Unread 07-29-2007, 09:41 AM   #9
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P.S. The tile changed to 12X12 porcelain tile. mondo
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Unread 07-29-2007, 10:01 AM   #10
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Quote:
Originally Posted by Mondo
cx what did you mean "Panel Adhesive version from LN"?
Hell, I dunno, Mondo. You wrote this:
Quote:
Originally Posted by Mondo
5/8 t&g plywood glued with panal adhesive and screwed to the planking.
and I guess I interpreted it to be a Liquid Nails product because they have one called Panel Adhesive, which is a pretty low-end product as I see it. If that's not what you meant, then I guess I was just making stuff up again.

But gluing subflooring layers with anything out of a tube is not a good idea. Can leave voids under the top layer that defeat the whole concept of adding the second layer for stiffness. And I just don't think there's a good way to glue plywood to an old board subfloor at all and make it effective. But let's hope yours will work out OK, eh?
Quote:
Originally Posted by Mondo
Is there a big advantage to using Ditra over ¼ cbu in this application?
I think so. I just like the whole design concept of the Ditra. Hasta give a fella more isolation than a CBU from the lateral stresses imposed by a wood subfloor as I see it. Especially in a remodel situation where you're dealing with a good bit of unknown construction under there.

Not to mention being able to carry hundreds of square feet of it over one of my skinny sholders.

My opinion; worth price charged.
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Last edited by cx; 07-30-2007 at 01:17 PM. Reason: typo
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Unread 07-29-2007, 11:54 AM   #11
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thanks cx. at this point should i screw or nail my plywood in the field also or just leave it as is nailed to the joists only if i use ditra ? As i have never used this product before and have been to their site for info, do use like the 5/16X5/16 v-notch over the 1/4X5/16 notched trowel? they also talk about no thinset with additives in it for laying the tile, is there a problem sticking to porcelain tile with out the additive. by the way, even though it's a mute point now i used PL® Premium Polyurethane Construction Adhesive. mondo
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Unread 07-29-2007, 12:23 PM   #12
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I just used Ditra for the first time this week in my MBR bath. With a door separating the sink from toilet & shower, and a closet too, I really appreciated how much easier it was to install than the 3/4 ply I put in below it, and many times easier than it would have been to put in CBU. Cut around obstacles with standard scissors and utility knife, saves hours and sore muscles.

The rolls at HD have good details on the trowels needed, and recommended thinset stocked there. You use a medium v notch for the modified beneath the Ditra, and a 1/4 x1/4 x 3/8 above it. You use a good bit of the unmodified above the Ditra because of filling the "pillars", but at less than $5 per 50# sack, no complaints. The $1.40 or so per sq. ft. is worth it, without even considering the reported superior resistance to cracking.
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Unread 07-30-2007, 08:45 AM   #13
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thanks ted. any body want to comment on if i need to put nails or screws in the field at this point? mondo
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Unread 07-30-2007, 08:54 AM   #14
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PL Premium is very good pookey, Mondo. But if you didn't use mechanical fasteners to squeeze it together, no tellin' what you ended up with. I'd certainly wanna screw the plywood down to the board flooring, but at this stage I'm not really sure how much good that'll do.

My opinion; worth price charged.
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Unread 07-30-2007, 09:00 AM   #15
Brian in San Diego
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TedL said...
Quote:
You use a good bit of the unmodified above the Ditra because of filling the "pillars", but at less than $5 per 50# sack, no complaints. The $1.40 or so per sq. ft. is worth it, without even considering the reported superior resistance to cracking.
I would advise against using thinset that only costs $5 per 50 lb. sack. You want to use a premium unmodified thinset above the ditra. Something like Laticrete 317 Floor and Wall or Mapei Kerabond would be good choices. They are probably closer to $15 a bag. You can use them with porcelain. I set my porcelain over kerdi using Laticrete 317.

I am wondering about the nailing or screwing the T&G in the field. Since you have already nailed into the joists, you have "lost" the ability to use this layer of plywood as a decoupling layer. I think I would make sure you are securely fastened to the joists. Maybe drill some pilot holes to the depth of the T&G and other subfloor material and use 3" deck screws and fasten everything to the joists. I would definitely use Ditra to provide some added isolation from the subfloor.

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