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Old 12-03-2007, 06:41 AM   #1
JB_Selig
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Cement Board vs Plywood w/ Deitra

I just removed the existing 12x12 tile floor in a kitchen I am re-doing. The tiles had cracked in numerous areas. There was a 3/8 CDX underlayment and 3/4" T&G subfloor on 2x8 joists 16" OC. I removed the 3/8" underlayment which had been stapled down.

I now need advice on what to install for underlayment before re-tiling with 12"x12" porcelain floor tile. My options seem to be:

1.- 1/2" CDX plywood thinset and screwed to existing subfloor. Followed by Deitra membrane thinset to the 1/2" CDX

2.- 1/2" Wonderboard thinset and screwed to existing T&G subfloor. Thinset tile directly to wonderboard.

Which option will provide the best base so that I can avoid having the new porcelain tiles crack.

Also, is anybody familiar with "Reflex" as a thinset product specified for laying porcelain tile. The local tile store told me that I needed to use a thinset that is specific for porcelain.

Thanks,

JB
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Old 12-03-2007, 07:37 AM   #2
Dave Taylor
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Howdy JB........

Thank you for joining our forum.

First things first. Use the (ta da) whirl' famus'' "Deflecto" tool at the top of this page to determine if your floor substructure is suitable for a ceramic covering.... let us know what deflection reading the tool gives..... then we will be better able to advise you on underlayment and thinset requirements which (BTY) will never include a CDX grade sheathing.

To use the "Deflecto" input your joist width, joist height, joist distance on center, longest unsupported joist span and your take on joist wood type and condition.

We await your response :---)
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Old 12-03-2007, 09:05 AM   #3
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Deflecto rating info

According to the Deflecto calculator, my structure rates an L/302 versus the recommended maximum allowable L/360. So I got the thumbs down for tile.

Is this likely why I had cracks in the exsting tile floor? Will adding 1/2 plywood or cement board alleviate this problem or is the only remedy to mid-span beam or "sister" the existing joists. Neither wou8ld be easy to do since the foundatiion is a concrete block crawl space with little room for manuevering material from underneath.

Also please advise on what grade plywood to use for underlayment and regarding "Reflex" thinset being recommended for porcelain.

Thanks,

JB
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Old 12-03-2007, 09:25 AM   #4
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JB,

You might have had a couple of reasons for the previous tile cracking. One could have been the CDX sheathing. There isn't enough structural support in CDX to use it as a tile substrate. Also you didn't mention if the tile was applied directly to the plywood, if so that could have caused it. Third, your joist structure could be inadequate for a tile installation. If you were to take up the subfloor could you sister from up above? Are there many interferences like plumbing, wiring, ducting and such? The Deflecto tool is a bit conservative so it's ultimately going to be your call.

CBU (cement board) adds no structural strength to a flooring system. And adding more of the correct plywood only strengthens between the joists and the deflecto is telling you the deflection along the length of the joists. When you did your deflecto calculations did you actually measure the length of the joist inside to inside between where it was supported? Measuring wall to wall of the room isn't necessarily the unsupported span.

I don't know that name of thinset. More on that after we solve your joist problem. Typically porcelain should be set with a modified thinset and there are many good ones out there. If you install Ditra, however, you would use an unmodified thinset to install the tile.

Brian
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Old 12-03-2007, 09:49 AM   #5
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What Brian says, JB.... also....

good flooring grade plywood underlayment costs $$ and is rated at AC or BC or CC Sanded and Plugged, Exterior Glued, (type 1) straight edge or T&G.
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Last edited by Dave Taylor; 12-03-2007 at 10:03 AM.
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Old 12-03-2007, 07:16 PM   #6
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More Info

The previous tile probably cracked due to poor adhesion. I was I able to pop most of the tile with a pry bar leaving practically no thinset residue on the plywood. Probably not a modified-thinset.

Unfortunately it is not feasible to pull up the subfloor since there is radiant heat tyubing stabled to the underside. I'll remeasure the inside span of the joists to double check the deflecto measurement. If I get any closer to meeting the deflection standard then I'll go ahead with tiling over Deitra on new 1/2" flooring plywood.

Otherwise I'll be forced to change plans and go with a hardwood floor instead of tile.

Thanks for all the input!

JB
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Old 12-03-2007, 07:21 PM   #7
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JB,

Where abouts in the Commonwealth are you? I'm originally from Needham...born and raised there. Good luck with your project. Any additional questions ask them here.

Brian
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Old 12-05-2007, 12:47 PM   #8
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Going ahead with plywood underlayment & Deitra

As suggested I measured the inside span between the sills supporting the 2x8 floor joists and it is 11'2". So I am going to go ahead with tiling rather engineered hardwood.

I plan to screw the esisting t&g 3/4" ply into the joists (only ring nailed originally) and then will put a plywood underlayment on top. I would prefer to go with 3/8" underlayment....for transition purposes. Or do people recommend going to 1/2". How much finished thickness does the Deitra add on top of the plywood? I am planning to using Singleflex thinset to bond the Deitra to the plywood.

As previously mentioned, I am using a 12x12 porcelain tile which itself is fairly thick. Any recommendation on what thinset to use for setting porcelain tile on the Deitra?

Thanks,

JB


PS-Brian in San Diego....I grew up in Brookline. Actually played in a band with a few guys from Needham during the late 60's.
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Old 12-05-2007, 01:04 PM   #9
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JB,

I left Needham for the West Coast in '68. I might have known your band mates...remember any names? Oh, forget it...that was the 60's!

The additional plywood should be 3/8" minimum. Ditra adds approximately 3/16". I don't recognize the product you are referring to, but you want to imbed your Ditra to the plywood with a modified thinset. You want an unmodified thinset to set your tile over the Ditra. If you can find a tile place that carries Bostik products, I'd highly recommend the Ditra Set mortar. Formulated for use over Ditra.

Brian
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Old 12-05-2007, 02:13 PM   #10
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Make sure that while you area attaching the next layer of plywood and not screwing it to the joists, you don't hit the radiant tubing.
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Old 12-06-2007, 08:59 AM   #11
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Wink A few more question about underlayment plywood

1. From reading posting in the forum library, I understand that the plywood grain should be laid perpendicular to the joist. If the szubfloor is not perpendicular, does that need to be torn up and redone or can I just get it right with the unmderlayment
2. Should I use a subfloor adhesive like Liquid Nails or PL4000 Urethane
3. What is the correct screwing pattern for underlayment...every 4 or 6".
4. To avoid hitting the radiant tubing I'd need to use 1" or 1.25" screws with 3/8 or 1/2 plywood respectively. Are coated exterior screws required...or can I use drywall screws (coarse or fine thread??)
4. Why not screw into the joists.

Thanks for the patience with so many questions!

JB
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Old 12-06-2007, 09:14 AM   #12
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JB,

If you read the thread in the Liberry I'm thinking of then you saw the photo of the different grain orientation. If the grain of the subfloor is not perpendicular to the joists then I would definitely take it up and put down new. Is that going to open a can of worms with the radiant heat?

If you are putting down new subfloor then you want an adhesive like Liquid Nails applied on the the joists when the plywood is set and screwed to the joists. You do NOT want anything like those products applied bewteen layers of plywood as they can create voids.

The reason you want to screw to the subfloor only and not to the joists is to help provide "uncoupling" from the joist structure. You want to use screws that will only make it into the subfloor so 1.25" deck screws would work. That's so if you set a screw over a joist it won't grab into it. If I remember the screw schedule, I think it's 4" on the perimeter and 6" in the field. If you want to make it really right, the holes should be predrilled so that the screws won't "jack" the second layer up as it's being screwed down. NO drywall screws for two reasons...they aren't heavy duty enough and if they get wet they rust and break.

Brian
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Old 12-06-2007, 10:18 AM   #13
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Hi JB.....

Just being curious............

What kind of subfloor material (floor directly above and fastened to the joists) do you have now..... I am just not sure. Is it dimensioned lumber (planks usually 1" x 6" and often Tongue and Groove -T&G- ).... or is it plywood sheets 4' x 8' straight edge or T&G?

Reason I ask is many a subfloor is installed at a 45 degree angle to joists (specially plank flooring) which is just fine. Underlayment above this may then be placed perpendicular to the joists with no problem.

Just what material do you have for your subfloor and..... is it really set parallel to your joists. If so.... I just don't recall when I last saw this?
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