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Old 12-26-2002, 03:21 PM   #1
Jon
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new construction tile

Hello everyone,

My question concerns cement board. My subfloor system consists of 3/4" T&G plywood nailed and glued to an I-Joist system on 16" centers. Largest room to get tile is 22'x12' (joist's span the 12' direction). I was going to use 1/4" Durock under the tile. Is this thick enough? Any other options beside cement board? The areas to be tiled are Bathrooms, Kitchen, foyer, laundry, etc.

I guess the main question is if there is a way to get around the cost of the cement board, without sacrificing stability?

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Jon
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Old 12-26-2002, 04:28 PM   #2
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Hi Jon

I'll wait for one of the engineers to comment on the span for the I-joists. I don't see many I-joist floors around here, and don't have a feel for how sturdy they are compared to conventional framing lumber.

How tall are the I-joists, BTW?

Assuming the span of the I-joists is okay, you should be able to use 1/4" cbu. Another option that is slightly more $$$ for the material, but much faster labor-wise, is Schluter Ditra. Check it out at www.schluter.com.
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Old 12-26-2002, 07:55 PM   #3
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Here's my 2 cents...

Your floor may be sturdy "as is" but BBcamp can confirm (or deny) if he finds this thread.

I have done new construction homes that have had sufficient floor support along with 1 1/8" Sturdifloor. This floor would stand up to anything you could throw at it . Nonetheless, I installed 1/2" CBU. Why ???

#1. There are thinsets rated for plywood but I just don't like plywood as a substrate. I have had tile pop loose from plywood even with good thinset ( for no apparent reason)


#2. 1/4" and 1/2" CBU are the same price. I pay $8.95 for both at the local HD.

#3. General specs mention a minimum floor thickness of 1 1/8" . Your 3/4 T&G plywood plus the 1/2" CBU gets ya' there ( and then some)

#4. With the exception of the weight difference, the same labor goes into installing 1/4" as 1/2"

If you floor height is an issue, you may be able to squeak by with the 1/4". If space allows, use the 1/2" as an insurance policy


Jason
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Old 12-26-2002, 08:11 PM   #4
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Jon:

If you can see those I joists, easiest thing to do is look'em over until you find one with the manufacturer's name on it. It should also have a part number or size designation and frequently there will be a toll-free phone number on it too. Call'em and axe'em for the rating of that particular joist. They are usually quite helpful and will give you whatever information you need.

Otherwise, measure as many dimensions as you can (the depth being most important as RobZ suggests) and our famous "bbcamp computer" will guess for you.
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Old 12-27-2002, 06:46 AM   #5
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Old 12-27-2002, 07:18 AM   #6
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The lightest I-joist I could find had a L/360 span rating of almost 17 feet, L/480 rating 15 feet (this is the rating recommended for I-joists under tile) so the 12 foot span is OK for tile.

None if the backerboard manufacturers I looked at cared if the joists were regular dimension lumber or trusses or I-joist, as long as they were 16" or less apart. Yours are OK.

Also, the backerboard folks specify 1/4" or thicker CBU over 3/4" plywood, 1/2" CBU over 5/8". So 1/4" is OK, more is better.

There are some manufacturers what make a epoxy-based adhesive and grout system that allows you to delete the backerboard. I have no idea of the costs involved, but unless you have a really good reason for using it, I'd stick with the backerboard. I believe the extra mass will be appreciated in in your large room (floor vibrations, noise transmition, etc.)
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Old 12-27-2002, 07:45 AM   #7
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Stay Away from the epoxy based system as a DIY er.Even Pro's dont like to use epoxy grouts.

You may consider looking into an SLC(self leveling cement) although it will cost more than CBU it is less time consuming and you can pour it around 3/8" nice flat level floor. Look in the library here under SLC .there is a good rundown on it by Flatfloor.
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Old 12-27-2002, 10:44 AM   #8
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Thanks for the replies guys!

These I-Joists are 9 1/2" tall by 2 1/2" wide. They are actually pretty long, but are supported every 12' , on pillars with laminated beams(this is the ground floor, over a crawl space).

I will go with the 1/2" CBU. I want to do it right, but just wanted to see if there was something out there that I didn't know about.

Here is another question...
When installing the CBU, should I apply a layer of thinset under the CBU, or just screw it down? or, use liquid nails?
Also, is there any benefit to using mesh tape and thinset over all joints in the CBU before installing the tile?

This looks like a very nice forum, with lots of fantastic information. I will also be installing a custom shower, and have found lots of good information in the archives. Thanks for the help!

Jon
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Old 12-27-2002, 01:07 PM   #9
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You should use a regular thinset under the CBU before screwing it down; it's just to fill the voids; no need to use a latex modified thinset (and it seems the recommendation is becoming DO NOT use a latex modified thinset).

DO NOT USE LIQUID NAILS UNDER CBU!

You're supposed to use fiberglass mesh tape on the seams with the thinset you use for the tile, and you can do this at the same time you lay the tile.
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Old 12-27-2002, 01:37 PM   #10
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Jon, this is what I would recommend.

Use 1/4" hardibacker board instead of 1/2" because 1/4" is easier to handle and cut. Once it is in place nobody will know the difference. use 1/2' if matching floor levels is an issue.

Use 1.5" ring-shanked double hot-dipped roofing nails with a few CBU screws on the high stress areas like washing machine. Screwing the whole floor is a waste of time and effort.

You HAVE to use thinset (at least 3/32") under the hardibacker board. Use either regular or modified. I used modified.

Use "USG Tile Backer Tape" to cover the seams of the CBU. This is another must-do item. Do NOT use mesh sheet-rock tape.

JMHO.
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Old 12-27-2002, 01:50 PM   #11
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1 1/2" nails seems like alot of nail for 1/4"+3/4" Plywood Garrett.Leaves a 1/2" of nail sticking out of the plywood.

Now,if you used utilicrete 1/4" backerboard and you went to thier website,you would see that they want a nonmodified thinset under utilicrete.Durock is still using a modified under thiers but they will come around eventually.Why would you want to thinset and bond your CBU to something that moves as much as a plywood subfloor.

Jon, use a nonmodified thinset under your CBU. nail every 3-4" on your edges and every 6-8" through the field of each sheet.
Personally I think nailing air isnt going to help much but seeing as you are going to use 1/2" cbu,1 1/2" will be fine .You will probobly be using a 1/4"x1/4" sq notch trowel for your tile installation.No need to go buy another trowel.use this notch to set your cbu just hold it at a lower angle.And do tape your seams as Garret pointed out.
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Old 12-27-2002, 03:53 PM   #12
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Todd, I like the idea of the nail coming out the other side, thinking that it holds better. 1.5" nail is too long in this case but finding a ring-shank double hot dipped roofing nail is not easy these days and I want to be able to use the nail for other purposes like nailing cdx to studs. Besides, one or two of those nails might just find a joist or block. The important thing is NOT to use those electro-plated roofing nails whose shanks are smoother than a baby's butt.

Liked your recommendation for nailing pattern and wished that I had followed it. Hardibacker recommended every 8" which I think is too far. I probably nailed every 5".


Concerning what kind of thin-set to use between the subfloor and cbu, I used the modified because the manufacturer said it was ok and because I though it would have more "flex" than the regular thinset. What is this "movement" in the subfloor that you want the cbu/tile to be separated from, assuming the movement is not deflection greater than L/360?
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Old 12-27-2002, 06:15 PM   #13
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I haven't argued with Todd since before Christmas, so I guess I can get away with it now.

Nails should go clear through and beyond the plywood. It's the same principle as when nailing roof shingles. The splinters from the final face ply of the panel pushing against the shank of the nail actually help hold it in.

And I'm no backer board pro, as everyone knows, but my buddy Dave Gobis always says that the manufacturer's specs should be followed over and above anything the TCA puts out.

And that is precisely why I don't go along with everything the TCA puts out. It's too arbitrary.

Jon,

Rob Z. did mention Schluter Ditra. It's fine over 3/4 in. ply and consumes only an eigth of an inch. You don't need the 1/2 inch if you don't want it. It's no stronger than the quarter-inch stuff.

http://www.schluter.com
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Old 12-27-2002, 10:03 PM   #14
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I ordered the 1/2" Durock today, is supposed to be delivered Monday.

I will be installing 900 sqft of tile. The rest of the house will be 3/4" prefinished hardwood, with a 15 pound roofing felt underlayment. The hardwood and tile will meet in several different areas. I want the flooring levels to end up as close to the same level as possible. So, 1/4" durock or 1/2"? I think I can catch the supplier Monday morning before they load the delivery truck if needed.

I was planning on screwing the entire Durock installation with left over sheetrock screws, I have about half of a 30 pound box left. Is this a no-no? I do have some 2" roofing nails left also, but they are the "smooth as a baby's but" kind...lol.
Or, should i just go and buy the CBU screws. The extra labor is not a problem with me.

Will do on the thinset under the Durock, and taping of the joints.

Will be spraying a primer/surfacer on all walls and ceilings this weekend, and starting the Durock installation Monday.

Thanks Guys!
Jon
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Old 12-28-2002, 01:41 AM   #15
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I don't think you want to be using drywall screws in CBU; they can corrode, plus they don't have the little extra ridges that help the screw cut into the CBU like the backerboard screws, so you'd probably never get them flush. Though it really seems people around here prefer the hot-dipped roofing nails over any type of screws for CBU installation. You want something coated so the screws don't easily pop up on their own over time.
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