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Unread 04-13-2002, 10:27 PM   #16
Harry
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I wanted to shut off the computer and go to bed, but this damn "flexural strength" terminology thing has been bugging me.

My understanding is that it is the stress measured in pounds "per square inch" required to create failure in the cbu. If 2100 psi is measured in one product whereas the other is measured at 900 psi I would conclude that the board measuring 2100 psi is the stronger of the 2. Can we follow this up? Until now I've been relying on one source and if I'm wrong I have a lot of editing to do in my forum.

JC, HardiBacker advertises their 1/4" primarily for floors and their 1/2" for walls (but either or is fine for the floor). It is the same with WonderBoard and Durock too. The others which I haven't used also use the 1/4" for floors too ... or so it shows in the specs I have. But of course the 1/2" is also used.

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Unread 04-14-2002, 07:47 AM   #17
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Time for Sunday morning coffee. I am going to let Chip explain the attributes of flexural and compressive strength on backerboards. An aquaintance of his is Chairman of that committee. Tests were run again with-in the past year.

On what constitutes proper thickness, the deflection critera of the substrate under all floors for backer board is the same, L/360. Panel size has some relevance but is not the sole consideration. 5/8 CDX will not make the critera. 5/8 CC will. 5/8 glued panels (to the floor joist)will make it, but 5/8 unglued won't, 3/4 PS1 will. Exterior glue panels are not as strong as exterior grade panels. It was the intention of the committee to have the panel recommendation be Exterior Grade. Determining what works and what doesn't is the function of a test known as ASTM C-627 or the Robinson Floor Tester. Manufacturers use this test on their panels for performance on a given substrate and centers. Here is a word of caution. If the plywood sheets are not properly adhered, fastened or gapped or if a moisture sensetive product, such as OSB swells, that is not a performance issue with the panel but with the substrate. Give that one some thought.
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Unread 04-14-2002, 08:10 AM   #18
JC
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Wow that is a scary thought too.

Dave is there any documentation on those plywood specs avaliable to me. Something I can give my builders.

That would also be a nice addition to the referance library.


The flexural part bugs me also. I can understand it bieng more flexable but how does PSI enter that equation? Seems to be a confusing way of measuring flexability if you ask me. If it measured resistance to flex it would make sense though.
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Unread 04-14-2002, 08:20 AM   #19
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Now Harry, you know I can't even say my name in five words or less.

Hardibacker probably adds something, but the others are so willowy they couldn't possible do anything except add height and continuity. You can almost punch holes with your fist through Durock and Wonderboard.

The other reason is because back when I was arguing that they added to the structure I was advocating bonding the product to the subfloor, and I now understand why this should not be done.

If you don't bond (laminate) something, it can't do much in the way of structure.

Did I stay under the 50 to 500 words?

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Unread 04-14-2002, 10:27 AM   #20
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Hardibacker advocates using, modified(latex or acrylic) dry-set mortar under their boards, except with natural stone, where modified thinset must be used. The 1/4" board has greater compressive and flexural strengths than the 1/2" board, whatever that means. The 1/4 " board put down with modified thinset absolutely stiffens the floor, and put against wall studs feels as strong or stronger than the 1/2" board. Neither does much for noise between walls.
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Unread 04-14-2002, 10:29 AM   #21
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The previous should read, Hardibacker advocates using modified(latex or acrylic)OR dry-set mortar under their boards, except with natural stone, where modified thinset must be used.
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Unread 04-14-2002, 02:01 PM   #22
Harry
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Agreed John ... I think we all came to that understanding awhile back here on your forum and when you give it thought, it makes practical sense. When I spent a day with a cbu rep I thought I was learning something but based on chip's description I may have in fact been lead into the wrong direction with the concept of "flexural strength"

I'd never advocate any cbu as a solution for a weak floor but knowing that a board will contribute in some added way to the structural strength seemed like a bonus and that is what I believed was being achieved with a higher flexural strength. Now if this isn't the case I'm gonna be hotter than hell at a certain little rep up here in my neck of the woods.

Chip ... I look forward to any info you have on this topic.
I'll owe you a cold beer ... Molson of course
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Unread 04-14-2002, 05:10 PM   #23
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I am not advocating anything contrary to manufacturers instructions but this I do know. When you bond a panel to the floor and make it part of the floor system it is more of the floor system than tile system. When the panel is put in compressive and tensile stress, having been bonded to the floor, that stress is transfered to the tile. When tile is subjected to enough stress it cracks at the weakest point over a joint or due to lack of thinset. Until they make tile with a flexural value you can have as much of or as little as you want. The strength must be in the floor system, not the tile system. A rigid panel in a bouncy house is a contradiction in terms. Over and over again I have been involved in projects where the panel was bonded or for extra measure, used 4'panels and spiked to the joist. The intent was a bulletproof floor. The result was cracked tile. Once dryset was used and the joist was avoided the problems disappeared. These are big projects for national builders. I can only ass-u-me that Hardie wants the panel bonded in the case of stone due to the L/720 requirement of stone products. As homes are built to L/360 and L/480 by code you won't ever find one stiff enough unless built for that purpose. Marble also has a maximum deflection allowance of either 7 or 9/32 ( hey, I do tile, can't remember everything ). In tract where they have a 40/10 rating it is even worse. For more info on wood floor systems go to http://www.apawood.com. If you want a good explaination of floor trusses, go to
http://www.trusjoist.com . Now it is all up to Chip to explain the rest, he sells the stuff. That takes care of my typing for a few days. Let me have boys, I'm ready!
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Unread 04-14-2002, 05:13 PM   #24
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Reps seem to lie to me all the time these days...they mean well but they get so filled up with sales brochure gimmicks that it somehow clouds there thinking.


So Hardi (not that I would ever use it) says to use any kind of thinset al all under the board but with natural stones to use modified????

Is'nt this a DIRECT contadiction to the theory of allowing independant movement under the board?

Seems to me that ESPECIALLY with nateral stones you need to be able to absorb slight movement rather then directly bond it...perhaps that is why Hardi has so many natural stone failures where the tile cracks on the seems.

I just don't believe Hardi has a clue as to what they are selling (aka pawning off on us)..Just don't like the stuff..it looks at me the wrong way.

Are not there more claims against Hardi then any other CBU?
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Unread 04-14-2002, 06:31 PM   #25
chip
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I'll be back....

With you tomorrow.

Harry, could you make it a Labatt Blue?

Sleep tight.

Chip
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Unread 04-14-2002, 06:40 PM   #26
Bri
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Talking

Geez...how 'bout picking a good beer!
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Unread 04-14-2002, 06:45 PM   #27
chip
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Bri,

It seems as the last time I brought that beer up, you said you wouldn't even clean you sponges with it.

As I sit here and type, drinking Old Milwaukee's Best Light, it sounds damn good to me.

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Unread 04-14-2002, 09:43 PM   #28
Harry
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Yeah ok Chip but I'll have to send someone else in to buy it .... I have an image to uphold you know.
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Unread 04-15-2002, 04:44 PM   #29
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Chip,

You're sick, man. Talking about a beer that none of the Canadians will even touch.

And then you sitting there with a Milwaukee's Best. I mean why don't you get a life man?

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Unread 04-15-2002, 06:06 PM   #30
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Hey I see you got your signature working John.
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