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Unread 04-12-2002, 05:42 PM   #1
JC
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When and what considerations are needed to decide which one to use for a floor?

I have always used 1/2 but really don't know why I do this.

If a CBU is not considered a structural member then what differance does it make? Having a lower profile is a big plus.

So what does everyone here use,reasons,specs if any.

Is using 1/4 0r 5/16 completely acceptable for floors and is it just as good as 1/2?

is the general rule of thumb 1/4 floors 1/2 walls?

Someone school me on this please..thanks
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Unread 04-12-2002, 06:18 PM   #2
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I don't know all brands, but I've used quite a few and in every case the 1/4" or 5/16" was stronger, both in compressive and flexural strength, and stiffer than the same brand of 1/2" or nominal 1/2" board. 1/2" board is used where the extra thickness is desirable, e.g. where cement board abuts drywall or to meet an existing floor.
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Unread 04-12-2002, 07:16 PM   #3
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JC

Harry Dunbar should show up soon. I think he has made a study of the various backer boards, and may have some more info for you.
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Unread 04-13-2002, 07:05 AM   #4
chip
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JC,

You must have a minimum 1 1/8" substrate for the installation of tile.

So If you have 5/8" subfloor, 1/2" backerboard will give you that 1 1/8".

If in remodeling, the additional hight of 1/2" is prohibitive, go with the 1/4".

1/2" backerboard is going to give you greater compressive strength than 1/4", just by sheer mass.

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Unread 04-13-2002, 04:09 PM   #5
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Well if.. I need to keep the hieght low and use 1/4 even though the sub-floor is 3/4 I am still under that 1 1/8 spec correct?

Now this is just not in remoldle work this happens amost every day in new homes..the half inch is to big and the 1/4 is under specs.

What ever happened to the train of thought that CBU are not to be used as structural membranes..just toss that concept or what?
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Unread 04-13-2002, 05:53 PM   #6
Harry
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When I started using HardiBacker I initially ordered 1/2" because it just made sense to go with what I thought was the stronger of the 2 sizes. I was told by the rep that I should go with 1/4" because it has almost twice as much "flexural strength".
For instance WonderBoard's flexural strength of the 1/2" board is 900 psi whereas the 1/4" board is 1500 psi.
All the various manufacturers have higher flexural strength in the thinner boards .... even DensShield.

I don't fully understand this and I hope someone can add to it because JC made a good point when he asked; "If a CBU is not considered a structural member then what differance does it make?" Well I think it does make a difference because there is always a noticable stable feeling after cbu is installed and if the board's specs say that there is a "measured" flexural strength then wouldn't that also apply to the substrate after it is installed?

Chip, unless I'm reading the specs wrong it shows that the "compression @ indentation (psi)" is the same for both 1/4" and the 1/2" boards. By the way ....HardiBacker has much higher compression resistance than all the other boards for instance both the 1/4" and 1/2" HardiBacker is 7000 psi while the Durock 1/4" and 1/2" is 2300 psi.

I hope someone with a technical insight can explain why flexural strength of cbu doesn't seem to be recognized as an attibute to the flexural strength of the combined substrate because my personal belief is that it does.

Harry
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Unread 04-13-2002, 06:09 PM   #7
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I have an answer for you all....leave the CBU's to the people who don't know any better. Can you say Ditra?
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Unread 04-13-2002, 06:12 PM   #8
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Let me try ..... Depitra???
Damn!
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Unread 04-13-2002, 06:16 PM   #9
chip
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JC,

I don't know of any one who is suggesting that the 1 1/8" rule be ignored.

Harry,

I "guess", and that is all it is, that a piece of tissue paper has greater flexural strength than a piece of news paper, due to the fact that is more flexible, and won't break at the same point as the news paper.

So 1/4" is more flexible than 1/2".

As far as flexural strength, I believe that it does add to a substrates flexural strength.

I checked ours and I have to clarify on Monday.

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Unread 04-13-2002, 06:37 PM   #10
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>>>1/2" backerboard is going to give you greater compressive strength than 1/4", just by sheer mass.<<<

Mass does not equate to compression strenth..density might but not mass..IMO.


>>>WonderBoard's flexural strength of the 1/2" board is 900 psi whereas the 1/4" board is 1500 psi. <<
To me this is saying that it would take more pressure to bend the thinner stuff...thus would this mean the thinner stuff is denser and stronger?

>>>I "guess", and that is all it is, that a piece of tissue paper has greater flexural strength than a piece of news paper, due to the fact that is more flexible, and won't break at the same point as the news paper.

So 1/4" is more flexible than 1/2".<<<

This is kinda contradictory to what Harry said above due to flexural strenth..stronger fexural psi would mean less flex..correct ..or am I misunderstanding the meaning of flexural strenth?

As far as CBU adding structural strenth I agree it does stiffen the floor..but I beleive it was Dave G. stating that it is not to be considered a structural member.
Now is this becuase the CBU companies are "playing it safe" or is there somekind of varience to the 1 1/8 thickness rule...ie. Does CBU count to the total thickness? Or do we have to have that thickness first to use CBU?

Just want to get my politically correct specifications understood.


Am I allowed to use 1/4 cement board over 3/4 plywood on 16 o.c. with a span that meets L/360...yes or no?






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Unread 04-13-2002, 06:52 PM   #11
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No!!!!

Yes, the 1/2 or 1/4" boards do count toward your 1 1/8".

No, they are not structural parts of the building, just the tile/stone installation.

Interesting info Harry gave on pull through strength of one of the boards out there.

Along the same train of thought, do think they put that vapor barrier on the face of their board, because they were so concerned about moisture vapor getting into the wall cavity? Or could they have been just a little concerned about it getting to the gypsum?

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Unread 04-13-2002, 06:53 PM   #12
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Chip, I'd like to find out more about this "flexural strength" thing and how it really applies to the substrate one way or another.

JC ...HardiBacker, Durock, WonderBoard, PermBase and Util-A-Crete all say they except 3/4" ext.plywood over 16" oc 2" x 10" joists for 1/4" and 1/2" boards, but like Chip has indicated.... it breaks the rules, so ..... ???
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Unread 04-13-2002, 07:23 PM   #13
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Well, I had a little email exchange with Dave G. this morning, and he's so packed in that he hardly has time to take a shower. He's doing a number of things at the same time and will find it difficult to stay up with the boards.

And this is great! It gives me a chance to act up without getting caught!

Art had the right idea on flexural strength. It simply means that thinner products will bend farther without breaking than thicker ones. And this is insignificant when it comes to a tile installation. We know that although somewhat elastic, tile and grout installations must remain rigid, or there'll be problems.

The 1-1/8 in. thing is a guide. Da Man has said on several occassions that the thing to do is go with the manufacturer's specs. Schluter says that Ditra, for example, can go down over one layer of plywood subflooring. (I'll bet Ditra has the highest flexural strenth of any of the products that have been mentioned.

I went through the structural argument long ago and lost when it comes to cbu. I now believe that with any of these products, including Ditra, that the subfloor must support the installation. If it won't, beef up the sub.

JC, One small correction. Mass does enhance compressive strength when you consider that it's only one-half of the issue. There is also tensile strength. Compression acts on the top of the product and tensile on the bottom. The combination of the two limits "flexural strength."

All of this simply says that thicker objects of any sort are harder to bend.

P.S. Yesterday I installed my first batch of Ditra. I used a 1/4 by 5/16 notch (recommended), and had no problem at all bonding the product. The key is to keep the thin set (I used Versa Bond) loose. Schluter says to keep it "fluid."
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Unread 04-13-2002, 09:14 PM   #14
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Yea I did that yesterday I used the 3/16 by 1/4 inch(reccomened of the literature) and used a better thinset Mapie ultra flex 2 and mixed it loose..no problems at all.

I think the reason I had problem in the first place was becuase I was using the old ditra and the roll was sitting in my gararge for over a year..and you know what happens to old plastic. The new ditra seems to be a "softer" plastic also..perhaps more plastisizers added to it IMHO...maybe..maybe not.


John you hit the nail on the head with this issue.

>>>HardiBacker, Durock, WonderBoard, PermBase and Util-A-Crete all say they except 3/4" ext.plywood over 16" oc 2" x 10" joists for 1/4" and 1/2" boards, but like Chip has indicated.... it breaks the rules, so ..... ???<<<

Are the rules and as John said manufacturer rules overide TCA guidelines...so 1/4 inch it is in the future for me!!!


BTW: my next question is...how many calls a year(claims) does the TCA get for 1/4 and for 1/2..that would be nice to know before I start smacking down 1/4 inch everywhere.
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Unread 04-13-2002, 09:45 PM   #15
Harry
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What made you change your mind about the cbu's ability to add to the structural strength of the substrate? .... 5 words or less.
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