# Laying cement board



## kawendtco (Oct 5, 2007)

we set our cement board in thinset to laminate the board to the subfloor. it makes the floor that much stiffer, which you are after. the stagger is to make the floor as solid as possible. no joints line up and every lap ties one side to the other. stagger not only the joints of cement board but stagger them from all of the joints in the subfloor as well.


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## Bud Cline (Mar 12, 2006)

ALL cement board manufacturers specify that their boards be installed in a bed of thinset - ALL OF THEM. This is to eliminate the voids that would otherwise be created when the boards are fastened to the substrate. The thinset is not there to attach the boards to the substrate.

Cement boards DO NOT stiffen a floor. Cement boards provide a suitable tooth for the tile adhesive.

Overlap the cement board joints from the subfloor joints about twenty-five percent.
Attach the cement boards to the subfloor sheets, not the joists.

You also MUST TAPE AND BED the joints using alkali resistant mesh tape.


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## kawendtco (Oct 5, 2007)

bud i beg to differ on the stiffening aspect. ANY time you laminate two materials together it becomes that much stiffer.i agree with the void filling, and the staggering you are talking about in your post, i disagree with the tape. isolation membrane on the floor if you are on concrete, to prevent telescoping concrete cracks, adding the tape and thinset to bed the tape creates a hump that for the average homeowner is going to cause them problems setting their tile flat. we have studied this in depth here in the k.c. metro area.


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## Daniel Holzman (Mar 10, 2009)

Kawendtco is correct in that laminating one material to another material creates a composite structure. The composite structure is stronger than either material alone in the technical sense that the composite structure can carry more load before failure than either individual element. The stiffness of the floor is also increased in the technical sense that the total deflection under load of the composite structure is less than the deflection of either element individually under the identical loading conditions.

Computing the actual increase in load capacity and stiffness is often very difficult for two materials with radically different properties such as cement backerboard and wood. The actual increase depends on the method used to fasten the two elements together, and will depend in a complex way on the tendency of the more brittle material (the cement board) to crack under flexure loading.

The most conservative assumption is to ignore the increase in stiffness and strength from the backer board, which is what is generally done. However, just because the increase is ignored DOES NOT mean there is no increase in strength and stiffness, simply means that the computation of the increase is too difficult to be worth it, and is therefore ignored.


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## Bud Cline (Mar 12, 2006)

ALL TRUE in a highly technical and scientific respect while at the same time anyone in the flooring industry will tell you there is no gain structurally from the addition of cement board to a substrate. Installing cement board is not enhancing the structural qualities of the substrate.

The use of the tape is a requirement of the industry. The tape can be applied as the tile is being installed thereby eliminating any rigid addition to the thickness at the seam. The compression of the fresh thinset under the application of the tile elevates the tile enough to make the thickness of the tape negligible.


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## oguevarra (Dec 13, 2009)

Thanks for all the great information. Have another question, when mixing the thinset mortar for the cement board can I just use water or do I need to use the special additive? When I apply the titles can I use water to mix the thinset or is it best to use that additive?


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## JazMan (Feb 17, 2007)

Well, you didn't say which thin set mortar you're using. Please reply cuz there are a few that are one small step about dirt. 

Thin set comes either regular "dry-set" which is unmodified and modified which already contains dry latex blended with the powder in the bag. 

With Hardie you can use either unmodofoed or modified to set the board. You must use modified to install the tiles however.

So, it's up to you. If it's a small job you may as well just buy one bag of modified for both. On large jobs you can use the cheaper unmodified for the board then switch to the modified for the tiles. 

Alternatively you could buy unmod & the latex additive and mix it with water for under and the "milk" for the tiles. 

Jaz


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## oguevarra (Dec 13, 2009)

JazMan said:


> Well, you didn't say which thin set mortar you're using. Please reply cuz there are a few that are one small step about dirt.
> 
> Thin set comes either regular "dry-set" which is unmodified and modified which already contains dry latex blended with the powder in the bag.
> 
> ...


Thanks for replying Jazman. I haven't purchased any thinset yet, I wanted to know the difference. For the hardiebacker they say use a modified thinset so I'm assuming that I can just use water to mix it. Since this is my first time doing this I'd say its a big job. I've read a 50LB bag of mortar should cover about 75 - 100 square feet so I'll grab 4 bags. I probably won't finish laying the cement board in one day since this is my first time will that be a problem?


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## oguevarra (Dec 13, 2009)

Also, after I've layed the cement board over the thinset how much higher will the floor be before I install the tiles? The hardieback I purchased is 1/4 inch.


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## JazMan (Feb 17, 2007)

If you want modified thin set, just buy modified thin set and mix with water only. Water in the pail first, then add powder.

I use a well worn 1/4x1/4" trowel and get about 100 ft. per bag for under the board. Make the mix a little thinner than when you set the tiles. You will probably need a different trowel for the tiles. And don't forget to tape the seams.

Jaz


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## Bud Cline (Mar 12, 2006)

*SIMULPOST*

Go to Home Depot and buy the fifty pound bags of *VersaBond*, then just add water. Don't screw around with adding additives or buying unmodified thinsets in this case.

How much your thinset application elevates your cement board depends on how thick you mix the thinset. Mix the thinset the consistency of creamy peanut butter. Spread the thinset so that you comb it all in one direction. Plop the board, position the board, walk it down, immediately nail it from the center outward to the edges. Scrape any purging thinset from along the edges before you spread and plop the next board. Typically the thinset will elevate the board about 1/16" or so, don't worry about that just go for it.


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## bob22 (May 28, 2008)

I don't think I'm hijacking this thread as I am doing the same job this weekend (if I am, my apologies in advance); just to clarify on spreading thinset below Hardibacker: use a 1/4" x 1/4" trowel for thinset on subfloor?

BudCline: Question re: "Attach the cement boards to the subfloor sheets, not the joists." Doesn't the Hardibacker have predetermined (stamped) spots indicating where to place the screws? If these fall over a floor joist what is one to do? I don't understand why you shouldn't screw into a floor joist as the subfloor is attached to the floor joists so there shouldn't be any difference in movement between screws placed in the subfloor fields between joists vs. on joists?

Also, is there any benefit to back-buttering 8" x 8" porcelain floor tiles or not needed?

Thanks, Bob


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## JazMan (Feb 17, 2007)

Hi Bob,

Yes use a 1/4x1/4" square notch trowel and make the thin set a little thinner than the normal consistency. This will add about 1/16" under the board.

You won't hit the joists if the screws aren't long enough to bite the joists. Generally we use 1 1/4" fasteners. Avoiding the joists helps to prevent full transfer of tresses the joists might produce under load and normal movement. 

Shouldn't be any need to back butter standard back tiles that are not warped, won't hurt though, just make it real thin. This of course is assuming you've got the correct trowel in your hands.

Jaz


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## oguevarra (Dec 13, 2009)

I probably won't finish laying the cement board in one day since this is my first time. If I do this over two days will that be a problem with the cement board being level?


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## JazMan (Feb 17, 2007)

You won't have any problems continuing the next day, just do everything the same way. How many sq. ft. is this anyway? 

Jaz


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## oguevarra (Dec 13, 2009)

JazMan said:


> You won't have any problems continuing the next day, just do everything the same way. How many sq. ft. is this anyway?
> 
> Jaz


Its about 220 sq. ft. I layed out the cement board to determine where I need to make cuts, so far its coming along.


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## Bud Cline (Mar 12, 2006)

I'll repeat: You should be sure to scrape any purging thinset from the edges of the boards. For sure don't leave it overnight then forget to do it the next day.

When thinset comes in contact with cement board the cement board rapidly wicks the moisture from the thinset and the thinset stiffens. If the purging thinset along the edges is allowed to remain, this is what causes lippage in adjacent cement boards. Keep up with it and it won't become an issue later. Even the short time it takes to go get the next board is sometimes long enough for the purge to stiffen and cause the edge of the next board to stand proud of the first.


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## bob22 (May 28, 2008)

Is hardibacker needed under where marble threshold will be in doorway?


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