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Unread 01-15-2006, 09:14 PM   #1
bwjohnson1927
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New member, First question

Hello to all. What a find this forum has been!
Problem set:
House in tropical climate. New construction. Pressure treated 2x12 joists (southern yellow pine?) on 24" centers. (drawings called for 12" centers but that's another story). 15' 6" span. About a 30' length, supported at 15' with a load bearing stud wall. 5/8" plywood deck, almost certainly just nailed, no glue. I might be able to access some joists to beef them up

Deflecto says no go for tile etc. Trouble is my wife has hand-made 400 11"x11" custom concrete tiles, aproximately 5/8 thick, which WILL be used as the floor!

I was hoping that by coming on top of the plywood with 1/2" durock, screwed and glued with a good construction adhesive, that I might acheive enough stiffness over the two 15'x15' sections. Might I get away with this? Would it help to orient the 5' length along the direction of the joists?

Any advice would be greatly appreciated!

Thanks in advance,
David
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Unread 01-15-2006, 10:07 PM   #2
Splinter
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Sorry David, unfortunately concrete underlayment doesnt add any structural strength to a floor. It's main function is to isolate the ceramic or stone tile from the seasonal movement of the plywood. Therefore it is especially bad to glue it down to the plywood. If you read through a few posts you'll find we almost always recommend an unmodified thinset (no latex additives) under the durock so there is no real bond to the plywood underneath. The recommended thinset under the durock is just meant to fill voids, and minimize deflection. Now the deflection on your floor system is going to be a big problem. Can you get underneath and add joists in each existing bay? 2x12's over 15 feet spaced 24 on center is really pushing the span limits to the edge.
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Unread 01-16-2006, 09:12 AM   #3
bwjohnson1927
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Thank you Alex. I was afraid that would be the answer. Thanks also for the explanation of the funtion of the Durock. I will take the time to look at threads which have already covered some of my questions.

I will add joists though the politics with the builder will be a little challenging. Would it be sufficient to add a 2x12 every other time? That is, ending up with a 24" then 2x 12", 24", 2x12" etc. spacing? Would the plywood/ durock resist flex across the 22.75" span?

Materials such as durock are quite expensive where this little project is taking place. What do you recomend as a the most cost effective substrate in this situation?
Thank you again,
David
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Unread 01-16-2006, 09:59 AM   #4
jgleason
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You would still end up with a 24" span in the bays where you didn't add a joist. Best bet would be to add the new joists in between each of the existing.

I would add another layer of 5/8" plywood followed by 1/4" Durock. The second layer of plywood would run perpendicular to the joists and the seams would need to be offset from the first layer of plywood. There are lots of posts about 2nd layer of plywood methods including great debates over whether or to glue or not. The pros can weigh in on the best method for your particular situation.

I also wonder if your deflection would need to meet the standard for stone rather than ceramic tile given the concrete tiles you plan on installing.

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Unread 01-16-2006, 10:16 AM   #5
bwjohnson1927
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Joe,
I have spent the last few hours reading past threads with disheartening results. I will for certain need 12" spacing and a second layer of 5/8 ply as you suggest. Cement tile apparently has ZERO flexural strength, so I'm very very wary. Man I wish I had stumbled here three months ago. Tough politics with all the effort gone into making, coloring, sealing the tile by my dear wife. We do have a ground floor that is to be tiled over a concrete slab, so perhaps they can be used there....
Anyone know of a source for cobalt blue ceramic floor tile 12x 12 or larger (not monochromatic)?
Thanks,
David
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Unread 01-16-2006, 02:47 PM   #6
bwjohnson1927
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Ok- After reading all day I realize the risks of using the concrete tiles here.
I will beef up the joists enough to allow use of ceramic tiles. This will still require the subfloor to span 23 1/4" in some locations. I am not certain at this time whether the subfloor is 5/8 or 3/4"! Regardless, what total thickness of plywood will be recommended for the spans between the 24" center areas? I plan on screwing the second layer of plywood to the subflooring not the joists as has been advised and then coming on top with 1/4" durok or equivalent. I should be able to install blocking. This forum is wonderful. Thank you John, I just ordered your book.
Thank You,
David
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Unread 01-16-2006, 03:22 PM   #7
jgleason
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I'm willing to bet that you've got at least 3/4" already. I can't see any contractor in their right mind going 5/8" with joists 24" apart.

I'd still add the second layer of ply.
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Unread 01-16-2006, 05:53 PM   #8
Theoderik
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David, might be cheaper and easier to just get a new wife. (joking! joking!)

sw (shawn)
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Unread 01-16-2006, 06:46 PM   #9
bwjohnson1927
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Shawn, you ever checked out what one of them new ones cost?

In my agitated state I misspoke (as former prez RR''s aids used to say) and recall my span is 14'-6" not 15'-6. At 14'-6" with 2x12's on 12" centers the deflecto indicates the substrate would be ok for stone. Anybody have an opinion on concrete tile? Found this by an estemed poster in the vaults which has me skeert, but with the politics here I've got to explore any option:
Tell you a story about cement tiles on a recent consultation of 4x8x1/2 brick look concrete pavers over a mortar bed of 2.5, 3/4" thick bed. Tiles were breaking all over after a few years. Took the tile back to the the lab and did a breaking strength test. Came back 375PSI. Standard tile is around 250PSI, porcelain is 375 to 400, on an average. So, did a modlus of rupture test. That tests the flexural properties of the tile. On a scale of 1 to 10 for the sake of arguement. Tile comes in around 4 or 5, porcelain tile 5 or 6 which means it has some flexural properties. The concrete tile came in at 1.5 which basically means it is absoultely rigid, with no flexural value. There are no standards for installing concrete tile over wood substrates. There is a concrete recommendation from the Concrete Tile Association based in CA and headed by Bud Newcomb.

I plead...Any more help out there???
Thanks,
David

Last edited by bwjohnson1927; 01-16-2006 at 07:36 PM.
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Unread 01-16-2006, 07:49 PM   #10
John Bridge
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Hi David,

The concrete tiles are going to be "soft" as compared to modern ceramic floor tiles, most of which are porcelain nowadays, which is a very strong ceramic composition. You're biggest problem is going to be subfloor, and doing as Joe suggests will go a long way. An inch and an eighth of plywood even over 24 in. centers becomes quite rigid. You won't get anybody around here to stand behind it, but it would probably work.

The problem is that it might not, and if it doesn't, there's really no cure short of complete tearout.
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