ditra under slate


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Old 04-22-08, 06:10 PM
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Smile ditra under slate

Hi, My question is I have new construction 15 x 50 deck 10' off ground it has 3/4 " plywood with a mapei 2 part waterproofer rolled on top. I worried over time about moisture getting to the wood and causing trouble.
the plan now is to put wet areas 1/2" hardee backer board over the waterproofed plywood then apply jamo water proofer over the hardee board then install slate tile over that.
Would this work?
Should i use Ditra over the waterproof plywood and skip the hardee board, To be protected better from moisture damage. Any suggestion appreciated...
 
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Old 04-22-08, 07:25 PM
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One thing for sure, you can not use Hardi backer outdoors, who recommended that?

If your framing passes the "test", you need to add a cement backer to the plywood, then install Ditra, followed by your tile.

Tell me about the framing. Type, size, spacing, species, grade and unsupported span. You also have to allow for wind and snow loads in addition to loads from furniture and people. Man you could have lots of people on a deck that size.

Jaz
 
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Old 04-22-08, 07:27 PM
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Slate is a natural stone and requires a very strong substrate of at least 1 1/4" thickness. Deflection is also a requirement.

What are your joists sizes, spacing, and unsupported span?

I would add at least another layer of 3/4" AC or BC rated ply, then thinset laydown your hardiebacker or Ditra. If you choose hardiebacker, you have to thinset it down to the substrate and screw it in place, then tile. No waterproofing on the CBU!

Jaz....you must be a fast typer....got me by 2 mins.
 
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Old 04-22-08, 08:11 PM
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Thanks for replies. It has 2x10 joists 16" apart . plenty of double 2x12 beams it seems very sturdy. The plwood is togue and groove. The building contractor brought the hardee board, but there is also the gray concrete backer board.
So i should use concrete backer board instead of hardee , even though it reads on the hardee board for wet areas.

Not a good idea to use the water proofing over the cbu?
I need the ditra over the cbu ? And if i use Ditra i use modified mortar under the ditra and regular thin set mortar on top to lay the slate with correct? This project is alot of money i want it to last a long time. Thanks again.
 
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Old 04-22-08, 08:14 PM
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Mark,

You're right about needing more plywood for slate. I just wanted to start with the basics.

Non,
AND......I forgot another thing, you need the deck to be slopped at least 1/4" per ft. So that means minimum of about 4" in 15 ft. Did you build the deck that way?

Jaz
 
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Old 04-22-08, 08:28 PM
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OK so, 2x10" what? Spaced 16", and the span is....?

Plywood subfloor is ALWAYS t&g, no extra points for that. You need another min. 1/2" ply over the sub because of slate.

If your contractor bought Hardi for outdoor use, I guess he has never installed tile over a CBU outside then?

You can use a waterproofing over the CBU, but there a many methods. I think you were doing with Ditra?

Among others things what are the contractors plans for expansion joints?

Jaz
 
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Old 04-23-08, 04:08 PM
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HI again,
I found out the joists are 2x12 with 16' span, then they have double 2x12 at the end of each set of joists. and they are sitting on 16x16 blocks poured solid with concrete and rebar. And the plywood turns out to be 3/4 OSB board t&g.
The contractor builds alot of metal commercial buildings. But not many houses. they already rolled mapeilastic 2 part waterproofer on the osb. Thats all thats done now.
Now the plan is to thinset cbu down and roll on jamo waterproofer ( almost pink in color ) on the entire surface. There is not much slope (around 3/4") due to transit being off as 1 worker told me. It's too late to fix the slope problem so i'm stuck with it. I'm just looking for the best way possible to keep it waterproofed. Thats why i was thinking Ditra because its like a solid plastic or something like that. Can i have the cbu put down then ditra over that and be able to sleep at nite not worrying about moisture damage. or would the cbu and jamo w/p work without the ditra. We are in north Florida . I will check on the expansion joints. what do you reccomend.. thanks again
 
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Old 04-23-08, 04:20 PM
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noname

16' span is overspanned for natural stone. Additionally, 3/4" osb is not gonna cut it.

You need to cut down the span of the joists with a center support system. You also need to add an additional layer of plywood, at least 1/2" and more would be better.

cement board then ditra makes no sense. Cement board wont make the deck any stronger. You can install ditra over the plywood. Not sure what the jamo stuff is that you are talking about and didnt bother to look it up. The ditra is your waterproofing.

And again you cant use hardibacker on exterior applications so if thats what you are calling cement board, you cant use it.

Oh yeah and the osb outside doesnt do it for me. Id replace the osb with exterior glue plywood.

3/4" in 15' run is not sufficient slope. The slope should be 1/4" per foot.
 
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Old 04-23-08, 05:57 PM
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NoName,

This is a misconceived project. Next time hire someone that knows what they're going to do. There is nothing right about the way you want to do the installation. Donate the $$$ to a charity instead....or start over.

Jaz
 
 

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