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Unread 03-19-2007, 11:00 PM   #1
Keeney
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Outdoor porch over PT 1" ply, 2x12 12" OC 10' span

Looking to tile an outdoor porch with 12x12 porcelain.

It was framed with tile in mind, so its 2x12's 12" O.C. for only a 10' span.

The decking is Pressure-treated plywood, I think its 1", but could be 3/4" (I will check).

I'd like to use Ditra and no additional plywood to keep the height change low enough to not have to adjust the attached stairs. Also, I like the idea of making it waterproof.

The PT plywood concerns me a bit. Its been in place for several years, so its pretty much dried as much as it is going to, but will I have problems getting thinset to stick to it?

- Rick
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Unread 03-19-2007, 11:09 PM   #2
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Hi Rick
Im still learning the schluter systems Ditra and Kerdi, but i believe
the general consensus among those who use it frequently is to use
a premium modified thinset to bond the ditra and an unmodified thinset
to bond your tile. Im pretty sure you will need kerdi band to lap your
Ditra seams in order to attain the best waterproofing for outdoors.
Hang tight for others to come along and set you straight. In the mean
time you can take the schluter link to the right. The schluter site
has a ton of good information.
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Unread 03-20-2007, 11:14 AM   #3
Scooter
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Ditra is not waterproof.

Kerdi is.

You can attain some measure of water resistence using Ditra with Kerdi bands.

I would membrane the whole thing with pvc, Noblecs, or Kerdi. I would not want a drop of water touching that ply.

Hopefully it is sloped, yes?
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Unread 03-20-2007, 03:41 PM   #4
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Yes, it is slightly sloped and has a full roof over it, but does get some rain and snow on it when its windy.

Why the concern of the plywood getting wet? Will the thinset let loose? The Pressure-Treated plywood is not going to rot or anything - its rated for ground contact...

- Rick
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Unread 03-20-2007, 04:09 PM   #5
Scooter
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Plywood will swell and break the bond, thinset will lose bond, tiles and grout will crack, your pick up truck will stop working, your wife will divorce you, your dog will run away, and it will be the end of the free world as we know it.
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Unread 03-20-2007, 04:22 PM   #6
jadnashua
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Schluter has some other products they recommend for decking. When banded with the Kerdi-band, ditra IS waterproof, but that won't prevent moisture from affecting the ply from the edges or bottom. With a roof, that might be enough. You would want to do something to prevent water from coming in from the sides, I think. Check out http://www.schluter.com/142.aspx for Tobra. It might be overkill...wait for one of the pros to offer their suggestions.
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Unread 03-20-2007, 06:21 PM   #7
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The sides are weather proof. They are finished exterior walls same as the remainder of the house. The stucco guys left it so I can slip some additional flashing (or kerdi) up under the last couple of inches of tar paper and lathe at the bottom of the stucco all around the deck.

So with sealed seams and flashed edges, would the Ditra be weather proof enough to keep itself stuck down to the plywood, or am I going to want CBU's screwed down to that plywood?

I suppose I could add a couple of drains at the low end to avoid any chance of puddles during rapid snow melt...

- Rick
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Unread 03-20-2007, 06:40 PM   #8
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Rick, have you found the Ditra Handbook? If so, turn to page 16. On a wood substrate over an unoccupied space, Schluter wants a layer of CBU under Ditra.




P.S. You can find it in the Liberry, here. http://johnbridge.com/vbulletin/showthread.php?t=39655
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Unread 03-20-2007, 09:01 PM   #9
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Rick,

You are right to be concerned about PT plywood used as a subfloor when installing tile. Pressure treated is NOT recommended even if covered by a cement board, (which is has to be) for this project. At this stage I don't know whether to add more plywood or remove the PT? BUT, there are other issues.

I would recommend, 3/4" ext. grade t&g ply exposure 1, then cement board with the recommended thinset under and fastened per manuf. recommendations. Now comes Ditra, with your favorite tiles. Now, back to those issues.

The deck must be slopped the min. of 1/4" per ft. 3/8" would be better. So I think your project is likely to fail in a few years. What I'm trying to say is you have to do every step correctly, 9 out of 10 is not good enough.

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Unread 03-20-2007, 09:49 PM   #10
Keeney
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The deck IS sloped 2" over the 10' span - a little under the 1/4" per foot standard.

Because its an exterior deck, the local building officials required PT decking and framing for the porch, even though its covered by a roof and was planned to be waterproofed and tiled.

All the seams in the plywood are supported by joists or blocking between the joists. Its glued and screwed down and is not going to move relative to the framing. I am not concerned about the lack of T&G (T&G PT Ply wasn't conveniently available).

I would imagine that one issue with PT plywood is due to the high moisture content left in it from the pressure treating - its going to shrink a lot when its new. However, its been drying for several years now, so I am not concerned about any additional shrinkage.

The surface really just doesn't get that wet - it probably stays dryer than my hardwood kitchen floor (we have toddlers who dump a lot of milk, juice, etc). With Ditra or some other membrane, its going to stay bone dry under there.

What I am concerned about is making things stick to the plywood, and having those things stay stuck through the seasonal temerature, freeze-thaw, and humidity swings we have here in MN.

So would 1/4" of Hardi screwed down over thinset suffice to keep the Ditra stuck?

- Rick
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Unread 03-20-2007, 10:17 PM   #11
Scooter
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The typical exterior spec for tile is

sloped mortar bed
membrane
drainage mat (troba)
sloped setting bed
thinset
tile

What you want to accomplish is to drain every drop of moisture that gets through the grout or tile, into the setting bed, through the drainage mat, then onto a sloped membrane and out the sides of the installation.

If any moisture gets stuck in there and it freezes, you will see your thousands of dollars go up in cracks and busted up tile.

I'm not a Ditra guy at all--I like old fashioned specs. I've seen fountains, darn near swimming pools made out of pvc membranes, and the stuff will hold water forever. But thats actually not the point.

You want that water to sit on that membrane only for a nano second and have it quickly drain out the sides.

Thats where plywood really presents an issue for me. I'd membrane the ply, and fold the membrane over the edges. Then make an aggressive mortar preslope and membrane it as well, then the drainage mat and a mortar based setting bed. That sand mix mortar will let water pass right through to the membrane, and out it goes.
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Unread 03-20-2007, 11:31 PM   #12
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Ok Rick,

You are talking yourself into doing it regardless if it's right or not! It needs to be pitched 3 inches. PT for the deck is ok unless you're gonna install something that requires a rigid base. It is going to get wet, it'll stay wet and then it'll freeze, guess what happens then? The whole structure is going to move alot! Alot might mean 1/4"? You're going to have a temp sling of close to or over 100 degrees, and the wind will move the deck too.

Then in the end you wanna put Hardi on there. You can NOT use HardiBacker outside! Listen to Scooter, find someone that know the right way to build it for you.

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Unread 03-23-2007, 12:08 AM   #13
Keeney
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OK, I get it - regardless of Ditra or drypack bed, I need a presloped membrane to get rid of ALL the water in freezing environment. And 1.5% is on the shy side of being enough slope. 2 to 3% being better.

So I obtain the required preslope.

That still leaves me with the issue that it can't drain out the edges - the deck is surrounded by walls all around (except the stair opening). Is there some kind of channel-drain system that can be positioned along the low edge?

- Rick
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Unread 03-23-2007, 12:31 AM   #14
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Well, I don't particularly like the PT plywood, but if it's old enough to actually be dry and the edges are blocked, I think you can survive that.

The 3/4" thickness meets Schluter's requirements for subflooring using Ditra.

The 1.5% slope meets Schluter's requirements for Ditra over un-occupied space.

Schluter requires an outdoor-rated CBU, installed per manufacturer's instructions, including mortar beneath and taped joints and all.

Schluter requires all seams in Ditra be seamed with KerdiBand and all perimeter/wall joints be properly flashed with KerdiBand for a waterproof installation.

And, of course, you'll need to do all the proper soft joints at the perimeter and in the field as necessary.

So, seems to me if that deck is as described, the only thing you were fixin' to do wrong for a Schluter approved installation was eliminate the CBU (which you'll need regardless the waterproofing membrane you select). That and do a proper job of flashing the low end down over the steps and other open areas as necessary.

My opinion; worth price charged.

Scooter:

Just because you don't like Ditra doesn't keep it from being completely waterproof when properly seamed with KerdiBand.
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Unread 03-23-2007, 07:52 AM   #15
Keeney
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I checked the plywood thickness. It measures out close to 1".

I am a bit concerned that the Ditra waould drain sufficiently to not have problems freezing no matter what the slope. All those little pockets worry me that they would collect moisture.

Anybody ever use Ditra outdoors in a freezing climate and can tell me it lasted 10+ years?

- Rick
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