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Unread 04-17-2009, 05:04 PM   #1
mikecarp
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Unheated tile floor

I am planning a ceramic tile job over an unheated sun room. This is a new construction with the following details. Joists will be pressure treated 2x8x16' with double 2x6x? beams spaced in between the ledger and the double 2x8 rim joist, so the largest span on the 2x8x16' joist will be 6'. I plan on a 3/4" T&G subfloor and a 1/2" underlayment (ie., Durock, or Hardi backer, ?). There is a small crawlspace underneath that will be exposed to New York State temperature fluctuations, ie. 0 to 100 degrees F. I plan on insulating the floor with R19 fiberglass. Does this sound like a workable environment for a tile floor ??? I guess my concern is the sub freezing temps along with the 100 deg fluctuation?
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Unread 04-17-2009, 05:10 PM   #2
jadnashua
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There's no advantage to use the 1/2" cbu unless it helps you with the desired thickness; 1/4" on a floor is fine. I'd use Ditra instead, it's much easier to handle and works better.

Will this area get wet? If it can get accumulating blown rain or snow with potential standing water in a freeze/thaw cycle, you need to account for slope to let things drain along with good moisture control. So, the full answer requires more details from you.

Some tile are better in freeze/thaw than others...you probably want a good porcelain. The reason for that is porcelain typically will absorb less than 1% moisture, so that really helps if it gets wet. Lesser tile could blow apart if they get wet then it freezes.
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Unread 04-17-2009, 05:27 PM   #3
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jadnashua, Thanks for the reply.
The room is a new addition (16'x22') that is fully enclosed with no plumbing or weather related moisture other than humidity changes. The room is accessed via patio doors from the main living area and will have 6 good size (energy efficient) windows. Ceiling and walls will have R-19 insulation with R-13 in the walls - but no space heating. I'm sure some heat gain (besides the sun) in the winter will be obtained through the 6' wide patio door unit.
I felt the 1/2" might help to minimize deflection of the floor since I have read that the floor thickness for tile should be a minimum of 1 1/4" (or something like that).

I have not seen the Ditra you mentioned but will look into it.
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Unread 04-17-2009, 06:19 PM   #4
jadnashua
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Industry standards call for a minimum of 5/8" for ceramic...now, most installers want more than that. With 3/4", you should be fine. More doesn't hurt. The pressure treated joists tend to warp, which could be a problem. CBU doesn't add any strength in practical terms...it is just a decoupling layer and Ditra does it better and is easier to install and carry home.
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Unread 04-17-2009, 10:00 PM   #5
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Welcome, Mike.

What Jim said.

For new construction where you have the option I would make allowance for an additional layer of half-inch plywood over your 3/4" subflooring. Here is a good article explaining the best way to install the second layer.

Unless you will have ground contact I would also change to a good grade of kiln-dried lumber or engineered wood joists rather than the pressure treated material you have planned.

I'd sure want to add some insulation under that floor, too.

With that room being unconditioned space, especially if it will see a lot of direct sunlight, you should give very serious thought to some movement joints in the tile surface, too.

My opinion; worth price charged.
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Unread 04-18-2009, 02:47 PM   #6
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That's another concern - the usually moist PT lumber can deform as it dries. Problem is with the floor joist's roughly 6" above the ground surface and the neighborhood noted for termites I thought PT was the way to go. Maybe the way to go is to double the plywood layers and then add the Ditra underlayment which sounds like a better decoupling method. I personally never heard of Ditra and wondered if there is anythng on the web touting its effectiveness in preventing cracks. It definitely appears to be considerably less labor to install - I have yet to price it.
On the other hand, is the 1/2" layer of plywood necessary?
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Unread 04-18-2009, 03:21 PM   #7
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No, the extra layer of ply isn't necessary...it's insurance (it would be required if you were using a natural stone tile, though). When you compare the time, screws, tape, weight, messy cutting of cbu to the easy installation of Ditra (you can carry all you'll need on your shoulder - try that with cbu), personally, I think it's a no-brainer. You may find a local source, but many people order on-line from the TYW store here at a decent price. They'll cut a roll so you don't have to buy the whole thing and ship it to you fast.

If you place a sheet of plastic on the ground, it should stop most moisture coming from there and maybe negate the need for PT lumber...local codes may dictate. PT stuff is often almost dripping wet when it first gets to the lumber yard and when it dries out, often checks, cracks, and warps. Not exactly something you want to tile to. You may be able to buy KDAT (Kiln-dried after treating) lumber, which would be a much better choice, if required at all.

In areas getting direct sunlight, you should plan for expansion joints. These can be a grout joint left unfilled, then fitted with a urethane caulk, or an engineered one. www.schluter.com also makes some of these in various colors and finishes. You'd probably want to plan for one every 8-10' or so. The only things tha tmay require PT are those in contact with say concrete piers. That will take some more research.
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Unread 04-19-2009, 12:07 PM   #8
mikecarp
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I will be checking with our building dept regarding the use of PT lumber. I had always planned on laying a moisture barrier on the ground (6 mil poly) even with the PT joists.

Mike
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Unread 04-20-2009, 09:24 PM   #9
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Ok - the local building dept inspector indicated a non PT floor framing must be a minimum of 18" above ground for the plans as described in my previous posts. Therefore, it looks like I'll be using PT joists.
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