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Unread 03-03-2012, 02:51 PM   #1
Ron in Seal Beach
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1.5" thick planks - how much more subfloor wood do I need for tile?

Hi everyone. Doing a bathroom project. Bathroom is small, 5X8. House is 1958 build. Single story with crawl space. Existing main floor is tongue and groove redwood planks 5" wide and 1.5 inches thick. The span underneath is about 6 feet between floor joists which are 4x10's also redwood and sitting on stem walls. No deflection that I can perceive. This floor sits only 3/8" below original finished oak floor which we'll keep. Therefore I'm trying to minimize the transition from the bathroom tiled floor down to the hallway oak floor. Plan to use 12x12 limestone tile 3/8" thick. Also plan to install a electric heat mat and use a ditra membrane. So..............

Current plan is:
1) main subfloor 1.5"
2) plywood to run across plank 3/8"
3) heated mat embedded in 3/8" thinset
4) ditra and thinset 1/4"
5) tile and thinset 1/2"

That gives me 1 7/8 inch wood under tile and puts the finished floor 1 1/8" above hallway wood floor. Seems like to much to me.............

1) Is the 3/8 ply necessary? Could I go with 1/4"?
2) Any other suggestions for combo of materials?

Tile is not negotiable...........wife picked it and loves it. Sigh........

Last edited by Ron in Seal Beach; 03-03-2012 at 02:52 PM. Reason: mis-spell title
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Unread 03-03-2012, 02:59 PM   #2
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Location: Boerne, Texas
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Welcome, Ron.

I have very serious doubts that your existing subfloor deflection qualifies for even a ceramic tile installation, but I have no way of calculating that (lacking in talent).

We do know from calculation by our resident engineer that pine flooring of that type spanning only 4 feet will qualify for ceramic tile, but not stone. Your far less rigid redwood spanning 50 percent farther just hasn't a chance even for ceramic tile and is not even gonna be in the right ballpark for natural stone.

If it did, the minimum plywood thickness you could add is nominal half-inch.

If you're unwilling to improve the framing, I think you should stick with the hardwood flooring. But we understand the spousal consideration.

But it's your house and your tile and your dinero and you're free to install stone tile over whatever you're comfortable with, eh?

My opinion; worth price charged.

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Unread 03-03-2012, 04:00 PM   #3
Ron in Seal Beach
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I did install travertine in the kitchen with 3/8 ply and a redguard before tiling. But I went under the house and toenailed a 2x4 under each plank (42 planks) running between the joists and then screwed the planks down to each 2x4 every 8 inches. Then laid (screwed into the planks not the joists) the 3/8 ply over that. I did that because I put a wolf range in and had to bring it in over the tile floor and was worried about the deflection with the huge weight (800 lbs I think). It worked and after 4 years I have no problems with the tile. I suppose I could do that in the bathroom to improve the load bearing and deflection. Its just that............I hate it under the house.............
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Unread 03-04-2012, 08:24 AM   #4
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Your idea for beefing up the subfloor is fine, but I would have used 2x6s on joist hangers.

I hate it under the house.............
Get someone else to do it.
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