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Unread 09-18-2008, 09:50 AM   #1
Michael B
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Join Date: Sep 2008
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Minimum Requirements For Underlayment

I am planning on installing 18x18 tile over a wood sub floor structure. This is what I have currently.

2x6 joist
2x6 Tongue and Grove
A layer of plywood ( thickness unknown)
1x6 plank over the plywood

From everything I read, it looks like I'll have to add about an 1-1/8" of plywood, hardibacker board, and thin set just to lay tile. This will cause two problems. One, it will raise appliances too high for the existing cabinets. Two, the transition from my hardwood floor to the tile will be almost an inch.

I am wondering if there are products out there or methods to minimize the thinckness of the underlayment?

5/8" - plywood
1/8" - thin set (approx.)
1/4" - hardibacker board
1/8" - thin set (approx)

Any help would be appreciated

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Unread 09-18-2008, 10:01 AM   #2
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If everythings flat and passes the deflecto. test you shouldn't need to add anything but thinset and backer board, or Ditra to help with the height. I'd rip up the 1x6 plank though and start from the plywood that's there. What's the joist span?
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Unread 09-18-2008, 01:06 PM   #3
Dave Taylor
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Location: St Marys County MD
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Hello Michael B.......

and welcome to TYW forums.

We have a Deflecto tool up top this page you should become familiar with. You can do some careful measuring of your subfloor components (actual joist height and width, joist distance on center and longest unsupported joist span)…. then load these stats into the Deflecto tool and it will tell you if your floor substructure will support tile or stone. Try it.

Gotta tell you, though Michael, that 2 x 6 (actual 1.5" x 5.5") joists gotta' have less than an 8' unsupported span to support ceramic. What kind of 18" x 18" tile or stone are you thinking of putting on this floor?

Also..... I don't know what part tongue and grove 2 x 6's play in your floor…. help me out here???

Also..... I don't recall ever seeing a floor where a 1 x 6 plank underlayment has been installed over the top of plywood..... usually its the other way around.

Lastly, why aren’t you able to determine the thickness of the plywood that is somewhere in your flooring mix?

Can you see markings on this plywood from the bottom…. or see it's thickness at a heat or cooling register edge… or a stair edge…. or a plumbing or other piping hole edge…. or measure it by drilling an inspection hole in some out-of-the-way place?

Hope this all helps Mike.
Dave T (DIY'er)
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Last edited by Dave Taylor; 09-18-2008 at 06:33 PM.
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