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Old 02-28-2009, 10:57 AM   #1
advent70
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subfloor - plywood panels between joists

I am installing 12" x 12" tile in the kitchen of my 1969 vintage house, 2" x 6" joists 4 ft. o. c. The area to be tiled is 10 ft. x 16 ft. Old flooring has been removed and blocking with 2" x 2" strips glued and screwed is installed 3/4" below top of joists so as to support all edges of 3/4" plywood panels inlayed between the joists. These panels are to be glued and screwed to the blocking and 2" x 2"s. I have not been able find any guidance on this method of installing a subfloor and the required thickness for underlayment over this subfloor. The plan is to use 1/2" or 5/8" underlayment, thin-set and 1/4" hardy backer that allows the finished tile floor to be even or 1/8" above the floor in the next room.
Can anyone give me a design reference for effectiveness for this type of subfloor and required thickness of underlayment?
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Old 02-28-2009, 12:37 PM   #2
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Welcome advent70, the tile gurus will be along shortly, in the meantime:

You can use the "Deflecto" tool (blue menu bar above) to check your joist structure for tile. I hope your 4 ft. o.c. joists are really spaced closer than that. Some information about the longest unsupported span would be useful too as well as what kind of tile you're planning on using.

You can also use the "User CP" (above) to enter your name on each post if you like.

Hope that's helpful.
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Old 02-28-2009, 12:41 PM   #3
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are you sure you have 2x6" on 48" oc? sounds odd to me, generally those post & beam floors are done with 4x material.
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Old 02-28-2009, 07:04 PM   #4
advent70
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Geoff and Jeremy,

Thanks for your observations and comments. I am a little slow learning how to use this forum.

You are correct, I wrote it wrong. I have 2x6 joists, 16" o.c., 4 ft. span. The house was built with double 2x8 outer and intermediate sills on the piers with 2x2 ledgers on the bottom sides of the 2x8 sills and 8 ft. joists. I added double 2x10 beams under mid span of the Joists to stop the bounce.

The deflecto tool says the 4 ft span is good for tile and stone.

The tile my wife and daughter selected is an American Florim tile, 12"x12", 8 m.m. thick with porcelain glaze.

Any other comments or suggestions will be appreciated.

John
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Old 02-28-2009, 07:14 PM   #5
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Welcome, John.

Did you cut your 3/4" plywood strips between the joist such that the face grain runs perpendicular to the joists, or parallel with them?
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Old 02-28-2009, 07:50 PM   #6
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CX,

The plywood grain is perpendicular to the joists. The gap between plywood and joists varies from 1/16" to about 3/16".

John
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Old 02-28-2009, 08:02 PM   #7
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I'd recommend a layer of half-inch plywood over what you've got.

Might wanna consider Ditra in lieu of your Hardibacker and get a little closer to the level of your adjacent flooring.
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Originally Posted by John
I have 2x6 joists, 16" o.c., 4 ft. span. The house was built with double 2x8 outer and intermediate sills on the piers with 2x2 ledgers on the bottom sides of the 2x8 sills and 8 ft. joists. I added double 2x10 beams under mid span of the Joists to stop the bounce.
The efficacy of that will depend to some extent upon the span of the doubled 2x10 beams.

Depending upon the type of wood, your 2x6s may not have been overspanned at 8 feet, though.

My opinion; worth price charged.
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Old 03-01-2009, 05:20 PM   #8
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The span of the doubled 2"x10" beams varies form 2'-4" to 3'-8" due to plumbing interferences. The span of the existing 2x8 sills is about 5 feet. The floor now passes my non-technical bounce test. (My wife and I take turns jumping on the floor and we don't feel the floor shake. Prior to installing the beams, I could feel her walking across the room, the stove rattled slightly and coffee in a cup on the table would not spill, but shake noticeably.)

The joists and sills are SYP and the board thickness is 1-5/8" vice the current standard of 1-1/2". This probably met code at the time the house was built.

I have never seen Ditra but I'll print the Ditra handbook and check dimensions.

RELATED ISSUE: One of the double 2x8 intermediate sills runs across the 16 ft. width of the kitchen at 4 ft. off the front kitchen wall. One row of boards of this sill were set on the piers, joists set in place and nailed through the 2x8s into the end of the joists. The other row of 2x8s were then set in place to overlap the end joints of the first board and a very few nails used to attach the 2 rows of boards together. The next set of joists were then toe nailed to the joist (3 16d nails, 1 at the top and 1 on each side.) The gap between the 2x8s through the length of the kitchen varies from about 1/16" to slightly over 3/16". The original 1/2" subfloor was the only real tie holding the joists on either side of the sill together. This seems to mean I should fastened the 1/2" underlayment you recommended to the top of the joists to control end to end movement of the joists on either side of the sill. However, both the TCNA and the Woeste & Nielson 2004 TileLetter on positioning underlayment say underlayment fasteners should not penetrate the joists. Since my inlayed subfloor panels are will be glued and screwed to the joists, what is the significance of fastening or not fastening the underlayment to the joists? What is the real reason for not fastening underlayment to the joists when there is a subfloor on top of the joists?
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Old 03-02-2009, 04:39 AM   #9
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John, If you use either Ditra or backerboard for your underlayment, you can attach the 1/2" plywood to either the joists or the recessed plywood (or both).

The confusion comes from the approved method of installing tile directly to plywood. In that case, the upper layer of plywood is the "underlayment" not the subfloor. The underlayment is not fastened to the joists to minimize the effects of movement of the floor framing on the tile.
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Old 03-02-2009, 07:26 AM   #10
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Geoff, Jeremy, CX & Bob.

Thanks for your comments and advice. I have learned a lot from you and other threads on this forum.

John
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