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jinxies 12-09-2009 05:29 PM

Glue tar paper over sub-floor or....?
We're remodeling our small 5'x5' bathroom and after removing the tile and plywood, we're down to 1x6s on top of the joists (house built in the 1950s). We got recommendations to put down tar paper, then *glue* 3/4" plywood and then 1/4" cement board in preparation for the tile.

It seems that the plywood should be glued to the sub-floor in order to form a more structurally sound layer but we need a barrier for the cold that comes up from the unfinished ground underneath (we have a ring foundation - cold, exposed soil under the middle of the house).
So the question is,
If we put down tar paper, will the glue on top of that be sufficient to hold the 3/4" plywood? Should the tar paper be glued, nailed, other? What's the best way to adhere the plywood on top of the tar paper?
Thanks for your help.

Bud Cline 12-09-2009 05:36 PM

Install the barrier (roofing felt) overlapping 3" at the seams, staple it a few times to hold it in place. No need to glue it.

Install the plywood using nails or screws DO NOT GLUE the plywood. Attach the plywood to the subfloor components only, NOT into the floor joists.

Install cement board or an isolation membrane according to the specifications of the manufacturer of the chosen product.:)

The "crawl space" should have breeches that will provide cross-ventilation and the ground should be covered with a plastic barrier also.:)

jinxies 12-09-2009 07:10 PM

Thank you for the response>>
I appreciate your advice and will proceed as you recommend. :)

msv 12-09-2009 10:25 PM

why not screw it to the joists, Bud? i thought you're supposed to hit the joists. that's what gives you stability. the 1x6's are probably a lil loose by now in that house... the joists on the other hand are as fixed as it gets...
i'm into wood, don't know jack about tile, so just i'm curious...

Bud Cline 12-12-2009 09:43 PM

You don't want stability in that fashion. Stay away from the joists and the tile installation will have a better chance of moving independently of the structure. That's what you want.:)

If the 1X6's are loose screw them to the joists first.

Wood structures have components that are obviously different from clay and cement components. All things are effected by seasonal changes and temperature fluctuations. Entire structures can even move in the wind. Tile moves (expands and contracts) at a different rate than wood does. Anything you can do to isolate the tile from the wood is the thing to do.

The seams of a second layer of plywood should be staggered from the seams of the first layer, not glued to the first layer and not attached to the joists with fasteners.:)

This is the principal that makes Schluter DITRA work as well as it does.:) ISOLATION.:)

26yrsinflooring 12-12-2009 11:47 PM

The same reason that ProvaFlex works better it has the overlaying mesh and you can use a preimium modified thinset with the installation.
And it is cheaper than ditra.

Gary in WA 12-13-2009 12:29 AM

Bud knows his stuff:
"Place underlayment panels (exposure 1, plugged-face plywood or OSB of minimum 3/8" (10 mm)
thickness) with long dimension perpendicular to floor joists such that the following conditions are
1. Abut all underlayment end joints at quarter points between joists.
Example: Abut underlayment panels on either side of the joist centerline at: 4" (102 mm) for 16"
(406 mm) o.c. joists, 5" (127 mm) for 19.2" (488 mm) o.c. joists, or 6" (152 mm) for 24" (610 mm)
o.c. joists (see figures 1 & 2).
Note: Underlayment end joints should be placed as far away from subfloor end joints as possible.
2. Underlayment to overlap edge joints of subfloor by 1/2 of the width of the subfloor panel (24" -
610 mm). At restraining surfaces, overlap may be less than 24" (610 mm) when the subfloor panel
is less than 48" (1.2 m)-wide (see figure 1).
3. Gap underlayment panels 1/8" (3 mm) on all ends and edges, and 1/4" (6 mm) at perimeter walls,
cabinetry, or other restraining surfaces." Check on the type and length of fasteners.

From the Ditra handbook, page # 23:

Be safe, Gary

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