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# Subfloor and Underlayment seam staggering

#1
07-04-08, 07:16 PM
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Join Date: Jun 2008
Location: USA
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Subfloor and Underlayment seam staggering

If the room is an oblong about 12' long and 70" wide, so that three sheets of 4x8 plywood would cover it, would the following be an OK way of staggering the underlayment above the subfloor?

The blue is the subfloor and red is the underlayment. The little circle is the shower drain.

#2
07-05-08, 08:15 AM
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What direction do the floor joists run? Are you down to joists now? You should be using glue and screwed tongue & groove plywood for the subfloor. What type of tile are you using? HAve you done a deflection calculation to make sure your subfloor meets L360 for ceramic/porcelain, and L720 for natural stone?

What are your plans for this shower?

#3
07-05-08, 09:47 AM
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Join Date: Jun 2008
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My main question is whether the staggering of the underlayment seams is OK. Is it OK to have a fairly narrow underlayment sheet on the wall opposite the shower, as shown in the drawing? What's the optimal distance from subfloor seam to underlayment seam?

DETAILS
The 2x10 joists 16" o.c. are 12' long, and run lengthwise. B ut they are supported at 10'. The unsupported span is 10'.

The plywood grain of both the 3/4" subfloor and the 3/4" underlayment will run perpendicular to the joists. Two layers of 3/4" B/C exterior grade. 2x10 blocking installed at the subfloor edges (i.e. because it's not t&g).

Staggered 2x10 blocking has also been installed between the joists.

Subfloor will be screwed and glued to the joists. Underlayment to be screwed to the subfloor with #8 1-1/4" deck screws.

The joists have been sistered extensively (using lag-bolts and deck screws) and the joist at the left long perimeter side has been sistered to a 2x10 LVL, glued, lag-screwed, and nailed.

Tile is granite 12x12. Deflection calculation shows OK for stone. The dimensions are well within spec for stone--the variable is the quality/condition of the wood, which is a judgment call. Some of the joists were not as good as others. But with the extensive sistering and blocking, the chances for stone ought to be good.

Floor will have 1/4" WEDI isolation membrane.
Shower will be prefab WEDI base, trimmed to size, 36x42.

Regards
T

#4
07-05-08, 10:06 AM
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Join Date: Jul 2006
Location: USA
Posts: 8,034
I'm unaware of a 3/4" underlayment...what is it?

You should not tile directly to plywood, maybe I misunderstood you?

You should have at least 1 1/4" thick subfloor, then a underlayment, like 1/4" cement board or a Ditra underlayment.

You board seams look fine to me.

#5
07-07-08, 12:11 PM
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Join Date: Jun 2008
Location: USA
Posts: 9
On top of the two layers of plywood there will be a 1/4" thick tile-ready isolation "membrane" -- from WEDICORP.COM -- though it's a panel not a flexible sheet of membrane.

The WEDI board is a lightweight, easily cut and easily positioned extruded polystyrene water-barrier panel, with a fiberglass-reinforced cementitious surface on both sides. Comes in wall-panels too.