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Unread 09-20-2021, 12:30 PM   #1
Mr_Stop
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24" O.C. I-joist with OSB subfloor

Getting ready for a master bath remodel. I remodeled the kitchen a couple of years ago and found the house was built with 24" OC I-joists. We had to replace the subfloor in places since I believe the tile trapped moisture from the basement and caused the subfloor to fail. The subfloor was ~3/4" nominal OSB IIRC.

In the master bath, I will be installing plank tiles in the main walking area with a pebble floor under a freestanding tub. In reviewing the TCNA manual, the closest spec I could find was F155-21. It recommends a 23/32" exterior glue plywood underlayment. However, it states 8"x8" and larger tile only. Should I be worried about the pebbles? Any recommendations for a more robust floor assembly? I'm going to have to remove some of the flooring to relocate the shower and tub drain, so subfloor replacement is within reason.
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Unread 09-20-2021, 03:50 PM   #2
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Welcome back, Brad.

I believe you'll find that the TCNA Method F155 requires a plywood underlayment of a minimum nominal 5/8th" plywood unless the method has changed since 2019.

Why the minimum tile size? I suspect that's what the testing has indicated will work and no manufacturer of plywood wants to do any further testing because the testing is costly and there is no real benefit for them.

Consider that your pebble floor is nothing more than a coating of thinset mortar with some stones pressed into it. Not even as strong as some of the better natural stone tiles and the stone industry won't approve installation over joist centers greater than 16 inches at all. And they much prefer concrete SOG.

That's my best guess. You wanna give it a try in your installation, that's entirely up to you, of course.

My opinion; worth price charged.
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Unread 09-21-2021, 06:34 AM   #3
Mr_Stop
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Thanks CX.

F155 does basically round to 5/8" nominal plywood underlayment (19/32").

It's interesting that F152 which calls out a plywood subfloor only (versus F155 being plywood or OSB) with a bonded membrane and 3/8" plywood underlayment allows 4"x4" tile. I'm guessing the added strength of the plywood subfloor + membrane is allowing the smaller plywood and tile.


Blanke Permat
specs out 3/4" ply/OSB subfloor on 24 O.C. with 3/8"underlayment plus their membrane for natural stone. They don't have tile size restrictions either. Maybe a good solution?

In a somewhat related digression. I have always wondered about natural stone and the higher deflection requirements. I understand it is due to minimize cracking on the softer/more brittle natural tile. However, wouldn't that mostly apply to larger tiles? Is a small mosaic just as likely to crack?
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Unread 09-21-2021, 07:49 AM   #4
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I thought we'd gotten over the OSB vs. Plywood when it comes to strength or rigidity, but, again, what you see in those TCNA Methods is testing that someone paid for. If the test (ASTM C627) was run with plywood, the accepted Method is going to specify plywood. If run with both plywood and OSB, it would say both or either. That's my understanding of it.

As for the tile size difference in F152, you'll note that it allows the smaller 4x4 tiles only if the top flange is 2 1/4" wide. If only 1 1/2" wide, you're back to 8x8 or larger. And it would be the membrane manufacturer(s) paying for the testing.

The tile installation product manufacturers can make any specification they want for their product. Especially true for the uncoupling membrane manufacturers where no standard has been established for their product.

As I said in Post #2, I agree that mosaics or pebbles should be considered more like natural stone than ceramic tile in regards to strength. But I don't vote and I don't pay for the testing.

My opinion; worth price charged.
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