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Unread 03-25-2020, 06:05 PM   #1
Future Tile Master
Join Date: Mar 2020
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Bathroom Deflection & Subfloor Advice

First off, thanks for all the great advice that many of you provide on here. I spent a few days reading dozens of threads on this board in preparation for a bathroom tile job, which answered many of my initial questions. But I still have a few that are specific to my situation, And I'm sure more will arise later.

A brief summary of what the floor is comprised of right now:
  • 2x8 dimensional lumber joists, 16" OC, 10' span between supports
  • 5/8" plywood on joists, 5/8" particle board on top of that (yes, im positive it's pb, blech)

The deflectometer reached the following conclusion:
For joists that are SYP or Douglas Fir, in good condition, 8 inches tall, 1.5 inches wide, 16 inches on center, and 10 feet long between supports, the deflection calculated is 0.197 inches.

This translates to a deflection of L / 610.

Since the maximum deflection for tile is L / 360, and for natural stone is L / 720, your floor is rated for Ceramic tile, Congratulations!
My plan is to tear out the particle board and replace it with either backerboard, 5/8" or 3/4" exterior grade plywood (whichever is ideal for structural integrity and gets closest to flush grade with hallway). Ditra underlayment with electric radiant heat to follow. And then tile on top.

My questions:

1. Natural stone requires max deflection of L / 720, my joists rate L / 610. Which technically rules out stone. Our plan was to tile with slate. Is this a firm no go due to current joists? Or is there some wiggle room there? Does tile size make any difference? If I beef up the second layer of subfloor and top that with ditra, does that help? I realize deflection starts with the joists, but it's hard to let go of the slate?

1a. Actually, I just realized that I gave the deflectometer the nominal measurement for the joist height and the actual measurement for the thickness. Clearly, one is wrong. Should it be actual size for both measurements? As in, a 2x8 should be entered as 7.25x1.5? If so, deflection is L / 480. Which is likely a definite non-starter for slate.

1b. My entry way is tiled with a natural stone that looks like slate. It shares a wall with the bathroom. No cracks or issues on that tile for the 10 years we've lived here. Not sure that this matter, though.
2. Setting aside the previous question...I know 3/4 ply is the ideal first subfloor layer. But there's only 5/8 in there right now. Is it reasonable to leave the 5/8" in place and install a stout second layer above it?

3. What is my best option for the second layer of subfloor? Keep in mind I plan to underlay the tile with ditra uncoupling. Is backerboard or plywood preferred in this scenario?

4. If backer is best, what thickness? If plywood is best, is 3/4 ply strongly recommended on top of a 5/8 base layer? Or am I fine going with 5/8 on 5/8? It will, of course, be exterior grade. I ask this last question because I'm not yet sure which tile we will select, so I want to know if I have options when it comes to grade transition.

I think that's all for now. Thanks! -- kb

Last edited by kbarbs4421; 03-25-2020 at 06:33 PM.
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Unread 03-25-2020, 07:00 PM   #2
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KB, welcome to the forum!

You are correct, the deflection rating for the numbers you proved are L / 480. Great for porcelain but a complete no go for natural stone.

Good plan discarding that particle board. I had a place many years ago that had particle board subfloor; that stuff was a nightmare.

Is the floor flat? That may impact the decision on how you will need to prepare the floors.
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Unread 03-25-2020, 07:28 PM   #3
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Location: Boerne, Texas
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Welcome, KB.

1. It's all a risk tolerance game. There is one (and I think only one) manufacturer of tile installation products that will give you a warranty for natural stone over joists with a deflection rating of only L/480 if you use all their products for your installation. Everyone else recognizes the industry standard of L/720 joist deflection for natural stone. And all manufacturers known to me require the industry standard of double layer plywood subflooring under a natural stone installation. And keep in mind that not all slate is equal.

2. If you go for the natural stone, you must install the second layer of plywood subflooring and it must be a minimum of nominal 1/2" exterior glue (there is no exterior grade) plywood with no face of grade lower than C. I would personally not install any tile, natural or man made, over a single layer of nominal 5/8ths" subflooring.

3. See #2.

4. See also #2. I would not recommend nominal 3/4" plywood as your second layer. Even if you pre-drill all the screw holes you'll still have difficulty fastening it to the thinner first layer without stripping out the holes in the lower layer.

My opinion; worth price charged.

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Unread 03-26-2020, 06:08 AM   #4
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If you're going to use Ditra I suggest going on Schluter's website and downloading the technical handbook/installation instructions. It will answer your questions about the appropriate subfloor construction. Same goes for any product you're thinking of using for that matter.

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