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Unread 07-14-2022, 07:08 PM   #1
wer
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Deflectolator question/Slate floor

Hi everyone!
I'd like to put a slate floor in our kitchen and want to be sure that the deflection score is greater than L/720. I put my info in the Deflectolator and got L/625 (see the screen shot).

I have two questions and would appreciate any help!

-My subfloor will be the original hardwood, plywood, and cement board (see cross section on drawing). Will this be enough to get me to "L/720"?

-In the Deflectolator, I entered "8 ft long between supports" because the slate will only go 7' 8". This is where the kitchen ends (See the drawing). However, the distance between the supports is 11'. Did I enter the correct length? I also attached two photos of the joists which may provide some assistance.

Thanks! Pete
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Unread 07-14-2022, 07:56 PM   #2
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Welcome, Pete.
Quote:
Originally Posted by Pete
Did I enter the correct length?
Nope.

If you'll look at your screen shot, you note that the result of your Deflectometer calculation says "...and 8 feet long between supports,.." It's that between supports length that you need to be using, which would be 11 feet in your case. That puts you just within range for a ceramic tile installation. Even if your joists are full sized, and not nominal (I see square edges), you're well short of L/720.

You will have a double layer subfloor, which is a requirement for natural stone tiles, and even though you don't have the proper materials (stone industry wants two layers of plywood), I generally consider your board floor an adequate first layer if it is nominal 1-inch material and well fastened. Second layer must be a minimum of nominal 1/2-inch plywood with no face of grade lower than C.

But your joist system is seriously lacking for stone tiles. You can, of course, tile over anything you want in your own home and with whatever tiles you choose, but natural stone is not a real good idea in your current situation.

My opinion; worth price charged.
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Unread 07-15-2022, 05:09 PM   #3
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Agree with CX, it’s a gamble using natural stone over that floor system. Some folks will say the older materials are stronger than new graded wood but that has not been my experience.

I recently added support columns to address the situation you described. The floor was full sized 100 year old pine 2x8 16”OC spanned 13’. Just barely made it to mid L/360. I built up a laminated beam using [strike three] two layers of 2x4 located 2’ off one end of the span. Reducing the span increased the deflection rating L/480. You may need to cut your span in half to get to L720.

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Unread 07-15-2022, 05:58 PM   #4
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Now, I don't wanna take issue with a guy who's got half a dozen or more initials after his title, and question his math abilities, but even with my reading glasses on I can't see that third 2x4, PC.
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Unread 07-15-2022, 06:16 PM   #5
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Haha, CX, sorry you caught me out! I couldn’t squeeze a third layer into that section. The triples are in another area and you can see it was a challenge finding a location amongst all of the existing piping.

My intention with these beam are to take some of the vibration out of a hardwood floor.

I would definitely go with 3 layers for a tile floor beam.

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Unread 07-18-2022, 11:37 AM   #6
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Thanks, PC7060 and CX! I'm glad I didn't install the slate and figure this out the hard way. I am going to go with engineered wood. Crisis averted! Pete
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