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Unread 03-10-2020, 03:10 PM   #1
puravida
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How to incorporate subfloor thickness into deflection calculation?

We are doing a forced remodel after a water pipe burst while we were away and pretty much destroyed most of our house. We are down to studs and subfloor. It's a one-floor home over a crawlspace.

There was ceramic tile in the kitchen before demo, and it was really solid. It felt almost like walking on slab on grade. No cracked tiles after 15 years. I'm guessing that it was due the following layers that were discovered during demo:
  • 1x12 subfloor on 2x8 joists (16" oc), 12.5' span
  • 3/4" oak hardwood floor
  • backer board
  • 12"x12" ceramic tile

We want to put tile back in the kitchen. I ran the joists through Deflecto, but it came back with only L/271:
Thank you for using the John Bridge Forums Deflect-O-Lator :-)

For joists that are SYP or Douglas Fir, in good condition, 7.25 inches tall, 1.5 inches wide, 16 inches on center, and 12.5 feet long between supports, the deflection calculated is 0.554 inches.

This translates to a deflection of L / 271.
So the layer of hardwood flooring must have contributed substantially to the stability of the floor. Since we're replacing the old 1x12 subfloor with 3/4" OSB, should I put in two layers to basically get back to where it was before and hope for the best?

Is there a way quantify the subfloor layer's effect on deflection?

Thanks for any thoughts.
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Unread 03-10-2020, 06:31 PM   #2
cx
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Welcome, Mark.

I'd recommend you buy a lottery ticket.

You were very lucky on two counts. First, your joist structure is substantially over spanned. Second, the hardwood flooring under the CBU is a no-no according to the ceramic tile industry and all manufacturers of any kind of ceramic tile substrate.

I'm really amazed that you had no failures, but we can't guarantee failures any more than we can guarantee success. We can only tell you what the industry standards and product manufacturers require and where the smart money is betting.

You say you're down to "subfloor." Does that include your hardwood layer?

My opinion; worth price charged.
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Unread 03-10-2020, 09:24 PM   #3
jadnashua
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Hardwood tends to move way too much with changes in humidity levels. That's the main reason why it is not recommended for use underneath tile.
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Unread 03-11-2020, 08:12 AM   #4
John Bridge
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Hi Mark,

In answer to your question, yes, two layers of OSB is an improvement over one layer and the hardwood flooring, but I agree with CX that we can't guaranty a successful tile installation. Of course, it seems to have worked before, so it should work again.

P.S. We have an article in the Liberry on how to install two layers of subflooring: https://www.johnbridge.com/vbulletin...d.php?t=114739
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Unread 03-11-2020, 11:33 AM   #5
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The major strength of plywood (and most OSB) is along the grain. So, it is primarily good for providing structural strength in between the joists, and only has a much smaller component along the joists. This is especially true if it is not properly bonded to the top of the joists with a good bead of a quality construction adhesive and fasteners. The subfloor's first layer must be intimately bonded to the top of the joists for that to work, and that usually takes (preferably) screws, or a good ring-shank nail (harder to ensure the subflooring is pulled down to the joists fully, but can work).

Depending on how the structure is used, you may not stress things. Throw a party and lots of people congregate in the kitchen (or room with the tile or an adjacent one supported by the same joists), and your odds of long-term success goes down. Do a conga line dance, and all bets are off. Transport in a new refrigerator, potential problems. Your 400-pound neighbor comes in...who knows?!
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Unread 03-11-2020, 02:23 PM   #6
puravida
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Thanks for your thoughts. I appreciate it.

Right now we're down to just 1x12 planks laid diagonally over the joists. Many of those are badly cupped due to the flood. So I'm thinking they need to come up.

I can think of two ways to strengthen the floor beyond 2 layers of OSB:
  • sister new joists to existing ones
  • put a support beam under the halfway point of the joists, reducing the span. The support beam would be 16' long, so it would probably need to be supported in the middle itself.

According to Deflecto, the last option gives the best results. But the "crawlspace" is only 10" high under most of the kitchen, which would make digging footers difficult even from above.
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Unread 03-11-2020, 04:37 PM   #7
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Hey Mark,

If you remove the floor boards you can dig footings from inside the house.
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