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Unread 01-14-2019, 05:03 PM   #1
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Yet another subfloor question

Hi, new to the list. trying to get some advice on my floor.

In the planning stages of a 2nd flr master bath with only studs and rough plumbing currently. The subfloor(3/4" T&G) sits on a 15' span made up of 2x10 12"OC. The deflecto tells me I'm L386. I'd like to add additional underlayment for some breathing room.
I can't find a calculator for how much I can improve the stiffness (glueing, screwing and offsetting edges) by adding another layer of plywood.

Any insight would be appreciated and look forward to the help I'm sure to get here. Thanks!

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Unread 01-14-2019, 06:30 PM   #2
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Welcome to the forum, Dan.

The Deflecto calulator calculates the joist structure only (which you're reporting at L/386). Adding plywood layers to the top doesn't increase this structural value. If you want the structural value to go up, you need to use more joist material (or add plywood to the underside of the joists using specialized, and rarely used techniques). I plugged in your figures into the Deflecto and am coming up with a different calculation. I must be using different values or wood species or spacing or span than you, so....

1) Can you clarify which values you're using for the Deflecto, and
2) I'll ask if there's anything specific you're concerned about, and
3) Can I confirm that you're planning to use man-made ceramic/porcelain tiles, and
4) Can I ask which tiling method you're planning to use?

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Unread 01-14-2019, 07:33 PM   #3
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Hi Dan welcome. I agree with Bubba. I ran your measurements through the Deflecto and come up with 424 which it good for ceramic or porcelain. Adding more subfloor doesn't add strength to the joists.
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Unread 01-14-2019, 08:01 PM   #4
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Thanks for the replies.

To be more specific the span is 15'6".
I plan to use a porcelain tile over Ditra. Not sure on the tile size since I'm assuming the smaller the better if the floor deflects a bit. wondering if 12x12 are too large.

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Unread 01-14-2019, 08:11 PM   #5
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Welcome, Dan.

First, do you actually know that your joists are Douglas Fir or Southern Yellow Pine and are of good grade? If not, your calculations might be much better than your joist structure.

Second, the size of your tiles has nothing at all to do with the floor deflection requirement. What will matter a great deal is the flatness of your floor, especially if you select a large format tile.

My opinion; worth price charged.

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Unread 01-15-2019, 09:58 AM   #6
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They are No. 2 SYP in good shape. The joists have 1x3 strapping 12"OC. It's 20yo construction.

Last edited by Olivrae; 01-15-2019 at 01:07 PM.
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