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Old 12-27-2007, 02:45 PM   #1
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Beefing up subfloor from basement


Hi all,

I'm looking to install a tile floor in my bathroom. Here's the problem:

The current floor is made up of 1/2" plywood covered by 1/2" particle board, then vinyl flooring. I'm planning to rip out the vinyl flooring and particle board leaving just the 1/2" plywood layer. I plan to top this with 1/4" USG Fiberock tile underlayment and then the tile. The remaining 1/2" plywood by itself is too thin a surface to properly support the tile underlayment and tile. I don't want to build on top of the existing floor because it would cause the bathroom floor to be at least 1/2" higher than the rest of the house, plus the bathroom door would have to be cut down to accommodate a higher floor.

So it seems like my only option here is to try to "beef up" the bathroom floor from below, in the basement. As you can see in the photo, there are many obstructions in the 2 joist bays where I need to work.



I've been thinking of cutting 14 inch wide pieces of 3/4" plywood to glue and screw to the underside of the floor, then "sistering" 2X4's to the joists for additional support of the added layer. I can easily remove the duct work in one of the joist bays but the PVC and water pipe are something I don't want to have to mess with unless absolutely necessary.

I'm wondering if anyone has any ideas, tips, tricks or suggestions on the best way to handle this situation.

Thanks!


Last edited by Richo; 12-27-2007 at 02:52 PM.
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Old 12-27-2007, 02:51 PM   #2
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Beefing up subfloor from basement


I think you may be still too thin with the 1/4" underlayment. If I were doing it, I would go with a 3/4" plywood over the 1/2" and then tile over that. I know it may take some extra work in cutting the doors, but the other way just won't be strong enough.

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Old 12-27-2007, 03:55 PM   #3
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Beefing up subfloor from basement


I agree with Jerry. It's no big deal installing a reducer threshold and cutting down the door. Use a 5/8" exterior grade plywood on top of existing 1/2" then 3/8" concrete board, then tile. It doesn't look like you can do a whole lot strengthening from the basement, but it wouldn't hurt to try and beef it up. I would also screw the plywood subfloor to the floorjoists.
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Old 12-27-2007, 05:24 PM   #4
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Beefing up subfloor from basement


So are you saying that if I were able to accomplish this despite the obstacles....

...that it would not be rigid enough to support a tile floor?

As is, my bathroom floor is already about 1/4" above the hallway. If I were to replace the 1/2" particle board with 3/4" plywood, then 3/8" tile underlayment, then the mortar bed and tile, I'm estimating that my bathroom floor would be more than an inch higher than the hallway. That's quite an extreme transition from one room to the next.

Thanks for your responses.
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Old 12-27-2007, 05:55 PM   #5
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Beefing up subfloor from basement


I'm not a pro, just a 50+year DIYer, but I have to go with Richo on this.
Although it is a lot of extra work, since it's DIY, it's just part of it (no out-of-pocket expense for labor). My flooring is 3/4" plywood subfloor with 5/8" flooring plus 1/2" cement board under my tile floors. We have no problems.
That's what I would do. Richo, that will definitely work.
Good Luck!
Mike

Last edited by Mike Swearingen; 12-27-2007 at 06:01 PM.
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Old 12-27-2007, 05:55 PM   #6
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Beefing up subfloor from basement


What size tiles are you planning to use?
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Old 12-27-2007, 06:34 PM   #7
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Beefing up subfloor from basement


The tiles I have are 12". The bathroom floor area is about 5' X 8', less the cabinet (21" X 62").
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Old 12-27-2007, 07:24 PM   #8
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Beefing up subfloor from basement


Are you sure the existing subfloor is only 1/2"?
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Old 12-27-2007, 08:14 PM   #9
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Beefing up subfloor from basement


Yes, absolutely certain, it's 1/2" plywood on the joists with a layer of 1/2" particle board on top of that. The particle board is in the bathroom only as a substrate for the vinyl flooring.
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Old 12-27-2007, 08:24 PM   #10
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Beefing up subfloor from basement


What is the unsupported span of those 2x8 floor joists?
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Old 12-27-2007, 09:31 PM   #11
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Beefing up subfloor from basement


The joists are 16 inches on center so it's about 14" to 14 1/2" unsupported.
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Old 12-27-2007, 10:18 PM   #12
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Beefing up subfloor from basement


Ron was asking for the span (the distance between supports on each joist), not the distance between joists
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Old 12-27-2007, 10:37 PM   #13
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Beefing up subfloor from basement


If I understand your question correctly, the span between the joist supports is about 11 feet. Are you referring to the sill and the ledger board that the joists rest on?

BTW, the joists are 2X10...

Last edited by Richo; 12-27-2007 at 10:44 PM.
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Old 12-27-2007, 10:45 PM   #14
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Beefing up subfloor from basement


That is correct. The span is important so the deflection(flex) can be calculated.Yours calculates at L377 which is good for ceramic tile, not enough fo natural stone. You still need a minimum of 3/4" plywood above joists. If you want to avoid the extra height of the cement backerboard, you can use Ditra instead. (2x10 joists would be L687 still not enough for stone)
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Last edited by Rehabber; 12-27-2007 at 10:48 PM.
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Old 12-27-2007, 11:05 PM   #15
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Beefing up subfloor from basement


Thanks for the calculations.

Can Ditra be used over the existing particle board underlayment (with vinyl flooring adhesive residue) or on top of vinyl flooring?

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