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Unread 01-17-2017, 07:59 AM   #1
devin.m
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Sub floor questions

Hey all,

Recently started the undertaking of redoing our bathroom. I've run into a slight question regarding the soundness of our OSB sub floor.

When pulling off the original tile we have 5/8th OSB attached to the joists, followed by a 5/8th PLY. Tile over the top.

We pulled up the layer of plywood in order to replace with 1/2" durock.
After walking on the OSB i noticed it is bouncy in spots.
Would the 1/2 durock with a solid thinset get rid of the bounce enough for a 4"x24" tile?
OR
Would you recommend putting down 1/4 ply to sturdy is up before adding durock?

PS.
Floor joists are 16"oc 2x10s
Bathroom floor is 6'x7'


Thanks in advance for help
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Unread 01-17-2017, 08:57 AM   #2
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Welcome Devin.

1. The Durock adds nothing in terms of structural rigidity. Think of it as an interface between tile assembly and substrate.

2. You need the additional layer of plywood to make things more ridgid between joists. If you're looking for minimal height gain, a membrane like Ditra on top of ply in lieu of cement board will help. The 1/4" ply just wouldn't add much bending resistance...I'd steer towards a minimum of 1/2"

3.The room size matters little, but rather the span of the joists between two points of support. If this has an open joist structure (like crawlspace), a better view of the joists and support can be had there. If not, more sleuthing is needed to determine suitability for tile installation.
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Unread 01-17-2017, 09:44 AM   #3
devin.m
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Thanks for your quick reply.

I was looking for a small height gain as the tiles I purchased are a bit thicker than the tile originally on the floor.

Would it be acceptable to brace the floor joists in sloppy spots around toilet drain go 1/2" ply and 1/4" durock? or stay consistent with the 1/2 durock that I currently have?

Ditra sounds like a great option but cost is a contributing factor. What would be an estimated cost be on ditra per sq ft?
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Unread 01-17-2017, 09:57 AM   #4
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Welcome, Devin.

Don't know what you might mean by "brace the floor joists in sloppy spots around toilet drain," but if you have a joist problem it certainly needs to be fixed before you tile.

As Peter pointed out above, using thicker CBU is of no advantage at all unless you simply want to raise the height of your tile installation, which doesn't sound like it's what you intend.

If you'll add a geographic location to your User Profile so it remains permanently in view, you might get someone to guess at the cost of Ditra in your area, but you'd be better off just going out and pricing the available materials and comparing them. Keep in mind the amounts of necessary thinset mortars and mechanical fasteners in your calculations.

My opinion; worth price charged.
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Unread 01-17-2017, 10:22 AM   #5
devin.m
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CX,

Sorry the play in the OSB sheet around the toilet is caused by the OSB edge not falling on the joist.

Best course of action sounds like add back the original size of plywood (5/8th) and replace cement backer with Ditra.

I'll update my profile, but for now Onalaska WI
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Unread 01-17-2017, 10:25 AM   #6
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OK, i will say it...If you are going to cut corners nobody here can help you end up with a job well done..

Follow Peters advice!!

If height is a concern, use the Ditra rather than the CBU or go to a 1/4" wonderboard CBU...

Since you already started, get out that credit card and put it to use
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Unread 01-17-2017, 02:20 PM   #7
devin.m
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Todd,

No corner cutting, just maybe assumed I had a better idea of this process than I actually do!

Went and priced Ditra and it looks like it'll cost me about 200 to cover the floor.

So with the 1/2 or 5/8th will do the trick to take out the bounce.

Next question. Slight slope in the parts of the floor.
What's the best course of action to negate that. its right in the middle of the floor and about 1/8" deep.

Would adding that 5/8th plywood take that out just by running the sheets in a different direction than that of the 5/8th OSB attached to the joists?

Thanks
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Unread 01-17-2017, 03:19 PM   #8
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You have not answered the critical question yet, what is the actual span of the joists? Not the size of the room, the unsupported span. All the plywood in the world won't take care of that, if it is inadequate.
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Unread 01-17-2017, 03:42 PM   #9
devin.m
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Sorry initial post had that info on it

2x10 spaced 16"oc
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Unread 01-17-2017, 04:37 PM   #10
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Quote:
Originally Posted by Devin
Sorry initial post had that info on it

2x10 spaced 16"oc
And those 2x10s are supported by.....?

There's more too it, Devin. The parts that connect the joists to the earth, specifically.

I'd also have some concern for OSB not ending on joists. Very unusual (and not good) if that's original to structure.

Here's a visual on floor framing.
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Unread 01-17-2017, 04:49 PM   #11
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The first layer of subflooring must either be T&G so the long edges are supported, or have blocking installed along all of them to prevent deflection along that seam between the sheets. The plywood primarily prevents deflection BETWEEN the joists, and does very little along the joists (it does some, but the strength is needed between the joists...the joists themselves MUST provide the overall strength needed on the floor). You MUST install all layers of ply so they cross the joists...you can't crisscross them- they have only one primary strength direction, and that must cross the joists, not be aligned along them. Without knowing how long those spans are with the joists, there's no way to tell if the joists, regardless of the subflooring, will provide for a suitable floor for tile. The magic here is how long are they between supports: either a load-bearing wall, or some beam, either wooden or metal. If the first layer, whether it's OSB or ply must be intact, not water damaged or improperly installed.
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Unread 01-17-2017, 04:50 PM   #12
devin.m
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My bad.... feeling really smart now....

I get what you guys are looking for now. Will update here shortly when I head over.

I'll snap a few pictures and see what you guys think.

Thanks again for all your help so far!
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Unread 01-17-2017, 05:52 PM   #13
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Few pictures. Second post coming
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Unread 01-17-2017, 05:53 PM   #14
devin.m
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second set of pictures

13'6" span of trusses unsupported between exterior concrete wall and beam

Last picture of first set of pictures shows seem not hitting truss. Thats where the most flex in the wood is
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Unread 01-17-2017, 06:25 PM   #15
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That deflection at the edge of the sheet in one of your pictures is the reason why they specify T&G underlayment! You'd need to install blocking to support all of those edges, or tear out that layer and put down an acceptable new one. Given it doesn't look all that healthy, I'd be more inclined to tear it out and replace.

But, given the lengths involved (span), it looks like you need to beef things up to support tile, as it's a bit over L/290, and needs to be at least L/360 for ceramic tile (porcelain is a type of ceramic). That's selecting unknown species, but in good condition for the joists. If there are significant holes bored into it, or notches for plumbing or ductwork, then it could be even lower. As I said, added subflooring will not correct for inadequate joists. Two choices, shorten their span by adding a beam or load bearing wall, or sister them.
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