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01-17-2017, 07:59 AM
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?
Would you recommend putting down 1/4 ply to sturdy is up before adding durock?

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

Thanks in advance for help

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01-17-2017, 08:57 AM
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.

01-17-2017, 09:44 AM
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?

01-17-2017, 09:57 AM
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.

01-17-2017, 10:22 AM

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

Todd Groettum
01-17-2017, 10:25 AM
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 ;)

01-17-2017, 02:20 PM

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?


01-17-2017, 03:19 PM
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.

01-17-2017, 03:42 PM
Sorry initial post had that info on it

2x10 spaced 16"oc

01-17-2017, 04:37 PM
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.

01-17-2017, 04:49 PM
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 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.

01-17-2017, 04:50 PM
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!

01-17-2017, 05:52 PM
Few pictures. Second post coming

01-17-2017, 05:53 PM
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

01-17-2017, 06:25 PM
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.

01-17-2017, 07:44 PM
Welcome, Devin. :)

I'm not seeing any trusses. Looks like dimension wood joists to me. How tall are the joists?

01-17-2017, 08:04 PM
tall as in dimension of truss? 2x10
or height from floor?

Sorry I'm a bit green with this whole process

01-17-2017, 08:12 PM

I'm thinking the best course of action is to tear out the underlayment and replace with a layer of 5/8th and 1/2 followed by ditra.

The joists seem in good condition. I'll look at adding a support post to split the difference between the joist span to decrease the deflection.

Thanks again guys for all the help!

01-17-2017, 08:27 PM
Devin, you don't have trusses, you have dimension wood floor joists. The dimension I'm asking about is the height of each of the joists, top to bottom.

Steve in Denver
01-17-2017, 08:29 PM
I haven't read the whole thread carefully, but if height is an issue, I'd probably pull the OSB that you have and put a layer of 3/4" T&G plywood then Ditra on top. I'm not sure if new 5/8" + 1/2" would be worth the extra money and effort, or not...but I'm sure the pros will have an opinion on that. :)

Edit: To clarify the OSB vs Plywood point...I'm actually pretty happy with how OSB has worked for me, but as far as adhering the Ditra to it with thinset, I haven't had much luck finding an approved combination...hence my preference for plywood for the top layer.

01-17-2017, 08:47 PM
Shoot CX I meant Joist when i actually typed Truss.

9 1/2" is height

01-17-2017, 09:09 PM
If they're 2x10s spanning 13 1/2 feet and of good grade and species, you should be fine from a deflection standpoint. from what I see in your photos, you appear to have some cracking and not a particularly good grade, but we can't see enough of your lumber to make a well calculated guess. That will be up to you. Finding a grade stamp would be helpful.

Given the apparent condition of your subflooring and the fact that not all your joints are supported, I'd be more worried about that than the joists, though. You say it's OSB and I can't even be sure of that looking at the photos. I'd be inclined to side with those recommending replacement.

My opinion; worth price charged.

01-17-2017, 09:36 PM
I'll dig around and see if I can find anything regarding the quality of joists.

Will look to replace the original subfloor before moving onto Ditra and tile.

Would you advise one way or the other when it comes to 3/4 TG or doing two sheets of 5/8th to follow along with what is in the rest of the house?

Can't thank you guys enough for the advice. You'd be surprised at how many contractors I've reached out to for work and can't even get a call back. So I've started to try and DIY it.

01-17-2017, 09:42 PM
I would recommend a first layer of nominal 3/4-inch T&G plywood or OSB followed by a second layer of nominal 1/2-inch plywood.

Here is a good article ( from our (Liberry) showing what I think is the best method of installing the layers.

My opinion; worth price charged.

Steve in Denver
01-17-2017, 10:11 PM
Keep in mind the panels should have an exposure 1 or exterior stamp on them - the normal glue can't handle the moisture from the thinset, and the panels can de-laminate. It's my understanding that this applies to all layers, even if they won't be in direct contact with the certainly applies to the top layer. Additionally you want your plywood to have no face grade lower than C. So BC or CC is what you are most likely to find (in other words, CDX won't cut it - the D layers can have internal voids that permit vertical movement, which makes the tile sad)

As far as the two layers vs one...I've been quite satisfied with a single layer of 3/4" T&G subfloor, but I certainly could tell a difference on the one floor where I added a 1/2" layer above that. I suspect, but am not certain, that the preference for 3/4 then 1/2 (compared to 2 layers of 5/8) is that the base 3/4" layer will hold the screws (or ring shank nails) more effectively than a 5/8" layer.

If you read the article that CX linked to, you will notice the details about the orientation of the strength axis, the position of the second layer vs the first, and not screwing the second layer into the joists...what you may not appreciate at first glance is just how many screws each panel will eat - make sure you have enough. (and don't over look the part about leaving a gap between panels and around the perimeter where it abuts a wall or other immovable object)

CX always recommends pre-drilling the top layer to prevent screw jacking - I'm not altogether convinced it's necessary (the screws I use have a smooth shank at the top), but I do it anyway. In any case you most certainly do want to get that top layer to lay flat to the bottom layer - no voids. Not only will tile not tolerate vertical movement, to get the full benefit of the two layers, they need to act as one...hence the 80 or so screws per panel. :) Start from the center of the panel with your screws, then work your way out radially...if you work outside in, you could have a wave or ripple that doesn't end up laying flat. You don't want that.

01-17-2017, 10:51 PM
Just a point of clarification, CX does recommend pre-drilling the top layer when fastening two layers of sheet goods when the top layer is thicker than nominal half-inch material. Your mileage may vary.

01-17-2017, 11:07 PM
Your deflection will be okay if the wood is SYP or Douglas Fir, but if not, then it is not at that span. The type of wood does make a difference in the strength, as does their state (extra holes if poorly placed or any notches in either the top of bottom). FWIW, something like Advantec is actually stiffer than typical ply in the same thickness. The issue will be whether you can find a thinset that works on it. NOt all OSB panels are created equal.

Todd Groettum
01-18-2017, 08:38 AM
Screw top layer using 1 1/4" screws every 3-4" at all edges seams and every 6-8" through the field of each sheet...Keep in mind More is better, Less is NOT!!

We are avoiding screwing top Underlayment panel to Joists....

Follow all manufacturer recommendations for installing Ditra..:tup1: