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01-25-2014, 02:27 PM
I'm a new poster with a mostly gutted bathroom waiting for surgery. My bathroom floor is 5' x 5.5' with water damage next to the tub and weak at the seams.

My plan is to pull out the 1/2, put down 3/4 T&G and cap that with 1/2 ply wood and 1/4 hardibacker..

My conditions are 2x8 joist on 16" center with a 11' 8" span. Attached is a photo of the wood stamp. I have plugged the numbers into the AWC maximum wood span calculator and get a max span of 12' 10". I'm planning on putting down 12"x24" porcelain.

Question 1 - when I cut the floor out I'm faced with one joist being located under a framing 2x4 located behind my drain/vent pipe (see attached). The other hidden joist in located about the same distance under the tub. Can I sister two 2x6 60" long so I have a ledge to anchor the 3/4 ply to on those perimeters? Is there a better work around for such a issue?

#2 - I know the new 3/4 sub floor should be installed perpendicular to the joist but is the 1/2 ply also the same direction with caution on overlaps?

Thanks in advance for your help.


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01-25-2014, 03:21 PM
Welcome, Michael. :)

Your joist structure, as described, is a little lean to satisfy our Deflectometer, but it likely passed code compliance inspection when built. Up to you whether you wanna install ceramic tile over it. I probably would in your application except I'm having difficulty figgering how you can have that joist spacing as I look at the nail pattern in your existing floor and read your room dimensions.

Yes, you can add 2x6 ledgers for nailers at the perimeter. You'll need similar support on the between-joist edges, too.

All layers of structural subflooring must be installed with the strength axis perpendicular to the joist structure.

My opinion; worth price charged.

01-25-2014, 03:45 PM
Wow CX, good eye. I just measured your questionable joist dimensions and the first two joist from the drain/vent pipe are on 12" centers. The other three to the tub are 16" center.

I know that variable can't be plugged into the calculator but what's your thought on how it strengthens the overall floor? Or does it even come into play?

Yes, I was planning on doing the in between support, too.


01-25-2014, 09:16 PM
No, usual bad eye, Michael. I was thinking you had 24" on center spacing with that subflooring joint kinda inna middle there. What you indicate you have does improve your situation in the area of the closer spacing. Wouldn't change the plan for changing out the subflooring, though, were it mine.

01-26-2014, 12:07 AM
For what it's worth, I just had to do the same thing along both walls in my bathroom, used a clamp to hold the first 2x6 tight up against the old sub under the sole plate, glued and screwed, then the same with the second 2x6, using screws that went all the way through to the joist, worked really well, was very solid. I used GRK Rugged Structural Screws for the final screwing (my new favorite screws, these are awesome!), the big washer head pulls everying nice and tight.

01-26-2014, 09:13 AM
Fred - thanks for the input,

If you went through two 2x6s what overall length screw did you buy? I'm assuming you predrilled both boards too?

Is there a best wood glue to use I should be aware off? We have some good wood working shops around so bypassing the box stores if need be isn't an issue for the product.

01-26-2014, 12:25 PM
Michael, I use the Grip Rite ( deck screws and Locktite PL ( polyurethane series adhesive (3x and 8x strenghts).

Re the length of the screws, I'd use the 2 1/2 or 3 inch (I prefer 3") length. Clamp glue and screw the first 2x6 into place and then attach the second ledger to the first one the same way. It would be great if you can drive screw through the wall plate into the ledger boards too.

No need for predrilling with the 3" screws, the screws have long shanks that will prevent screw jacking.

Good luck.

01-27-2014, 12:17 AM
I used a 4 1/2" for the second board, and I did not predrill anything, but I did do a little mock-up test with some scrap lumber in my garage to see if jacking would be an issue, and because I developed some what of a fastener fetish haha I tried a few different screws. I, like PC used PL adhesive. I could easily reach the bottom of the two boards just to make sure everything stayed tight. I did find it much easier to start the screws in the board before I put it in place, where I could anyway.

02-27-2014, 12:46 PM
Thanks for all the help to date.

I have an underlayment question now and my picture shows what I'm thinking of doing.

I took out the very weak 1/2 sub and put down a nice sturdy 3/4 and now want to confirm my thoughts about the underlayment panels. I've read the article referred to on this forum and think my layout covers the concept, but only on a smaller scale. No 4x8' sheet in this bathroom but rather two 30" x 67".

The dash lines are 1/2 panels, does my layout look correct? Of course I'd follow the screw pattern referred to in the paper.

02-27-2014, 02:36 PM
I would not make an extra seam just to facilitate 1/2 panel overlaps, Michael. Can't see your dimensions, but I'd be more inclined to lay a full 48" width and an 18" piece on the terlit end.

You installed blocking under the joint between your first two panels?

My opinion; worth price charged.

02-28-2014, 09:31 AM
What CX said. The article does say to stagger seams, but it is assuming a normally staggered subfloor, in my humble learnings a more accurate rule would be to have any subfloor seams straddled evenly by the underlayment, if that makes sense. Which would go along with the earlier suggestion to put a full width piece centered over the seam in the subfloor.

02-28-2014, 12:54 PM
Yes CX, I have blocking for that seam. In fact, that's them in the corner.

Are you suggesting laying a full 48x67" panel down over the seam and then cover the remaining top 12" with one strip? No 4" over the joist and underlayment end joint butt rule?

The picture shows what I think you're suggesting, let me know different is so.

I've read the underlayment end joint butt rule several times and under stand the concept but might have a harder time actually seeing it used in my small 5x5.5' area, that is without all the seams from the photo before.

Less seams would be much easier but I'm not about the easiest but rather the correct way.

Thanks for the input.

02-28-2014, 02:30 PM
You'd be better off with a full sheet in the area where all the foot traffic is gonna be, at least that's my take on it. A seam that runs under the toilet 12" from the wall isn't going to get much foot traffic. The quarter-point rule is more applicable in a larger room.

02-28-2014, 03:01 PM
Michael, if you don't need seams, don't make seams. That article is for floors where multiple panels must be used.

05-27-2014, 06:32 PM
Once again I'm so glad I found this site.

I'm back with situation and need some advice. I'm laying 12 x 24 porcelain tile on ditra and started this process yesterday. I mixed Laticrete 317 for my bonder before knowing this thin set can't be use with porcelain without adding 333.

Upon putting the thin set to ditra I realized my mix was too loose. I could comb it with the trowel and get the ridges but after a minute they started to spread. Using the rule of not adjusting the mix after it has sat I ditched the bucket. Volume lost was half a bag. Total thin set put on ditra was enough to lay one tile and a few rows of waffles past the tile.

I'm now going to find Ditra Set and use what works.

My question,

1) can I put the new ditra set over the 317?

2) do I need to remove the 317 from the pockets before putting ditra set down?

The total area of prefilled waffles is 15" x 31". I'll also add that the area I put this down is behind the toilet at the valve end of the tub. No one will ever step on this tile if it matters.

Thanks for the help, again...

05-27-2014, 06:43 PM
1. Michael, the Laticrete 317 will work fine with your porcelain tiles. See Post #7 here. (

2. No.

My opinion; worth price charged.

05-27-2014, 06:45 PM
You can go right over it, provided the Ditra cavities are properly filled.

Many times, I fill the cavities the day before I set tile. Could be because I have thinset left over, or some extra time, or just so I can pop some chalk lines and not have them disappear later in the day. Doesn't hurt a thing.

You will still need to key thinset into the area where you filled the cavities. Not so if they haven't been filled.

05-27-2014, 07:19 PM
I can't argue with your bond there cx, I hope to never need a crowbar in my bathroom, too.

If you were able to create such a great bond on your tile then why is it mentioned in Laticrete's 317 data sheet that porcelain is not a suitable tile?

I don't mean to beat a dead horse but there's peace in understanding one is doing things correctly.

05-27-2014, 07:29 PM

doesn't the fact I had a little more water than desired play a role to the negative? Wouldn't what's down be weaker that what will be on top?
The fact that the tile will never see any pressure a plus too?

I did get the waffles filled fully.

One side note- the unmodified thin set dried to a very light gray color compared to when I used the 333 additive and drying to a very dark gray.
Is this normal. Could this be from the type of water I have coming from the tap?

05-27-2014, 09:39 PM
A little bit of difference in the mixture can make a big difference, not to mention the drying times and color.

But unless you made soup that you could pour onto the floor and it ran into the cavities very quickly and filled them just perfectly with no effort from you, I wouldn't worry about. Too much water can make for a weak bond, but you're not bonding to anything here. You're just filling some holes.

Next batch, mix thoroughly and check the consistency before you let it slake. Doesn't hurt to spread a little out to see how it's going to work, then put it back in the bucket and find something else to do for 10 minutes.