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Old 05-20-2019, 08:17 PM   #1
willkyl
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1950's bath reno - issue with tub/wall/floor

I've spent months reading similar posts to mine and I've finally hit a point where I need to rely on the forum experts for advice. The background on my situation is, i have a ~1955 house that I'm preparing to rent but before doing so it needed a serious bathroom remodel so I ripped out everything to see the bare bones. So far I've resized a window, corrected some crazy plumbing, and I was about to start working on the shower wall tiles when...

...I saw my first issue (1) inconsistent tub spacing to wall studs.
When I started to size my CBU to the wall I noticed some studs were shimmed, some were flush, some were not even touching the tub. I think my best course of action is to sister new studs that meet the tub and secure my CBU to those so I get the appropriate tub-to-lip spacing, coat with Redgard, followed by tiles.

BUT

My second issue (2) I looked at my door threshold and my tub to see how my flooring tile will work. I noticed my tub is shimmed with a 1x board and an EXTRA 1/2" on at the right. I had anticipated adding Advantech 3/4" plywood and then Ditra but I think I will have an issue on one side of the tub with the extra shimming.
I tried to determine if a floor joist was sagging and from what I can tell...it possible...but the diagonal slats are so uneven and have been patched before that its difficult to be certain. (see pictures)

My question is what is the recommended course of action? What would you do in this situation?


1) I've considered removing the tub, removing the 1X6's to the last stud before the walls, lay new sub floor plywood (no idea what I need beyond the 3/4" advantech), and sister a new joist to the one i suspect has been weakened. Would I still lay Ditra? Or use a CBU instead?
Note: I'm a little concerned with removing the tub as I'm not 100% sure I can install it better than it is, hence my hesitation.

OR

2) Leave the tub, sister the joist (it cant hurt, right?), add a 1/2" plywood + Advantech plywood to run tiles over the tub shims, try to level the floor with leveling thinset (because I feel like that what most people would do in this). Would I still lay Ditra? Or use a CBU instead?

OR

3) A combination of the above...(guided by the knowledgeable community on this forum)

I should also note, I've measured my joists to be 1 3/4" x 7 1/2" x 12'. I suppose that's really a 1950's 2" x 8" x 12'? I'm still unsure what dimensions to put in the deflecto calc so if there's advice on that to confirm what to put down, that would be appreciated since it looks like i'm on the edge for ceramic tiles being "ok" with the numbers I used. (I also assumed SYP in good condition)

Thank you for reading!
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Old 05-20-2019, 09:39 PM   #2
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Welcome, Kyle.

1.
Quote:
Originally Posted by Kyle
I've considered removing the tub, removing the 1X6's to the last stud before the walls,
I trust you meant to the last joist before the walls? If you plan to replace your subfloor, I'd recommend you remove the boards entirely, adding joists or blocking at the floor perimeter as needed before installing your new subflooring.
Quote:
Originally Posted by Kyle
I had anticipated adding Advantech 3/4" plywood...
Again I'm guessing you mean Advantech's OSB. The only problem I have with that is that we've heard reports that thinset mortar does not like to bond well with the coating on the surface of that material. If you used the CBU you suggested, their would be no such issue, but with the Ditra you might want to discuss the plan with your thinset mortar manufacturer before you commit.

While the subflooring is removed, you should have no difficulty sistering additional joists to the existing to bring them up to the required L/360 deflection. You don't indicate your joist spacing, but even if it is 16" centers, you're below the requirement by your description, even if your guess as to the joist species is correct. And guessing at that is not a good idea. You must sister, at an absolute minimum, the center two-thirds of the joist span to be at all effective in reducing deflection.

You could sister the joists to improve the design deflection while also bringing the joist tops into plane and making the floor level if you want. The tub installation will go much easier on a level subfloor.

My opinion; worth price charged.
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Old 05-21-2019, 08:57 AM   #3
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CX, I did mean to say the last joist. I believe I have a joist on each side of the room that stops 4"-6" from the wall...which is weird since I thought it would be closer to the wall. I will need to check this.

Ok, this plan has me excited...and nervous haha

I'm going to gut the remove the remaining parts (tub and original subfloor old slats) and see how my joists sit, as far as being level.

I'm inclined to try and sister the joists the full 12' span and I'm hoping I don't have to worry about leveling the floor. Otherwise, I will have some questions to clarify how that is best done during the sistering (if that's a word) process.


I assume I would use a 2"x8" despite the existing joist being 1.75"x7.5"?
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Old 05-21-2019, 09:50 AM   #4
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Hi Kyle,

2x8 would be the way to go.
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Old 05-22-2019, 05:49 AM   #5
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I tore everything up...and took some pictures because what I found has made me uncertain about the best course of action. The smallest bit of good news is my joists are actually closer to being level than I thought. It's really only one joist that sits higher on the left side, so it should be straight forward to fix with sistering the joists.

I've labeled one picture to point out the areas of the water damage and to show the obstructions I have to avoid with my HVAC system. Unfortunately, you will see the previous contractor did some poor work when sistering the joists...I'm glad to find this now rather than later.

Question 1) When I sister joists, do I have to anchor the joists on the same side of every joist? The way my joists, HVAC, and other obstructions are it would be all but impossible to mount every joist on the same side of existing joists.

Question 2) How should I manage the left side joists? They are very close together, (maybe because of the furnace vent in the wall?) and I would say itis the "strongest" part of my floor.

Question 3) Should I be removing the right side, water damaged, joist section due to the excessive rot? I've seen this done before, just not sure how to best repair that. Same kind of applies to the left side.

Note, the HVAC will be completely redesigned in the coming months...which makes this even more frustrating because I can't exactly postpone this remodel.
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Old 06-07-2019, 08:11 AM   #6
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Quick update followed by a few questions.

I have reinforced all joists across the full span and I am now searching for flooring material and fasteners suggestions.

I have the 3/4" T&G OSB cut to size. about ready to secure but I am not sure if the #8 1 3/4" construction screws I have will be adequate (it seems like very few people use this size). It seems like the majority of people use nails+glue for cost/time savings. Can anyone recommend an appropriate screw to use?

Also, I plan on using a CBU over the plywood.
I plan on following the installation guide for hardibacker, but i don't see mentioning of avoiding or hitting joists when securing the CBU. Am I supposed to avoid the joists to allow "float"?
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Old 06-07-2019, 08:54 AM   #7
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I've used a 2.5" #8 or #9 coated deck screw with torx head with decent results.

Have you thought about using Ditra? it is much easier to install and lay tile on and it's waterproof. Would be an advantage in a rental where folks don't care about water spillage from a tub. From what I've read on here, Hardie is very 'thirsty' i.e. thinset will skin over pretty quickly on it.

If you decide to use hardiebacker, you apply thinset to the floor, bed the hardiebacker, then use screws. Screw placement over joists shouldn't matter.
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Old 06-07-2019, 09:28 AM   #8
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I had originally intended for ditra, but I also wanted to utilize the 1/2" CBU for bridging the height to the new tub I'm putting in for ease of entry/exit. It will be taller than my previous one. I also thought the ditra might not bond well to the subfloor and i would had to have delamination...but if its a better way to go i will reconsider.

Is there some thickness of plywood i can put down before the Ditra to add to the finished floor height?

I also read that people felt the CBU gives some added stiffness (maybe a false perception) to the floor.

I'm just trying to make a sold floor here with decent tile work
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Old 06-07-2019, 10:04 AM   #9
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Substitute properly attached 1/2" plywood (not CDX) for CBU and you will indeed stiffen the structure. Ditra and plywood are friends.

While CBU might have some perceptible stiffening effect, manufacturers and likewise engineers and inspectors will assign zilch, as would I. Anything else and you become the engineer.
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Old 06-07-2019, 11:36 AM   #10
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Agree with Ali, Kyle, the 1 3/4" screws aren't likely long enough. Go for the #9's at no less than 2", but not more than 2.5". The torx heads (the screws will come with a torx bit in the box) are definitely your friend, as would be an impact driver. I know Depot sells "construction" screws with torx heads in various lengths. Those are what I used. A whole mess of 'em.

As for building floor height, you can add another layer of ply over your OSB without issue, 1/2", 3/4", no problem. I'm not a fan of OSB so I'd use standard plywood. Be sure it's exterior rated, (Exposure1, etc) and no face grade lower than C. If you use 1/2" ply you'll be able to use 1 3/4" or 2" screws.
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Old 06-07-2019, 01:58 PM   #11
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Ok that confirms it, I will go get the 2" screws and use the 1 3/4" for a 1/2" underlayment for the Ditra.


For my clarification, is the RTD plywood what I should be looking for? I know I read a thread where RTD was basically discouraged but it appears to the only suitable alternative.

Thanks for all the help!

Edit:
Sorry, another followup question!
Is there reasons to NOT lay the Ditra on the OSB subfloor? Is it best to add another layer of plywood (RTD plywood 1/2" or 3/4")
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Old 06-08-2019, 05:28 AM   #12
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I'm getting a little confused, Kyle, thought your plan was to lay the 3/4" OSB, then 1/2" ply, then Ditra. That's how I'd do it if I could put up with the added height that the 1/2" ply adds.

As far as Ditra over OSB, I dunno. But I'm pretty certain Schluter knows, and addresses it in their Ditra install spec sheet.

As far a RTD - haven't ever heard of it.
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Old 06-08-2019, 05:40 AM   #13
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You can Ditra over OSB, however, I would stick with using 1/2" exterior glue plywood with face grade no less than C to build up your floor for ditra.

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Old 06-08-2019, 10:51 AM   #14
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Kyle, Schluter is perfectly happy to have their Ditra installed directly over OSB, but the bonding mortar manufacturer's may not agree. And I wouldn't do it unless there was no other option. If you plan on a second layer of subflooring, I would make it plywood.

As for the RTD, if you hafta pay a nickle extra for it, I wouldn't bother. An AC or BC plywood with an exposure rating of Exterior or Exposure 1 is more than adequate for your application.

My opinion; worth price charged.
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