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Unread 01-21-2019, 12:35 AM   #1
Yes si
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TH with 2nd floor concrete over wood subfloor deflection concerns

Hi all. Jesse from Houston here. handyman thrown into reconstruction/remodeling after Harvey. Still 'green" so I'm here looking for advice on how to proceed with a tub to shower conversion on a second floor Townhome bath when it comes to the shower floor structure.
the removal of the cast iron tub unveiled 2" thick concrete over plywood (i'll try and attach a pic) .The concern is the concrete has cracks through it.
I believe its safe to assume that there is enough deflection to potentially cause problems and the question is what do I need to do before I install a pan in this case?. I'll measure the joist tomorrow but by the looks its probably 2x10 or 2x12. the "head scratcher" for me is how to enter this scenario in the deflecto calculator. joists hang from a beam (sistered joist) under the shower floor. Any suggestions or recommendations would be greatly appreciated.Name:  concrete floor .25.jpg
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Unread 01-21-2019, 08:39 AM   #2
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I’m hoping it is as straight forward as installing a reinforced mud bed with a cleavage membrane?
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Unread 01-21-2019, 12:08 PM   #3
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I wouldn't assume there is deflection especially if the 2" deck mud was not properly installed (cleavage membrane + diamond lath or other approved lath). You didn't mention how it was constructed.

The only way you'll know if there's deflection is to measure the joists, subfloor thickness and unsupported span and use the deflection calculator on the site. With your specific scenario, I'd wait for one of the pros to chime in on the structure.

I would tear up the deck mud in the area over the plywood and either add 1/2" exterior grade plywood over it and 1.5" deck mud with tar paper + lath and use a thinset slurry to bond old deck mud to new.

Or

Tear up the deck mud, 2" deck mud with tar paper + lath and use a thinset slurry to bond old deck mud to new.

Both of these options are providing the floor passes the deflection test.

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Unread 01-21-2019, 01:20 PM   #4
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Thanks for replying Ali.
The floor joist are 2x9.25 on 16” c . Unknown species With 1/2 “ plywood. It does not have a mud deck . The entire 2nd floor has what appears to be a 2” thick un-reinforced concrete layer. Concrete runs under wall framing and under adjacent rooms’ carpet. The deflecto meter is giving me an L655 but my concern is the actual way the joist are laid out. (See pic) the bathroom complete rectangular dimensions are 11x5 and the joist run parallel to the longest side The 3rd and final joist is spaced 8” oc. From the last one (it is sistered) to accommodate the shower and toilet plumbing past it and beyond that point the next joist is 4 feet apart. Although that unsupported span is divided up by the adjacent bathroom which mirrors the one in question there is still a 22”x22” unsupported span directly under the front of this shower space to be. This might be basic stuff for some of you pros but those joist mentioned are hung on a sistered joist running perpendicular that . So to me most the load is on the perpendicular joist. Hope this makes some sense.
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Unread 01-21-2019, 01:49 PM   #5
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If it helps, my shower pan floor joist spacing was 16"OC with the exception of the area just below the shower which was 24"OC. IIRC CX (resident pro builder) recommended 1/2" exterior grade plywood with a face grade no less than C glued and screwed to the existing subfloor (not to the joists) to eliminate lateral deflection. I have a 1.875" deck mud pan tapering down to about 1.125" with no issues.

Again, I'd wait for the pros to chime in and make suggestions as my experience falls under the DIY realm.

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Unread 01-21-2019, 02:46 PM   #6
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Welcome, Jesse.

You do understand that the joist deflection is calculated over the entire unsupported span of the joists, not just the portion under the room to be tiles, yes?

I would also like to know more about that "concrete" layer you have. It's much more common to see poured gypsum-based underlayment in applications such as yours.

Quote:
Originally Posted by Ali
CX (resident pro builder) recommended 1/2" exterior grade plywood with a face grade no less than C glued and screwed to the existing subfloor (not to the joists) to eliminate lateral deflection.
I bet if you remember further, Ali, CX told you that might be an option if your existing layer of plywood was in near pristine condition and the second layer was laminated with a full spread of wood glue and lots of mechanical fasteners. With the glue-lamination it is not important that the mechanical fasteners do or do not penetrate the joist structure.

And please note that the purpose was to decrease the between-joist deflection, a vertical component, and has nothing to do with "lateral deflection." We do wanna be careful not to confuse new visitors, eh?

Tell us more about that two-inch layer of material, Jesse, and we'll work from there.

You are/are not planning to tile the bathroom floor as well as the shower?

My opinion; worth price charged.
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Unread 01-21-2019, 03:16 PM   #7
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Thanks again Ali.
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Unread 01-21-2019, 03:42 PM   #8
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Hi Cx. Thanks .. and thank you for your help.
1. Yes
2. Only installing tile in the shower. The bathroom has travertine in good condition. You would think that would be a sign. Just need to make sure.
3. As far a it being gypsum based , that’s a probability. I just don’t know what that looks like. I imagine drywall mud white.
I’ll describe what I see in a piece I just took off and magnified with my phone. (Pic failed to attach)
It’s a hard light gray substance with what appears to be sand sprinkled throughout. I’m sure that’s not very helpful . I’ll post a pic this evening. But Its tough and running a deck screw across the surface barely scratches it . In fact takes the coating off the screw.
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Unread 01-21-2019, 03:52 PM   #9
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Quote:
Originally Posted by cx View Post
I bet if you remember further, Ali, CX told you that might be an option if your existing layer of plywood was in near pristine condition and the second layer was laminated with a full spread of wood glue and lots of mechanical fasteners. With the glue-lamination it is not important that the mechanical fasteners do or do not penetrate the joist structure.

And please note that the purpose was to decrease the between-joist deflection, a vertical component, and has nothing to do with "lateral deflection." We do wanna be careful not to confuse new visitors, eh?
That is correct on the glue/screws. Apologies on the nomenclature, by lateral deflection I meant between the joists.

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Unread 01-21-2019, 08:22 PM   #10
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ok
Here is a close up. looks on the lighter side.
could it be gypcrete?
and how does it make a difference if it is or if its concrete ?
easier to tear out maybe?
its a monolithic pour throughout the 2nd floor and runs under bathroom framing.
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Unread 01-21-2019, 09:25 PM   #11
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Gypcrete can be scratched fairly easily with something like a screwdriver.

In a multi-dwelling building, that layer helps with both sound and fire resistance, so you need a similar material back down to preserve that.

It looks like there's a bunch of aggregate in that mix, which may lean more towards concrete verses gypcrete.
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Unread 01-21-2019, 10:47 PM   #12
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Thanks for chiming in Jim.
Your guess is better then mine.
So the question is if I reinforce the joist do I still need to rip this out and redo due to the cracks or could I install a "pan" over it
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Unread 01-23-2019, 11:30 AM   #13
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see draft ? .
Option 1: demo the 2" concrete/gypcrete and add a couple layers of plywood then mud or prefab pan
challenge with that is the wall framing sits on top so in order to not disturb the wall "base" I would have to cut it with a grinder. ok more work but do-able.
BUT, should I be concerned that the loaded longer side of the shower wall that is sitting between the joist may affect load distribution to adjacent joist?
option 2: leave concrete subfloor and lay "floating" mud bed 1 1/2 minimum with kerdi membrane on top.
got to get it done very soon
I realize it's difficult to give direction on something with many variables without seeing it for yourself.
But if someone see this post and the rough draft of the subfloor layout and can make sense of it all and most importantly have a recommendation , I would greatly appreciate it.
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Last edited by Yes si; 01-23-2019 at 12:28 PM.
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Unread 01-31-2019, 01:55 PM   #14
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ok to SLC between subfloor plys?

I so appreciate the wealth of information here and have been digging in and the more I read the more I realize that I don't know squat. I've been staying up late reading and researching and I realize that I am barely scratching the surface. lol
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So, I ended up ripping out the 2" concrete subfloor ,

added another joist where the shower wall rested between two joist ( since this wall shares the load from the water heater above in the attic ,
added blocking under the plywood joint where the floor tile end and where the curb will be,
the plywood subfloor where the drain hole is at minimum 4'x4' section that does NOT have joist under it to support due to the drain intersections but rather has 2x4 cleats across from each other with a 2x4 sideways bridging the span as support holding up the plywood (had 2 original 2x4 but apparently the plumbers cut thru one. I did add another one but not convinced the 2x4's are adequate AND this section is not surprisingly warped and dips down 1/2 from the beam support.

Question 1: can I use Slc to level out the 1/2 original subfloor then install another layer or two over that?

So, my thought are to add 2 layers of sturdie type of 23/32 plywood over this half inch subfloor but not sure if 2 is overkill and


question 2: what will be the ideal fastening method in this application?
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Unread 01-31-2019, 05:58 PM   #15
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Jesse, you can either remove the useless 1/2" plywood subflooring you have or at least pretend it's not there. You could not use SLC to flatten it and then install additional plywood over it even if it were thick enough and of the correct edge style, but you needn't worry about that. Adding a layer of nominal 3/4" plywood over that area that is to be a shower should be adequate. I presume you'll be making a mud bed for your shower floor, yes?

Difficult/impossible to evaluate the structure under your floor from what we see in your photos.

My opinion; worth price charged.
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