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Unread 10-23-2018, 08:43 AM   #1
Phidelt414
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Joist Deflection Calculation

Hello all, I am new user and have found all the information on her super helpful.

I am in the middle of a major renovation of a master bathroom and am down to the subfloor. (the previous owner had ceramic flooring that looked to be in good shape, but i don't know how long it was down or when they did it. I've only been in the house a little over a year.) I am hoping to put a 12x24 porcelain tile on the floors, 10.5' long vanity cabinets (21")deep, with quartz counter top and a kohler, acrylic shower base with 5 x 10 (or around that ballpark) ceramic tile shower walls.

I looked under my floor and saw my floor joist (bathroom is on a 2nd floor) are 2x10 (9.25)16" on center and look to be fir (just like the dimensional lumber lowes sells but how can I be sure; I can't find a stamp?) They seem to span 14.27 feet, with one end (Middle of the house) using joist hangers and the other end sitting on a exterior 2x4 wall it looks like. At 14.27 feet it doesn't seem the minimum deflection criteria is there for ceramic/porcelain. If I measure to the edge of the wall it sits on (hence measuring just the unsupported part, its measures at 14.02'..which is very close to 14' which is viable.

My question is (1) am i measuring just the part of the joist the wall isn't under and (2) will the .02 be a deal breaker itself?

I really appreciate all the insight guys!

Hope your having a good day!

Mike
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Unread 10-23-2018, 09:47 AM   #2
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(1) The span is just the unsupported length, so the part of the joist without any structure to bear on.

(2) 0.02" is negligible. In fact, I doubt you can measure the unsupported span to that accuracy.

BTW, the (fairly conservative) Deflecto tells you that at 14', your DF 2x10s @ 16" o.c. will have a deflection of L/380, and at 14' 6", the deflection would be L/347. So the allowable span for L/360 deflection is somewhere in between, probably 14' 3".

Cheers, Wayne
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Unread 10-23-2018, 10:10 AM   #3
Phidelt414
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So are you saying I should be find to put the porcelain tile on there???

Also, how can I verify the type of wood the joists are since I can't seem to find a stamp?

This DIY'r really appreciates the info and help!
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Unread 10-23-2018, 10:16 AM   #4
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Welcome, Mike.

I don't buy lumber at Lowe's and you don't have a geographic location in your User Profile, so it's difficult to tell from your description or a best guess what you might have for joists.

If you can't identify the wood species and grade by visual inspection, finding someone who can would be your best resource.

If your house was built in a code compliance jurisdiction during construction, it's probable that your floor structure met building code and that would, technically, qualify it for a ceramic tile installation. I'd rather see it meet the requirements of our Deflectolator, which, as Jim pointed out, is rather conservative.

My opinion; worth price charged.
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Unread 10-23-2018, 10:23 AM   #5
Phidelt414
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I am in the Midwest; Missouri to be exact. The house was built in 1978.

Assuming my house was built to code and NOT taking the 3 inches of joists that is sitting on the wall, out of the measurement, I measured 171.25". That equates to 14.2708333 feet. The Deflectator doesn't go in small increments....14' is ok, 14.5' isn't if I am reading it right. Is there a way to know for sure? HOW conservative is it? I am scared to get all this in and then have to do a complete gut job down the road....the wife's wrath is real

Do you believe a structural engineer and/or a carpenter, would be able to define the wood species just by looking at it?

Thanks,

Mike
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Unread 10-23-2018, 02:14 PM   #6
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If you don't add that geographic location to your User Profile it'll be lost before we leave this page, Mike.

I can likely find you any number of lumber span tables that will tell you your joist structure meets L/360 deflection requirements, but only if you know the wood species and grade.

You can't assume your house was built to code unless you at least know it was built within a code compliance jurisdiction. I've built a number of houses in my time and only a couple were within such a jurisdiction. Mine all meet or exceed code requirements, but there is no record of that anywhere nor were there ever any compliance inspections on most of them.

If you post some photos of your joists, showing lots of the joist, someone here may be able to give you a good guess. Operative word there is may. Having someone actually look at your joist structure would be a better option.

My opinion; worth price charged.
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Unread 10-30-2018, 12:29 PM   #7
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Ok...I haven't had the opportunity to get a photo yet, but have had a guy who does restoration work, as well as a carpenter, come by separately and both tell me the joists are pine (one said "standard yellow pine?")

I'm having a structural engineer out this afternoon. We will see...
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Unread 11-07-2018, 02:37 PM   #8
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Good Afternoon All, hope your week is going well.

Update: I had a carpenter tell me my joists are Yellow Pine. I had a structural engineer agree. This puts the deflection within good parameters for the porcelain tile I was wanting to put down. Its 3/4 plywood down currently over 2x10's at 16oc.

New problem: I want to make sure this sub floor is level and secure (not only for appearances, but a low profile shower base requires it for proper draining.) but we have found that the last joist at the end of the room is lower than the rest. It supports the plumbers chase that spans that area (apparently common for 2 story homes to provide room for plumbing between floors). The structural engineer said the joists were in good shape so there isn't a strength problem to be fixed, its is probably just deflection from when it was build 40 years ago.

My carpenter recommended, since there is no risk of it slipping off the joist (having the chase there), to glue and screw a plywood board the appropriate thickness to the joist/chase floor and then lay and secure the 3/4 inch plywood back on top of that to bring the subfloor to even.

I don't want to do the wrong thing, but it sounds like a reasonable solution; wanted to get your opinions. Attached is a picture of part of the area the subfloor was removed.

Appreciate your insight and support!
Attached Images
  
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Unread 01-10-2019, 03:23 PM   #9
Phidelt414
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Missouri Bathroom Project

I thought I had a thread on here at one point, but using the search tools I haven't been able to find it, so if I am simply doing something wrong, I appologize for starting a new one with a more encompassing title.

The good news is I have had my sub floor deflection quetsions resolve and appear to be within the requirements I need.

I have been doing a Master Bath remodel and am on the the Shower now. I have a Kohler shower base in place and Durock on the walls with the seams all taped and embedded in thin-set. As of right now, my plan is to utilize the Kerdi Membrane on the walls and have researched all the requirements and installation process for that.

Two questions that are not connected, but perhaps someone could help with....

1. I heard that once Kerdi-fix dries, it can only be removed by mechanical means......would a razor blade cut it or is it so hard only a grinder would work? Just curious as to HOW permanent it becomes....

2. I haven't decided yet, but i might actually carry my Kerdi Membrane out past the shower base a bit and under the bullnose there, if I do so, it would be going over a seam that was mudded by my drywall guy, between the Durock and the drywall. (After talking with USG Durock, I was told to seal the Durock with type 1 ceramic adhesive before he used the compound. Then I had him utilize a Alk-resistant mesh tape over the seam to be sure and he used setting compound.) If i wanted to carry the membrane/thin set over the drywall joint, i'm told that is possible if i seal it first... What type of sealant am i to use? i saw someone once mention something like Zinssers? Is there a certain type and/or a reason for it?

Appreciate any insight, you all are a wealth of knowledge!
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Unread 01-10-2019, 05:47 PM   #10
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I've combined you with your original thread, Mike.

1. Kerdi Fix remains flexible and can certainly be cut with a razor blade if desired.

2. Not quite sure just what USG might have had in mind for that joint, but "sealing" the Durock with a "type 1 ceramic adhesive" (organic adhesive or mastic) makes no sense to me at all.

That said, if you have covered the joint with regular drywall compound you will have no difficulty applying Kerdi membrane over that. Some folks like to use a drywall primer (paint). Zinsser makes such a primer. Your local home center might carry Kilz products. And all actual paint stores will have one or more such products. I've never primed any drywall mudded joints prior to installing Kerdi.

My opinion; worth price charged.
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Unread 01-10-2019, 06:32 PM   #11
Phidelt414
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yeah, believe it or not, they say:

"3.5 Joint Treatment Application

For Dry Untiled Areas—For small areas where the USG Durock® cement board will not be tiled, such as a board
extending beyond the tiled area and abutting another surface, treat joints as follows. Seal USG Durock® cement
board with Type 1 ceramic tile adhesive. (Mix four parts adhesive with one part water.) Embed USG Sheetrock® Brand
joint tape over joints and treat fasteners with USG Sheetrock® Brand Durabond® setting-type joint compound (45 or
90) applied in conventional manner. Flat trowel USG Sheetrock® Brand setting-type joint compound over board to
cover fasteners and fill voids to a smooth surface. Finish joints with at least two coats USG Sheetrock® Brand ready mixed joint compound. Do not apply ready-mixed or setting-type joint compound over unsealed board."
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Unread 01-10-2019, 10:11 PM   #12
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Well, I must admit that's a new one on me. Never seen that before. All the products in that scenario would be from the same manufacturer, USG, so following their recommendations would be the usual rule. Not entirely sure you could actually get me to do that, though.

Maybe they foresee the formation of something like ettringite as a reaction of the gypsum and Portland cement in the board, but that sounds completely backward from what little I know about that.

But if you've got some mastic laying about and wanna mix up some of their primer and finish the joint per those instructions, couldn't anyone fault your method.

And now you've got me very curious about their reasoning. Unfortunately we no longer have a USG technical rep as a member here to help us with the question. Hell, in just a few months there won't even be a USG since they've been bought out by Knauf and I dunno if they'll keep the USG name in this country. But I know a former USG rep that might be able to shed some light. I'll axe him.

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Unread 01-11-2019, 05:32 AM   #13
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Talking Bathroom project

Once again, ol’ CX is on the right path. I’ve been gone from USG over 5 years now, and literally ALL my contacts inside that knew anything about Durock are gone, my how that company has changed. That being said, I imagine some new MBA college folk have determined that they need to only sell USG products and they decided to “concoct” that lovely statement plan they provided you.
In my experience, it will work, however, unless they’ve changed the formulation of Durock it just isn’t needed. They are trying to modify a drywall level 5 finish plan to use on Durock. Gypsum Association is their “guiding light” document instead of Tile industry data. Recommendation of the “paper type” drywall tape in a potential high moisture area proves this to me, PLEASE DON’T DO THAT! Use the wet area mesh and forget the drywall mud compound as both will attract and hold moisture to form future mold,
Like CX, I’d just use the Schluter products over the Durock and know you’re safe and be completely comfortable that you’ll have no problems.
Finally, again as CX stated, soon the USG won’t really exist as Knauf will absolutely cut all liabilities on prior sales of products so you’ve got zero warranty from them anyway.
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Unread 01-11-2019, 08:23 AM   #14
Phidelt414
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Yeah, I was just curious and that i why I called. Since after hanging the Durock and taping the other seams that were within the shower, i noticed that the Durock extended past where the wife decided she NOW didn't want tile. So i was just making sure I would be able to finish that in a way to match the rest of the drywall since its out of the shower. This is what the technical specialist said i had to do.

That being said, I was just asking about the primer in the case the ole' lady changed her mind and I decide to put some tile a bit further past the shower than planned. Someone suggested that priming the section first would keep the water in the thinset from turning the compound back to mud again?

Not sure, I think i have heard people say that's why you use thin set on floor underlayment seams....though i always thought it was just because it was stronger.

Have a great day gents, 7" of snow coming at noon here!
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Unread 01-11-2019, 11:00 AM   #15
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Mike where you at in Missouri? Just curious. I am in the Kansas City metro area. We got a touch of snow coming through here before it crushes St Louis
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